OF · MINUTE BODIES, · MADE BY · MAGNIFYING GLASSES; · WITH

OBSERVATIONS and INQUIRIES thereupon.

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Observ. I. _Of the Point of a sharp small Needle._

As in _Geometry_, the most natural way of beginning is from a Mathematical _point_; so is the same method in Observations and _Natural history_ the most genuine, simple, and instructive. We must first endevour to make _letters_, and draw _single_ strokes true, before we venture to write whole _Sentences_, or to draw large _Pictures_. And in _Physical_ Enquiries, we must endevour to follow Nature in the more _plain_ and _easie_ ways she treads in the most _simple_ and _uncompounded bodies_, to trace her steps, and be acquainted with her manner of walking there, before we venture our selves into the multitude of _meanders_ she has in _bodies of a more complicated_ nature; lest, being unable to distinguish and judge of our way, we quickly lose both _Nature_ our Guide, and _our selves_ too, and are left to wander in the _labyrinth_ of groundless opinions; wanting both _judgment_, that _light_, and _experience_, that _clew_, which should direct our proceedings.

We will begin these our Inquiries therefore with the Observations of Bodies of the most _simple nature_ first, and so gradually proceed to those of a more _compounded_ one. In prosecution of which method, we shall begin with a _Physical point_; of which kind the _Point of a Needle_ is commonly reckon’d for one; and is indeed, for the most part, made so sharp, that the naked eye cannot distinguish any parts of it: It very easily pierces, and makes its way through all kind of bodies softer then it self: But if view’d with a very good _Microscope_, we may find that the _top_ of a Needle (though as to the sense very _sharp_) appears a _broad_, _blunt_, and very _irregular_ end; not resembling a Cone, as is imagin’d, but onely a piece of a tapering body, with a great part of the top remov’d, or deficient. The Points of Pins are yet more blunt, and the Points of the most curious Mathematical Instruments do very seldome arrive at so great a sharpness; how much therefore can be built upon demonstrations made onely by the productions of the Ruler and Compasses, he will be better able to consider that shall but view those _points_ and _lines_ with a _Microscope_.

Now though this point be commonly accounted the sharpest (whence when we would express the sharpness of a point the most _superlatively_, we say, As sharp as a Needle) yet the _Microscope_ can afford us hundreds of Instances of Points many thousand times sharper: such as those of the _hairs_, and _bristles_, and _claws_ of multitudes of _Insects_; the _thorns_, or _crooks_, or _hairs_ of _leaves_, and other small vegetables; nay, the ends of the _stiriæ_ or small _parallelipipeds_ of _Amianthus_, and _alumen plumosum_; of many of which, though the Points are so sharp as not to be visible, though view’d with a _Microscope_ (which magnifies the Object, in bulk, above a million of times) yet I doubt not, but were we able _practically_ to make _Microscopes_ according to the _theory_ of them, we might find hills, and dales, and pores, and a sufficient bredth, or expansion, to give all those parts elbow-room, even in the blunt top of the very Point of any of these so very sharp bodies. For certainly the _quantity_ or extension of any body may be _Divisible in infinitum_, though perhaps not the _matter_.

But to proceed: The Image we have here exhibited in the first Figure[1], was the top of a small and very sharp Needle, whose point _aa_ nevertheless appear’d through the _Microscope_ above a quarter of an inch broad, not round nor flat, but _irregular_ and _uneven_; so that it seem’d to have been big enough to have afforded a hundred armed Mites room enough to be rang’d by each other without endangering the breaking one anothers necks, by being thrust off on either side. The surface of which, though appearing to the naked eye very smooth, could not nevertheless hide a multitude of holes and scratches and ruggednesses from being discover’d by the _Microscope_ to invest it, several of which inequalities (as A, B, C, seem’d _holes_ made by some small specks of _Rust_; and D some _adventitious body_, that stuck very close to it) were _casual_. All the rest that roughen the surface, were onely so many marks of the rudeness and bungling of _Art_. So unaccurate is it, in all its productions, even in those which seem most neat, that if examin’d with an organ more acute then that by which they were made, the more we see of their _shape_, the less appearance will there be of their _beauty_: whereas in the works of _Nature_, the deepest Discoveries shew us the greatest Excellencies. An evident Argument, that he that was the Author of all these things, was no other then _Omnipotent_; being able to include as great a variety of parts and contrivances in the yet smallest Discernable Point, as in those vaster bodies (which comparatively are called also Points) such as the _Earth_, _Sun_, or _Planets_. Nor need it seem strange that the Earth it self may be by an _Analogie_ call’d a Physical Point: For as its body, though now so near us as to fill our eys and fancies with a sense of the vastness of it, may by a little Distance, and some convenient _Diminishing_ Glasses, be made vanish into a scarce visible Speck, or Point (as I have often try’d on the _Moon_, and (when not too bright) on the _Sun_ it self.) So, could a Mechanical contrivance succesfully answer our _Theory_, we might see the least spot as big as the Earth it self; and Discover, as _Des Cartes_[2] also conjectures, as great a variety of bodies in the _Moon_, or _Planets_, as in the _Earth_.

But leaving these Discoveries to future Industries, we shall proceed to add one Observation more of a _point_ commonly so call’d, that is, the mark of a _full stop_, or _period_. And for this purpose I observed many both _printed_ ones and _written_; and among multitudes I found _few_ of them more _round_ or _regular_ then this which I have delineated in the third figure of the second Scheme, but _very many_ abundantly _more disfigur’d_; and for the most part if they seem’d equally round to the eye, I found those points that had been made by a _Copper-plate_, and Roll-press, to be as misshapen as those which had been made with _Types_, the most curious and smothly _engraven strokes_ and _points_, looking but as so many _furrows_ and _holes_, and their _printed impressions_, but like _smutty daubings_ on a matt or uneven floor with a blunt extinguisht brand or stick’s end. And as for _points_ made with a _pen_ they were much _more ragged_ and _deformed_. Nay, having view’d certain pieces of exceeding curious writing of the kind (one of which in the bredth of a _two-pence_ compris’d _the Lords prayer, the Apostles Creed, the ten Commandments, and about half a dozen verses besides of the Bible_, whose _lines_ were _so small_ and _near together_, that I was unable to _number_ them with my _naked eye_,) a very ordinary _Microscope_, I had then about me, inabled me to see that what the Writer of it had asserted was _true_, but withall discover’d of what pitifull _bungling scribbles_ and _scrawls_ it was compos’d, _Arabian_ and _China characters_ being almost as well shap’d, yet thus much I must say for the Man, that it was for the most part _legible_ enough, though in some places there wanted a good _fantsy_ well _preposest_ to help one through. If this manner _of small writing_ were made _easie_ and _practicable_ (and I think I know such a one, but have never yet made tryal of it, whereby one might be inabled to write _a great deale_ with _much ease_, and _accurately_ enough in a very _little roome_) it might be of very good use to convey _secret Intelligence_ without any danger of _Discovery_ or _mistrusting_. But to come again to the point. The _Irregularities_ of it are caused by three or four _coadjutors_, one of which is, the _uneven surface_ of the _paper_, which at best appears no smother then a very course piece of _shag’d cloth_, next the _irregularity of the Type_ or _Ingraving_, and a third is the _rough Daubing_ of the _Printing-Ink_ that lies upon the instrument that makes the impression, to all which, add the _variation_ made by the Different _lights_ and _shadows_, and you may have sufficient reason to ghess that a _point_ may appear much more _ugly_ then _this_, which I have here presented, which though it appear’d through the _Microscope_ _gray_, like a great splatch of _London_ dirt, about three inches over; yet to the _naked eye_ it was _black_ and no bigger then that in the midst of the Circle A. And could I have found Room in this Plate to have inserted an O you should have seen that the _letters_ were not more distinct then the _points_ of Distinction, nor a _drawn circle_ more exactly _so_, then we have now shown a _point_ to be a _point_.

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Observ. II. _Of the Edge of a Razor._

The sharpest _Edge_ hath the same kind of affinity to the sharpest _Point_ in Physicks, as a _line_ hath to a _point_ in Mathematicks; and therefore the Treaty concerning this, may very properly be annexed to the former. A Razor doth appear to be a Body of a very neat and curious aspect, till more closely viewed by the _Microscope_, and there we may observe its very Edge to be of all kind of shapes, except what it should be. For examining that of a very sharp one, I could not find that any part of it had any thing of sharpness in it; but it appeared a rough surface of a very considerable bredth from side to side, the narrowest part not seeming thinner then the back of a pretty thick Knife. Nor is’t likely that it should appear any otherwise, since as we just now shew’d that a _point_ appear’d a _circle_, ’tis rational a _line_ should be a _parallelogram_.

Now for the drawing this second Figure[3] (which represents a part of the Edge about half a quarter of an inch long of a Razor well set) I so plac’d it between the Object-glass & the light, that there appear’d a reflection from the very Edge, represented by the white line abcdef. In which you may perceive it to be somewhat sharper then elsewhere about d, to be indented or pitted about b, to be broader and thicker about c, and unequal and rugged about e, and pretty even between ab and ef. Nor was that part of the Edge ghik so smooth as one would imagine so smooth bodies as a Hone and Oyl should leave it; for besides those multitudes of scratches, which appear to have raz’d the surface ghik, and to cross each other every way which are not half of them exprest in the Figure, there were several great and deep scratches, or furrows, such as gh and ik, which made the surface yet more rugged, caus’d perhaps by some small Dust casually falling on the Hone, or some harder or more flinty part of the Hone it self. The other part of the Razor ll, which is polish’d on a grinding-stone, appear’d much rougher then the other, looking almost like a plow’d field, with many parallels, ridges, and furrows, and a cloddy, as ’twere, or an uneven surface: nor shall we wonder at the roughnesses of those surfaces, since even in the most curious wrought Glasses for _Microscopes_, and other Optical uses, I have, when the Sun has shone well on them, discover’d their surface to be variously raz’d or scratched, and to consist of an infinite of small broken surfaces, which reflect the light of very various and differing colours. And indeed it seems impossible by Art to cut the surface of any hard and brittle body smooth, since _Putte_, or even the most curious _Powder_ that can be made use of, to polish such a body, must consist of little hard rough particles, and each of them must cut its way, and consequently leave some kind of gutter or furrow behind it. And though Nature does seem to do it very readily in all kinds of fluid bodies, yet perhaps future observators may discover even these also rugged; it being very probable, as I elsewhere shew, that fluid bodies are made up of small solid particles variously and strongly mov’d, and may find reason to think there is scarce a surface _in rerum naturâ_ perfectly smooth. The black spot mn, I ghess to be some small speck of rust, for that I have oft observ’d to be the manner of the working of Corrosive Juyces. To conclude, this Edge and piece of a Razor, if it had been really such as it appear’d through the _Microscope_, would scarcely have serv’d to cleave wood, much less to have cut off the hair of beards, unless it were after the manner that _Lucian_ merrily relates _Charon_ to have made use of, when with a Carpenters Axe he chop’d off the beard of a sage Philosopher, whose gravity he very cautiously fear’d would indanger the oversetting of his Wherry.

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Observ. III. _Of fine Lawn, or Linnen Cloth._

This is another product of Art, A piece of the finest Lawn I was able to get, so curious that the threads were scarce discernable by the naked eye, and yet through an ordinary _Microscope_ you may perceive[4] what a goodly piece of _coarse Matting_ it is; what proportionable cords each of its threads are, being not unlike, both in shape and size, the bigger and coarser kind of _single Rope-yarn_, wherewith they usually make _Cables_. That which makes the Lawn so transparent, is by the _Microscope_, nay by the naked eye, if attentively viewed, plainly enough evidenced to be the multitude of square holes which are left between the threads, appearing to have much more hole in respect of the intercurrent parts then is for the most part left in a _lattice-window_, which it does a little resemble, onely the crossing parts are round and not flat.

These threads that compose this fine contexture, though they are as small as those that constitute the finer sorts of Silks, have notwithstanding nothing of their glossie, pleasant, and lively reflection. Nay, I have been informed both by the Inventor himself, and several other eye-witnesses, that though the flax, out of which it is made, has been (by a singular art, of that excellent Person, and Noble Virtuoso, M. _Charls Howard_, brother to the _Duke of Norfolk_) so curiously dress’d and prepar’d, as to appear both to the eye and the touch, full as _fine_ and as _glossie_, and to receive all kinds of colours, as well as Sleave-Silk; yet when this Silken Flax is twisted into threads, it quite loseth its former luster, and becomes as plain and base a thread to look on, as one of the same bigness, made of common Flax.

The reason of which odd _Phenomenon_ seems no other then this; that though the curiously drest Flax has its parts so exceedingly small, as to equallize, if not to be much smaller then the clew of the Silk-worm, especially in thinness, yet the differences between the figures of the constituting filaments are so great, and their substances so various, that whereas those of the _Silk_ are _small_, _round_, _hard_, _transparent_, and to their bigness proportionably _stiff_, so as each filament preserves its proper _Figure_, and consequently its vivid _reflection_ intire, though twisted into a thread, if not too hard; those of Flax are _flat_, _limber_, _softer_, and _less transparent_, and in twisting into a thread they joyn, and lie so close together, as to lose their own, and destroy each others particular reflections. There seems therefore three Particulars very requisite to make the so drest Flax appear Silk also when spun into threads. First, that the substance of it should be made more _clear_ and _transparent_, Flax retaining in it a kind of opacating brown, or yellow; and the parts of the whitest kind I have yet observ’d with the _Microscope_ appearing white, like flaw’d Horn or Glass, rather then clear, like clear Horn or Glass. Next that, the filaments should each of them be _rounded_, if that could be done, which yet is not so very necessary, if the first be perform’d, and this third, which is, that each of the small filaments be _stifned_; for though they be square, or flat, provided they be _transparent_ and stiff, much the same appearances must necessarily follow. Now, though I have not yet made trial, yet I doubt not, but that both these proprieties may be also induc’d upon the Flax, and perhaps too by one and the same Expedient, which some trials may quickly inform any ingenious attempter of, who from the use and profit of such an Invention, may find sufficient argument to be prompted to such Inquiries. As for the _tenacity_ of the substance of Flax, out of which the thread is made, it seems much inferiour to that of Silk, the one being a _vegetable_, the other an _animal_ substance. And whether it proceed from the better concoction, or the more homogeneous constitution of _animal_ substances above those of _vegetables_, I do not here determine; yet since I generally find, that _vegetable_ substances do not equalize the _tenacity_ of _animal_, nor these the _tenacity_ of some purified _mineral_ substances; I am very apt to think, that the _tenacity_ of bodies does not proceed from the _hamous_, or _hooked_ particles, as the _Epicureans_ and some modern _Philosophers_ have imagin’d; but from the more exact _congruity_ of the constituent parts, which are contiguous to each other, and so bulky, as not to be easily separated, or shatter’d, by any small pulls or concussion of heat.

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Observ. IV. _Of fine waled Silk, or Taffety._

This[5] is the appearance of a piece of very fine Taffety-riband in the bigger magnifying Glass, which you see exhibits it like a very convenient substance to make Bed-matts, or Door-matts of, or to serve for Beehives, Corn-scuttles, Chairs, or Corn-tubs, it being not unlike that kind of work, wherewith in many parts in _England_, they make such Utensils of Straw, a little wreathed, and bound together with thongs of Brambles. For in this Contexture, each little filament, fiber, or clew of the Silk-worm, seem’d about the bigness of an ordinary Straw, as appears by the little irregular pieces, ab, cd, and ef; The _Warp_, or the thread that ran crossing the Riband, appear’d like a single Rope of an Inch Diameter; but the _Woof_, or the thread that ran the length of the Riband, appear’d not half so big. Each Inch of six-peny-broad Riband appearing no less then a piece of Matting Inch and half thick, and twelve foot square, a few yards of this, would be enough to floor the long Gallery of the _Loure_ at _Paris_. But to return to our piece of Riband: It affords us a not unpleasant object, appearing like a bundle, or wreath, of very clear and transparent _Cylinders_, if the Silk be white, and curiously ting’d; if it be colour’d, each of those small horney _Cylinders_ affording in some place or other of them, as vivid a reflection, as if it had been sent from a _Cylinder_ of Glass or Horn. Insomuch, that the reflections of Red, appear’d as if coming from so many _Granates_, or _Rubies_. The loveliness of the colours of Silks above those of hairy Stuffs, or Linnen, consisting, as I else-where intimate, chiefly in the transparency, and vivid reflections from the _Concave_, or inner surface of the _transparent Cylinder_, as are also the colours of Precious Stones; for most of the reflections from each of these _Cylinders_, come from the _Concave_ surface of the air, which is as ’twere the foil that incompasses the _Cylinder_. The colours with which each of these _Cylinders_ are ting’d, seem partly to be superficial, and sticking to the out-sides of them; and partly, to be imbib’d, or sunck into the substance of them: for Silk, seeming to be little else then a dried thread of Glew, may be suppos’d to be very easily relaxt, and softened, by being steeped in warm, nay in cold, if penetrant, juyces or liquors. And thereby those tinctures, though they tinge perhaps but a small part of the substance, yet being so highly impregnated with the colour, as to be almost black with it, may leave an impression strong enough to exhibite the desir’d colour. A pretty kinde of artificial Stuff I have seen, looking almost like transparent Parchment, Horn, or Ising-glass, and perhaps some such thing it may be made of, which being transparent, and of a glutinous nature, and easily mollified by keeping in water, as I found upon trial, had imbib’d, and did remain ting’d with a great variety of very vivid colours, and to the naked eye, it look’d very like the substance of the Silk. And I have often thought, that probably there might be a way found out, to make an artificial glutinous composition, much resembling, if not full as good, nay better, then that Excrement, or whatever other substance it be out of which, the Silk-worm wire-draws his clew. If such a composition were found, it were certainly an easie matter to find very quick ways of drawing it out into small wires for use. I need not mention the use of such an Invention, nor the benefit that is likely to accrue to the finder, they being sufficiently obvious. This hint therefore, may, I hope, give some Ingenious inquisitive Person an occasion of making some trials, which if successfull, I have my aim, and I suppose he will have no occasion to be displeas’d.

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Observ. V. _Of watered Silks, or Stuffs._

There are but few _Artificial_ things that are worth observing with a _Microscope_, and therefore I shall speak but briefly concerning them. For the Productions of art are such rude mis-shapen things, that when view’d with a _Microscope_, is little else observable, but their deformity. The most curious Carvings appearing no better then those rude _Russian_ Images we find mention’d in _Purchas_, where three notches at the end of a Stick, stood for a face. And the most smooth and burnish’d surfaces appear most rough and unpolisht: So that my first Reason why I shall add but a few observations of them, is, their mis-shapen form; and the next, is their uselessness. For why should we trouble our selves in the examination of that form or shape (which is all we are able to reach with a _Microscope_) which we know was design’d for no higher a use, then what we were able to view with our naked eye? Why should we endeavour to discover mysteries in that which has no such thing in it? And like _Rabbins_ find out _Caballisms_, and _ænigmâs_ in the Figure, and placing of Letters, where no such thing lies hid: whereas in _natural_ forms there are some so small, and so curious, and their design’d business so far remov’d beyond the reach of our sight, that the more we magnify the object, the more excellencies and mysteries do appear; And the more we discover the imperfections of our senses; and the Omnipotency and Infinite perfections of the great Creatour. I shall therefore onely add one or two Observations more _artificial_ things, and then come to the Treaty concerning such matters as are the Productions of a more curious Workman. One of these, shall be that of a piece of water’d Silk, represented in the second Figure of the third _Scheme_,[6] as it appear’d through the least magnifying Glass. _AB_ signifying the long way of the Stuff, and _CD_ the broad way. This Stuff, if the right side of it be looked upon, appears to the naked eye, all over so waved, undulated, or grain’d, with a curious, though irregular variety of brighter and darker parts, that it adds no small gracefulness to the Gloss of it. It is so known a propriety, that it needs but little explication, but it is observable, which perhaps everyone has not considered, that those parts which appear the darker part of the wave, in one position to the light, in another appears the lighter, and the contrary; and by this means the undulations become transient, and in a continual change, according as the position of the parts in respect of the incident beams of light is varied. The reason of which odd _phænomena_, to one that has but diligently examin’d it even with his naked eye, will be obvious enough. But he that observes it with a _Microscope_, may more easily perceive what this _Proteus_ is, and how it comes to change its shape. He may very easily perceive, that it proceeds onely from the variety of the _Reflections_ of light, which is caus’d by the various _shape of the Particles_, or little protuberant parts of the thread that compose the surface; and that those parts of the waves that appear the brighter, throw towards the eye a multitude of small reflections of light, whereas the darker scarce afford any. The reason of which reflection, the _Microscope_ plainly discovers, as appears by the Figure. In which you may perceive, that the brighter parts of the surface consist of an abundance of large and strong reflections, denoted by a, a, a, a, a, &c. for the surfaces of those threads that run the _long way_, are by the Mechanical process of watering, _creas’d_ or _angled_ in another kind of posture then they were by the weaving: for by the weaving they are onely _bent round_ the warping threads; but by the watering, they are _bent with an angle, or elbow_, that is in stead of lying, or being bent _round_ the threads, as in the third Figure, a, a, a, a, a, are about b, b, b (b, b, b representing the ends, as ’twere, of the cross threads, they are bent about) they are creas’d on the top of those threads, with an _angle_, as in the fourth Figure, and that with all imaginable variety; so that, whereas before they reflected the light onely from one point of the round surface, as about c, c, c, they now when water’d, reflect the beams from more then half the whole surface, as de, de, de, and in other postures they return no reflections at all from those surfaces. Hence in one posture they compose the brighter parts of the waves, in another the darker. And these reflections are also varied, according as the particular parts are variously bent. The reason of which creasing we shall next examine; and here we must fetch our information from the Mechanism or manner of proceeding in this operation; which, as I have been inform’d, is no other then this.

They double all the Stuff that is to be water’d, that is, they crease it just through the middle of it, the whole length of the piece, leaving the right side of the Stuff inward, and placing the two edges, or silvages just upon one another, and, as near as they can, place the wale so in the doubling of it, that the wale of the one side may lie very near parallel, or even with the wale of the other; for the nearer that posture they lie, the greater will the watering appear; and the more obliquely, or across to each other they lie, the smaller are the waves. Their way for folding it for a great wale is thus: they take a Pin, and begin at one side of the piece in any wale, and so moving it towards the other side, thereby direct their hands to the opposite ends of the wale, and then, as near as they can, place the two opposite ends of the same wale together, and so double, or fold the whole piece, repeating this enquiry with a Pin at every yard or two’s distance through the whole length; then they sprinkle it with water, and fold it the long-ways, placing between every fold a piece of Pastboard, by which means all the wrong side of the water’d Stuff becomes flat, and with little wales, and the wales on the other side become the more protuberant; whence the creasings or angular bendings of the wales become the more perspicuous. Having folded it in this manner, they place it with an interjacent Pastboard into an hot Press, where it is kept very violently prest, till it be dry and stiff; by which means, the wales of either contiguous sides leave their own impressions upon each other, as is very manifest by the second Figure, where ’tis obvious enough, that the wale of the piece ABCD runs parallel between the pricked lines ef, ef, ef, and as manifest to discern the impressions upon these wales, left by those that were prest upon them, which lying not exactly parallel with them, but a little athwart them, as is denoted by the lines of, oooo, gh, gh, gh, between which the other wales did lie parallel; they are so variously, and irregularly creas’d that being put into that shape when wet, and kept so till they be drie, they so let each others threads, that the Moldings remain almost as long as the Stuff lasts.

Hence it may appear to any one that attentively considers the Figure, why the parts of the wale a, a, a, a, a, a, should appear bright; and why the parts b, b, b, b, b, b, b, should appear shadowed, or dark; why some, as d, d, d, d, d, d, should appear partly light, and partly dark: the varieties of which reflections and shadows are the only cause of the appearance of watering in Silks, or any other kind of Stuffs.

From the variety of reflection, may also be deduc’d the cause why a small breez or gale of wind ruffling the surface of a smooth water, makes it appear black; as also, on the other side, why the smoothing or burnishing the surface of whitened Silver makes it look black; and multitudes of other phænomena might hereby be solv’d, which are too many to be here insisted on.

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Observ. VI. _Of Small Glass Canes._

That I might be satisfi’d, whether it were not possible to make an _Artificial_ pore as _small_ as any _Natural_ I had yet found, I made several attempts with small _glass pipes_, melted in the flame of a Lamp, and then very _suddenly_ drawn out into a great length. And, by _that means_, without much difficulty, I was able to draw some almost as small as a _Cobweb_, which yet, with the _Microscope_, I could plainly perceive[7] to be _perforated_, both by looking on the _ends_ of it, and by looking on it _against the light_ which was much the _easier way_ to determine whether it were solid or perforated; for, taking a small pipe of glass, and closing one end of it, then filling it _half full_ of water, and holding it _against the light_, I could, by this means, very easily find what was the _differing aspect_ of a _solid_ and a _perforated_ piece of glass; and so easily distinguish, without seeing either end, whether any _Cylinder_ of glass I look’d on, were a _solid stick_, or a _hollow cane_. And by this means, I could also presently judge of any small _filament_ of glass, whether it were _hollow_ or _not_, which would have been exceeding tedious to examine by looking on the end. And many such like ways I was fain to make use of, in the examining of divers other particulars related in this Book, which would have been no easie task to have determined meerly by the more common way of looking on, or viewing the Object. For, if we consider first, the very _faint light_ wherewith the object is enlightened, whence many particles appear _opacous_, which when more enlightned, appear very _transparent_, so that I was fain to _determine_ its _transparency_ by one glass, and its _texture_ by another. Next, the _unmanageableness_ of most _Objects_, by reason of their _smalness_, 3. The _difficulty of finding_ the desired point, and of _placing_ it so, as to reflect the _light conveniently_ for the Inquiry. Lastly, ones being able to view it but with _one eye_ at once, they will appear no small _obstructions_, nor are they easily _remov’d_ without many _contrivances_. But to proceed, I could not find that water, or some _deeply ting’d_ liquors would in small ones rise so high as one would expect; and the _highest_ I have found it yet rise in any of the pipes I have try’d, was to 21 _inches_ above the level of the water in the vessel: for though I found that in the small pipes it would _nimbly enter_ at first, and run about 6 or 7 _inches_ upwards; yet I found it then to move upwards _so slow_, that I have not yet had the _patience_ to observe it above that height of 21 _inches_ (and that was in a pretty _large Pipe_, in comparison of those I formerly mentioned; for I could observe the _progress_ of a _very deep ting’d liquor_ in it with my _naked eye_, without much trouble; whereas many of the _other pipes_ were so _very small_, that unless in a _convenient posture_ to the light, I could not perceive _them_:) But ’tis very probable, that a greater _patience_ and _assiduity_ may discover the liquors to _rise_, at least to remain _suspended_, at heights that I should be loath now even to _ghess_ at, if at least there be any _proportion_ kept between the height of the ascending liquor, and the _bigness of the holes_ of the pipes.

_AN ATTEMPT FOR THE EXPLICATION OF THIS EXPERIMENT._

My Conjecture, _That the unequal height of the surfaces of the water, proceeded from the greater pressure made upon the water by the Air without the Pipes_ ABC, _then by that within them_[8]; I shall endeavour to confirm from the truth of the two following _Propositions_:

The first of which is, _That an unequal pressure of the incumbent Air, will cause an unequal height in the water’s Surfaces_.

And the second is, _That in this experiment there is such an unequal pressure_.

That the first is true, the following _Experiment_ will evince. For if you take any Vessel so contrived, as that you can at pleasure either _increase_ or _diminish_ the _pressure_ of the Air upon this or that part of the _Superficies_ of the _water_, the _equality_ of the height of those parts will presently be _lost_; and that part of the _Superficies_ that sustains the _greater pressure_, will be _inferior_ to that which undergoes the _less_. A fit Vessel for this purpose, will be an inverted Glass _Syphon_, such an one as is described in the _Sixth Figure_. For if into it you put Water enough to fill it as high as _AB_, and gently blow in at _D_, you shall _depress_ the Superficies _B_, and thereby _raise_ the opposite Superficies _A_ to a _considerable height_, and by gently _sucking_ you may produce clean _contrary_ effects.

Next, That there is such an _unequal pressure_, I shall prove from this, _That there is a much greater incongruity of Air to Glass, and some other Bodies, then there is of Water to the same_.

By _Congruity, I mean a property of a fluid Body, whereby any part of it is readily united with any other part, either of itself, or of any other Similar, fluid, or solid body: And by Incongruity a property of a fluid, by which it is hindred from uniting with any dissimilar, fluid, or solid Body._

This last property, any one that hath been observingly conversant about fluid Bodies, cannot be ignorant of. For (not now to mention several _Chymical Spirits_ and _Oyls_, which will _very hardly_, if at _all_, be brought to _mix_ with one another; insomuch that there may be found some 8 or 9, or more, several distinct Liquors, which _swimming_ one upon another, will not presently _mix_) we need seek no further for Examples of this kind in _fluids_, then to observe the _drops of rain_ falling through the _air_ and the _bubbles of air_ which are by any means conveyed under the surface of the _water_; or a drop of common _Sallet-Oyl_ swimming upon water. In all which, and many more examples of this kind that might be enumerated, the _incongruity_ of two _fluids_ is easily discernable. And as for the _Congruity_ or _Incongruity_ of Liquids, with several kinds of _firm_ Bodies, they have long since been taken notice of, and called by the Names of _Driness_ and _Moisture_ (though these two names are not comprehensive enough, being commonly used to signifie only the adhering or not adhering of _water_ to some other _solid Bodies_) of this kind we may observe that _water_ will more readily _wet some woods_ then _others_; and that _water_, let fall upon a _Feather_, the whiter side of a _Colwort_, and some other leaves, or upon almost any _dusty_, _unctuous_, or _resinous_ superficies, will not _at all adhere_ to them, but easily _tumble off_ from them, like a solid _Bowl_; whereas, if dropt upon _Linnen_, _Paper_, _Clay_, _green Wood_, &c. it will not be taken off, without leaving some part of it behind _adhering_ to them. So _Quick-silver_, which will very _hardly_ be brought to _stick_ to any _vegetable body_, will _readily adhere_ to, and _mingle_ with, several clean _metalline bodies_.

And that we may the better finde what the _cause_ of _Congruity_ and _Incongruity_ in bodies is, it will be requisite to consider, First, what is the _cause_ of _fluidness_; And this, _I conceive_, to be nothing else but a certain _pulse_ or _shake_ of _heat_; for Heat being nothing else but a very _brisk_ and _vehement agitation_ of the parts of a body (as I have elswhere made _probable_) the parts of a body are thereby made so _loose_ from one another, that they easily _move any way_, and become _fluid_. That I may explain this a little by a gross Similitude, let us suppose a dish of sand set upon some body that is very much _agitated_, and shaken with some _quick_ and _strong vibrating motion_, as on a _Milstone_ turn’d round upon the under stone very violently whilst it is empty; or on a very stiff _Drum_-head, which is vehemently or very nimbly beaten with the Drumsticks. By this means, the sand in the dish, which before lay like a _dull_ and unactive body, becomes a perfect _fluid_; and ye can no sooner make a _hole_ in it with your finger, but it is immediately _filled up again_, and the upper surface of it _levell’d_. Nor can you _bury_ a _light body_, as a piece of Cork under it, but it presently _emerges_ or _swims_ as ’twere on the top; nor can you lay a _heavier_ on the top of it, as a piece of Lead, but it is immediately _buried_ in Sand, and (as ’twere) sinks to the bottom. Nor can you make a _hole_ in the side of the Dish, but the sand shall _run out_ of it to a _level_, not an _obvious property_ of a fluid body, as such, but this dos _imitate_; and all this meerly caused by the vehement _agitation_ of the conteining vessel; for by this means, _each_ sand becomes to have a _vibrative_ or _dancing_ motion, so as no other heavier body can _rest_ on it, unless _sustein’d_ by some other on either side: Nor will it suffer any Body to be _beneath_ it, unless it be a _heavier_ then it self. Another Instance of the strange _loosening_ nature of a violent jarring Motion, or a strong and nimble vibrative one, we may have from a piece of _iron_ grated on very strongly with a _file_: for if into that a pin _screw’d_ so firm and hard, that though it has a convenient head to it, yet it can by no means be _unscrew’d_ by the fingers; if, I say, you attempt to unscrew this whilst _grated on by the file_, it will be found to undoe and turn very _easily_. The first of these Examples manifests, how a body actually _divided_ into small parts, becomes a _fluid_. And the latter manifests by what means the agitation of heat so easily _loosens_ and _unties_ the parts of _solid_ and _firm_ bodies. Nor need we suppose heat to be any thing else, besides such a motion; for supposing we could _Mechanically_ produce such a one _quick_ and _strong_ enough, we need not spend _fuel_ to _melt_ a body. Now, that I do not speak this altogether groundless, I must refer the Reader to the Observations I have made upon the shining sparks of Steel, for there he shall find that _the same_ effects are produced upon small chips or parcels of Steel by the _flame_, and by _a quick and violent motion_; and if the body of _steel_ may be thus melted (as I there shew it may) I think we have little reason to doubt that almost _any other_ may not also. Every Smith can inform one how quickly both his _File_ and the _Iron_ grows _hot_ with _filing_, and if you _rub_ almost any two _hard_ bodies together, they will do the same: And we know, that a sufficient degree of heat causes _fluidity_, in some bodies much sooner, and in others later; that is, the parts of the body of some are so _loose_ from one another, and so _unapt to cohere_, and so _minute_ and _little_, that a very _small_ degree of agitation keeps them always in the _state of fluidity_. Of this kind, I suppose, the _Æther_, that is the _medium_ or _fluid_ body, in which all other bodies do as it were swim and move; and particularly, the _Air_, which seems nothing else but a kind of _tincture_ or _solution_ of terrestrial and aqueous particles _dissolv’d_ into it, and agitated by it, just as the _tincture_ of _Cocheneel_ is nothing but some finer _dissoluble_ parts of that Concrete lick’d up or _dissolv’d_ by the _fluid_ water. And from this Notion of it, we may easily give a more Intelligible reason how the Air becomes so capable of _Rarefaction_ and _Condensation_. For, as in _tinctures_, one grain of some _strongly tinging_ substance may _sensibly_ colour some _hundred thousand_ grains of _appropriated_ Liquors, so as every _drop_ of it has its proportionate share, and be sensibly ting’d, as I have try’d both with _Logwood_ and _Cocheneel_: And as some few grains of _Salt_ is able to infect as great a quantity, as may be found by _præcipitations_, though not so easily by the _sight_ or _taste_; so the _Air_, which seems to be but as ’twere a _tincture_ or _saline substance, dissolv’d and agitated by the fluid and agil Æther_, may disperse and _expand_ it self into a _vast space_, if it have room enough, and infect, as it were, every part of that space. But, as on the other side, if there be but some _few grains_ of the liquor, it may _extract all_ the colour of the tinging substance, and may _dissolve_ all the Salt, and thereby become _much more impregnated_ with those substances, so may _all_ the air that sufficed in a _rarify’d state_ to fill some _hundred thousand_ spaces of Æther, be compris’d in only _one_, but in a position proportionable _dense_. And though we have not yet found out such _strainers_ for Tinctures and Salts as we have for the Air, being yet unable to _separate_ them from their dissolving liquors by any kind of _filtre_, without _præcipitation_, as we are able to _separate_ the Air from the Æther by _Glass_, and several other bodies. And though we are yet unable and ignorant of the ways of _præcipitating_ Air out of the Æther as we can Tinctures, and Salts out of several _dissolvents_; yet neither of these seeming _impossible_ from the nature of the things, nor so _improbable_ but that some happy future industry may find out ways to effect them; nay, further, since we find that Nature _does really perform_ (though by what means we are not certain) both these actions, namely, by _præcipitating_ the Air in Rain and Dews, and by supplying the Streams and Rivers of the World with fresh water, _strain’d_ through secret subterraneous Caverns: And since, that in very many other _proprieties_ they do so exactly _seem_ of the _same nature_; till further observations or tryals do inform us of the _contrary_, we may _safely enough conclude_ them of the _same kind_. For it seldom happens that any two natures have so many properties _coincident_ or the _same_, as I have observ’d Solutions and Air to have, and to be _different_ in the rest. And therefore I think it neither _impossible_, _irrational_, nay nor _difficult_ to be able to _predict_ what is _likely_ to happen in other particulars also, besides those which _Observation_ or _Experiment_ have declared thus or thus; especially, if the _circumstances_ that do often very much conduce to the variation of the effects be duly _weigh’d_ and _consider’d_. And indeed, were there not a _probability_ of this, our _inquiries_ would be _endless_, our _tryals vain_, and our greatest _inventions_ would be nothing but the meer _products_ of _chance_, and not of _Reason_; and, like _Mariners_ in an Ocean, destitute both of a _Compass_ and the sight of the _Celestial guides_, we might indeed, _by chance_, Steer _directly_ towards our desired Port, but ’tis _a thousand to one_ but we _miss_ our aim. But to proceed, we may hence also give a plain reason, how the Air comes to be _darkned_ by _clouds_, &c. which are nothing but a kind of _precipitation_, and how those _precipitations_ fall down in _Showrs_. Hence also could I very easily, and I think truly, deduce the cause of the curious _sixangular figures_ of Snow, and the appearances of _Haloes, &c._ and the sudden _thickning_ of the Sky with Clouds, and the _vanishing_ and _disappearing_ of those Clouds again; for all these things may be very easily _imitated_ in a _glass of liquor_, with some slight _Chymical preparations_ as I have often try’d, and may somewhere else more largely relate, but have not now time to set them down. But to proceed, there are other bodies that consist of particles more _Gross_, and of a more _apt_ figure for _cohesion_, and this requires _somewhat greater_ agitation; such, I suppose ☿, _fermented vinous_ _Spirits_, several _Chymical Oils_, which are much of kin to those Spirits, &c. Others yet require a _greater_, as _water_, and so others _much greater_, for almost infinite degrees: For, I suppose there are very _few_ bodies in the world that may not be made _aliquatenus_ fluid, by _some_ or _other_ degree of agitation or heat.

Having therefore in short set down my Notion of a Fluid body, I come in the next place to consider what _Congruity_ is; and this, as I said before, being a _Relative property_ of a fluid, whereby it may be said to be _like_ or _unlike_ to this or that other body, whereby it _does_ or _does not mix_ with this or that body. We will again have recourse to our former Experiment, though but a rude one; and here if we mix in the dish _several kinds_ of sands, some of _bigger_, others of _less_ and finer bulks, we shall find that by the agitation _the fine sand_ will _eject_ and _throw out_ of it self all those _bigger_ bulks of small _stones_ and the like, and those will _be gathered_ together all into _one_ place; and if there be _other_ bodies in it of other natures, those also will be _separated_ into a place by themselves, and _united_ or _tumbled_ up together. And though this do not come up to the _highest property_ of _Congruity_, which is a _Cohæsion_ of the parts of the fluid together, or a kind of _attraction_ and _tenacity_, yet this does as ’twere _shadow_ it out, and somewhat resemble it; for just after the same manner, I suppose the _pulse_ of heat to _agitate_ the small parcels of matter, and those that are of a _like bigness_, and _figure_, and _matter_, will _hold_, or _dance_ together, and those which are of a _differing_ kind will be _thrust_ or _shov’d_ out from between them; for particles that are _similar_, will, like so many _equal musical strings equally stretcht_, vibrate together in a kind of _Harmony_ or _unison_; whereas others that are _dissimilar_, upon what account soever, unless the disproportion be otherwise counter-ballanc’d, will, like so many _strings out of tune_ to those unisons, though they have the same agitating _pulse_, yet make quite _differing_ kinds of _vibrations_ and _repercussions_, so that though they may be both mov’d, yet are their _vibrations_ so _different_, and so _untun’d_, as ’twere to each other, that they _cross_ and _jar_ against each other, and consequently, _cannot agree_ together, but _fly back_ from each other to their similar particles. Now, to give you an instance how the _disproportion_ of some bodies in one respect, may be _counter-ballanc’d_ by a _contrary disproportion_ of the same body in another respect, whence we find that the subtil _vinous spirit_ is _congruous_, or does readily _mix_ with _water_, which in many properties is of a very _differing nature_, we may consider that a _unison_ may be made either by two _strings_ of the same _bigness_, _length_, and _tension_, or by two strings of the same _bigness_, but of _differing length_, and a _contrary differing tension_, or _3ly._ by two strings of _unequal length_ and _bigness_, and of a _differing tension_, or of _equal length_, and _differing bigness_ and _tension_, and several other such varieties. To which _three properties_ in _strings_, will correspond _three proprieties_ also in _sand_, or the _particles_ of bodies, their _Matter_ or _Substance_, their _Figure_ or _Shape_, and their _Body_ or _Bulk_. And from the _varieties_ of these _three_, may arise _infinite varieties_ in fluid bodies, though all agitated by the _same pulse_ or _vibrative_ motion. And there may be as many ways of making Harmonies and Discords with these, as there may be with _musical strings_. Having therefore seen what is the cause of Congruity or Incongruity, those relative properties of fluids, we may, from what has been said, very easily collect, what is the _reason_ of those Relative proprieties also between _fluid bodies_ and _solid_; for since all bodies consist of _particles_ of such a _Substance_, _Figure_, and _Bulk_; but in some they are _united_ together more _firmly_ then to be _loosened_ from each other by every _vibrative_ motion (though I imagine that there is no body in the world, but that some degree of agitation may, as I hinted before, agitate and loosen the particles so as to make them fluid) those _cohering_ particles may _vibrate_ in the same manner almost as those that are _loose_ and become _unisons_ or _discords_, as I may so speak, to them. Now that the _parts_ of all _bodies_, though never so _solid_, do yet _vibrate_, I think we need go no further for proof, then that _all_ bodies have some _degrees_ of _heat_ in them, and that there has not been yet found any thing _perfectly cold_: Nor can I believe indeed that there is any such thing in Nature, as a body whose particles are at _rest_, or _lazy_ and _unactive_ in the great _Theatre_ of the _World_, it being quite _contrary_ to the grand _Oeconomy_ of the Universe. We see therefore what is the reason of the _sympathy_ or uniting of some bodies together, and of the _antipathy_ or flight of others from each other: For _Congruity_ seems nothing else but a _Sympathy_, and _Incongruity_ an _Antipathy_ of bodies, hence _similar_ bodies once _united_ will not _easily part_, and _dissimilar_ bodies once _disjoyn’d_ will not _easily unite_ again; from hence may be very easily deduc’d the reason of the _suspension_ of _water_ and _Quick-silver_ above their usual _station_, as I shall more at large anon shew.

These properties therefore (alwayes the concomitants of fluid bodies) produce these following visible _Effects_:

First, They _unite_ the parts of a fluid to its _similar_ Solid, or keep them _separate_ from its _dissimilar_. Hence _Quick-silver_ will (as we noted before) _stick_ to _Gold_, _Silver_, _Tin_, _Lead_, &c. and _unite_ with them: but _roul_ off from _Wood_, _Stone_, _Glass_, &c. if never so little scituated out of its _horizontal level_; and _water_ that will _wet salt_ and _dissolve_ it, will _slip_ off from _Tallow_, or the like, without at all _adhering_; as it may likewise be observed to do upon a _dusty_ superficies. And next they cause the parts of _homogeneal fluid_ bodies readily to _adhere_ together and _mix_, and of _heterogeneal_, to be exceeding _averse_ thereunto. Hence we find, that _two_ small _drops_ of _water_, on any superficies they can roul on, will, if they chance to touch each other, _readily unite_ and _mix_ into one 3d _drop_: The like may be observed with two small _Bowls_ of _Quick-silver_ upon a Table or Glass, provided their surfaces be not _dusty_; and with two drops of _Oyl_ upon fair water, _&c._ And further, _water_ put unto _wine_, _salt water_, _vinegar_, _spirit_ of _wine_, or the like, does immediately (especially if they be shaken together) _disperse_ it self all over them. Hence, on the contrary, we also find, that _Oyl of Tartar_ poured upon _Quick-silver_, and _Spirit of Wine_ on that _Oyl_, and _Oyl of Turpentine_ on that _Spirit_, and _Air_ upon that _Oyl_, though they be stopt closely up into a Bottle, and _shaken_ never so much, they will by no means long suffer any of their bigger parts to be _united_ or included within any of the other Liquors (by which recited Liquors, may be plainly enough represented the four _Peripatetical Elements_, and the more subtil _Æther_ above all.) From this property ’tis, that a drop of _water_ does not mingle with, or vanish into _Air_, but is _driven_ (by that Fluid equally protruding it on every side) and forc’t into as little a space as it can possibly be contained in, namely, into a _Round Globule_. So likewise a little _Air_ blown under the _water_, is _united_ or thrust into a _Bubble_ by the ambient water. And a parcel of _Quick-silver_ enclosed with _Air_, _Water_, or almost any other _Liquor_, is _formed_ into a _round Ball_.

Now the cause why all these included Fluids, newly mentioned, or as many others as are wholly included within a heterogeneous fluid, are not _exactly_ of a _Spherical Figure_ (seeing that if caused by these Principles only, it could be of no other) must proceed from some other kind of _pressure_ against the two opposite flatted sides. This _adventitious_ or _accidental pressure_ may proceed from _divers causes_, and accordingly must _diversifie_ the Figure of the included heterogeneous fluid: For seeing that a body may be included either with a fluid only, or only with a solid, or partly with a fluid, and partly with a solid, or partly with one fluid, and partly with another; there will be found a very great variety of the terminating _surfaces_, much differing from a _Spherical_, according to the various resistance or pressure that belongs to each of these encompassing bodies.

Which Properties may in general be deduced from two heads, _viz._ _Motion_, and _Rest_. For, either this Globular Figure is altered by a _natural Motion_, such as is _Gravity_, or a _violent_, such as is any _accidental motion_ of the fluids, as we see in the _wind_ ruffling up the water, and the _purlings_ of _Streams_, and _foaming_ of _Catarracts_, and the like. Or thirdly, By the _Rest_, _Firmness_ and _Stability_ of the ambient _Solid_. For if the including _Solid_ be of an _angular_ or any other _irregular_ Form, the included _fluid_ will be near of the _like_, as a Pint-_Pot_ full of _water_, or a _Bladder_ full of _Air_. And next, if the including or included fluid have a greater _gravity_ one than another, then will the _globular_ Form be deprest into an _Elliptico-spherical_: As if, for example, we suppose the Circle _ABCD_, in the _fourth Figure_, to represent a _drop of water_, _Quick-silver_, or the like, included with the _Air_ or the like, which supposing there were no _gravity_ at all in either of the _fluids_, or that the _contained_ and _containing_ were of the _same weight_, would be _equally comprest_ into an exactly _spherical_ body (the ambient fluid _forcing equally_ against every side of it.) But supposing either a greater _gravity_ in the included, by reason whereof the parts of it being _prest_ from _A_ towards _B_, and thereby the whole put into _motion_, and that _motion_ being _hindred_ by the _resistance_ of the _subjacent_ parts of the ambient, the _globular_ Figure _ADBC_ will be _deprest_ into the _Elliptico-spherical_, _EGFH_. For the side _A_ is _detruded_ to _E_ by the _Gravity_, and _B_ to _F_ by the _resistance_ of the subjacent medium: and therefore _C_ must necessarily be thrust to _G_; and _D_ to _H_. Or else, supposing a greater _gravity_ in the _ambient_, by whose more then ordinary _pressure_ against the under side of the included globule; _B_ will be forced to _F_, and by its _resistance_ of the motion _upwards_, the side _A_ will be _deprest_ to _E_, and therefore _C_ being thrust to _G_ and _D_ to _H_; the _globular_ Figure by this means also will be made an _Elliptico-spherical_. Next if a fluid be included _partly_ with one, and _partly_ with another fluid, it will be found to be shaped _diversly_, according to the proportion of the _gravity_ and _incongruity_ of the 3 _fluids_ one to another: As in the _second Figure_, let the upper _MMM_ be _Air_, the middle _LMNO_ be common _Oyl_, the lower _OOO_ be _Water_, the _Oyl_ will be form’d, not into a _spherical_ Figure, such as is represented by the _pricked Line_, but into such a Figure as LMNO, whose side LMN will be of a flatter _Elliptical_ Figure, by reason of the great disproportion between the _Gravity_ of _Oyl_ and _Air_, and the side LOM of a rounder, because of the smaller difference between the weight of _Oyl_ and _Water_. Lastly, The _globular_ Figure will be changed, if the _ambient_ be partly _fluid_ and partly _solid_. And here the termination of the incompassed _fluid_ towards the incompassing is shap’d according to the proportion of the congruity or incongruity of the _fluids_ to the _solids_, and of the gravity and incongruity of the _fluids_ one to another. As suppose the subjacent _medium_ that hinders an included fluids descent, be a _solid_, as let KI, in the _fourth Figure_, represent the smooth superficies of a _Table_; EGFH, a parcel of _running Mercury_; the side GFH will be more flatted, according to the proportion of the incongruity of the _Mercury_ and _Air_ to the _Wood_, and of the _gravity_ of _Mercury_ and _Air_ one to another; The side GEH will likewise be a little more deprest by reason the subjacent parts are now at rest, which were before in motion.

Or further in the _third figure_, let AILD represent an including _solid_ medium of a cylindrical shape (as suppose a small _Glass Jar_) Let FGEMM represent a contain’d _fluid_, as water; this towards the bottom and sides, is figured according to the concavity of the _Glass_: But its upper _Surface_, (which by reason of its gravity, (not considering at all the Air above it, and so neither the congruity or incongruity of either of them to the Glass) should be terminated by part of a _Sphere_ whose diameter should be the same with that of the earth, which to our sense would appear a straight _Line_, as FGE, Or which by reason of its having a greater congruity to Glass than Air has, (not considering its Gravity) would be thrust into a _concave Sphere_, as CHB, whose diameter would be the same with that of the concavity of the Vessel:) Its upper Surface, I say, by reason of its having a greater gravity then the Air, and having likewise a greater congruity to Glass then the Air has, is terminated, by a _concave Elliptico-spherical Figure_, as CKB. For by its congruity it easily conforms it self, and adheres to the Glass, and constitutes as it were one containing body with it, and therefore should thrust the contained Air on that side it touches it, into a _spherical_ Figure, as BHC, but the motion of Gravity depressing a little the Corners B and C, reduces it into the aforesaid Figure CKB. Now that it is the greater congruity of one of the two _contiguous fluids_, then of the other, to the containing _solid_, that causes the separating surfaces to be thus or thus figured: And that it is not because this or that figurated surface is more proper, natural, or peculiar to one of these fluid bodies, then to the other, will appear from this; that the same _fluids_ will by being put into differing _solids_, change their _surfaces_. For the same water, which in a Glass or wooden Vessel will have a concave surface upwards, and will rise higher in a smaller then a greater Pipe, the same water, I say, in the same Pipes greased over or oyled, will produce quite contrary effects; for it will have a _protuberant_ and _convex_ surface upwards, and will not rise so high in small, as in bigger Pipes: Nay, in the very same solid Vessel, you may make the very same two contiguous _Liquids_ to alter their Surfaces; for taking a small Wine-glass, or such like Vessel, and pouring water gently into it, you shall perceive the _surface_ of the water all the way _concave_, till it rise even with the top, when you shall find it (if you gently and carefully pour in more) to grow very _protuberant_ and _convex_; the reason of which is plain, for that the _solid_ sides of the containing body are no longer extended, to which the water does more readily adhere then the air; but it is henceforth to be included with air, which would reduce it into a _hemisphere_, but by reason of its _gravity_, it is flatted into an _Oval_. _Quicksilver_ also which to _Glass_ is more incongruous then _Air_ (and thereby being put into a _Glass-pipe_, will not adhere to it, but by the more _congruous air_ will be forced to have a very _protuberant_ surface, and to rise higher in a greater then a lesser Pipe) this _Quicksilver_ to clean _Metal_, especially to _Gold_, _Silver_, _Tin_, _Lead_, &c. _Iron_ excepted, is more _congruous_ then _Air_, and will not only stick to it, but have a _concave_ Surface like _water_, and rise higher in a less, then in a greater Pipe.

In all these Examples it is evident, that there is an _extraordinary_ and _adventitious force_, by which the _globular_ Figure of the contained _heterogeneous_ fluid is altered; neither can it be imagined, how it should otherwise be of any other Figure then _Globular_: For being by the _heterogeneous_ fluid equally _protruded_ every way, whatsoever part is _protuberant_, will be thereby _deprest_. From this cause it is, that in its effects it does very much resemble a _round Spring_ (such as a _Hoop_.) For as in a _round Spring_ there is required an additional _pressure_ against two opposite sides, to reduce it into an _Oval_ Form, or to force it in between the sides of a _Hole_, whose _Diameter_ is less then that of the _Spring_, there must be a considerable force or _protrusion_ against _the concave_ or inner side of the _Spring_; So to alter this _spherical_ constitution of an included fluid body, there is required more pressure against opposite sides to reduce it into an _Oval_; and, to press it into an _Hole_ less in _Diameter_ then it self, it requires a greater _protrusion_ against all the other sides, What degrees of force are requisite to reduce them into longer and longer _Ovals_, or to press them into less and less _holes_, I have not yet experimentally calculated; but thus much by experiment I find in general, that there is alwayes required a greater pressure to close them into longer _Ovals_, or protrude them into smaller _holes_. The necessity and reason of this, were it requisite, I could easily explain: but being not so necessary, and requiring more room and time then I have for it at present, I shall here omit it; and proceed to shew, that this may be presently found true, if Experiment be made with a _round Spring_ (the way of making which trials is _obvious_ enough.) And with the fluid bodies of _Mercury_, _Air_, _&c._ the way of trying which, will be somewhat more difficult; and therefore I shall in brief describe it. He therefore that would try with _Air_, must first be provided of a _Glass-pipe_, made of the shape of that in the _fifth Figure_, whereof the side AB, represents a straight _Tube_ of about three foot long, C, represents another part of it, which consists of a _round Bubble_; so ordered, that there is left a _passage_ or _hole_ at the top, into which may be fastened with _cement_ several _small Pipes_ of determinate _cylindrical_ cavities: as let the _hollow_ of

F. ¼ G. ⅙ H. ⅛ I. be ¹⁄₁₂ of an inch. K. ¹⁄₁₆ L. ¹⁄₂₄ M. ¹⁄₃₂ &c.—— There may be added as many more, as the Experimenter shall think fit, with holes continually decreasing by known quantities, so far as his senses are able to help him; I say, so far, because there may be made _Pipes_ so small that it will be impossible to perceive the _perforation_ with ones naked eye, though by the help of a _Microscope_, it may easily enough be perceived: Nay, I have made a _Pipe_ perforated from end to end, so small, that with my naked eye I could very hardly see the body of it, insomuch that I have been able to knit it up into a knot without breaking: And more accurately examining one with my _Microscope_, I found it not so big as a sixteenth part of one of the smaller hairs of my head which was of the smaller and finer sort of hair, so that sixteen of these _Pipes_ bound faggot-wise together, would but have equalized one single hair; how small therefore must its _perforation_ be? It appearing to me through the _Microscope_ to be a proportionably _thick-sided Pipe_.

To proceed then, for the trial of the Experiment, the Experimenter must place the _Tube_ AB, perpendicular, and fill the _Pipe_ F (cemented into the hole E) with water, but leave the _bubble_ C full of _Air_, and then gently pouring in water into the Pipe AB, he must observe diligently how high the water will rise in it before it protrude the _bubble_ of Air C, through the narrow passage of F, and denote exactly the height of the _Cylinder_ of water, then cementing in a second Pipe as G, and filling it with water; he may proceed as with the former, denoting likewise the height of the _Cylinder_ of water, able to protrude the _bubble_ C through the passage of G, the like may he do with the next _Pipe_, and the next, &c. as far as he is able: then comparing the several heights of the _Cylinders_, with the several _holes_ through which each _Cylinder_ did force the _air_ (having due regard to the _Cylinders_ of water in the small _Tubes_) it will be very easie to determine, what force is requisite to press the _Air_ into such and such _a hole_, or (to apply it to our present experiment) how much of the pressure of the _Air_ is taken off by its ingress into smaller and smaller _holes_. From the application of which to the entring of the _Air_ into the bigger _hole_ of the _Vessel_, and into the smaller _hole_ of the _Pipe_, we shall clearly find, that there is a greater pressure of the air upon the water in the _Vessel_ or greater _pipe_, then there is upon that in the lesser _pipe_: For since the pressure of the _air_ every way is found to be equal, that is, as much as is able to press up and sustain a _Cylinder_ of _Quicksilver_ of two foot and a half high, or thereabouts; And since of this pressure so many more degrees are required to force the _Air_ into a smaller then into a greater _hole_ that is full of a more congruous fluid. And lastly, since those degrees that are requisite to press it in, are thereby taken off from the _Air_ within, and the _Air_ within left with so many degrees of pressure less then the _Air_ without; it will follow, that the _Air_ in the less _Tube_ or _pipe_, will have less pressure against the superficies of the _water_ therein, then the _Air_ in the bigger: which was the minor Proposition to be proved.

The Conclusion therefore will necessarily follow, _viz._ That _this unequal pressure of the Air caused by its ingress into unequal holes, is a cause sufficient to produce this effect, without the help of any other concurrent_; and therefore is probably the principal (if not the only) cause of these _Phænomena_.

This therefore being thus explained, there will be divers _Phænomena_ explicable thereby, as, the rising of _Liquors_ in a _Filtre_, the rising of _Spirit of Wine_, _Oyl_, _melted Tallow_, &c. in the _Week_ of a _Lamp_, (though made of small _Wire_, _Threeds_ of _Asbestus_, _Strings_ of _Glass_, or the like) the rising of _Liquors_ in a _Spunge_, piece of _Bread_, _Sand_, &c. perhaps also the ascending of the _Sap_ in _Trees_ and _Plants_, through their small, and some of them _imperceptible pores_, (of which I have said more, on another occasion) at least the passing of it out of the earth into their roots. And indeed upon the consideration of this Principle, multitudes of other uses of it occurr’d to me, which I have not yet so well examined and digested as to propound for _Axioms_, but only as _Queries_ and _Conjectures_ which may serve as _hints_ toward some further _discoveries_.

As first, Upon the consideration of the _congruity_ and _incongruity_ of Bodies, as to _touch_, I found also the like _congruity_ and _incongruity_ (if I may so speak) as to the _Transmitting_ of the _Rates_ of Light: For as in this regard, _water_ (not now to mention other Liquors) seems nearer of affinity to _Glass_ then _Air_, and _Air_ then _Quicksilver_: whence an _oblique Ray_ out of _Glass_, will pass into _water_ with very little _refraction_ from the _perpendicular_, but none out of _Glass_ into _Air_, excepting a _direct_, will pass without a very great refraction from the perpendicular, nay any oblique Ray under thirty degrees, will not be admitted into the Air at all. And _Quicksilver_ will neither admit oblique or direct, but reflects all; seeming, as to the transmitting of the Raies of Light, to be of a quite differing constitution, from that of _Air_, _Water_, _Glass_, &c. and to resemble most those opacous and strong reflecting bodies of Metals: So also as to the property of cohesion or congruity, Water seems to keep the same order, being more congruous to Glass then Air, and Air then Quicksilver.

A Second thing (which was hinted to me, by the consideration of the included fluids globular form, caused by the protrusion of the ambient heterogeneous fluid) was, whether the _Phænomena_ of gravity might not by this means be explained, by supposing the _Globe_ of Earth, Water, and Air to be included with a _fluid_, heterogeneous to all and each of them, so subtil, as not only to be every where _interspersed_ through the _Air_, (or rather the _air_ through it) but to _pervade_ the bodies of _Glass_, and even the _closest Metals_, by which means it may endeavour to _detrude_ all earthly bodies as far from it as it can; and partly thereby, and partly by other of its properties may move them towards the Center of the Earth. Now that there is some such fluid, I could produce many Experiments and Reasons, that do seem to prove it: But because it would ask some time and room to set them down and explain them, and to consider and answer all the Objections (many whereof I foresee) that may be alledged against it; I shall at present proceed to other _Queries_, contenting my self to have here only given a hint of what I may say more elswhere.

A Third _Query_ then was, Whether the _heterogeneity_ of the _ambient fluid_ may not be accounted a _secondary cause_ of the _roundness_ or _globular form_ of the _greater bodies_ of the world, such as are those of the _Sun_, _Stars_, and _Planets_, the _substance_ of each of which seems altogether _heterogeneous_ to the _circumambient fluid æther_? And of this I shall say more in the Observation of the Moon.

A Fourth was, Whether the _globular form_ of the _smaller parcels_ of matter here upon the _Earth_, as that of _Fruits_, _Pebbles_, or _Flints_, &c. (which seem to have been a _Liquor_ at first) may not be caused by the _heterogeneous ambient fluid_. For thus we see that melted _Glass_ will be naturally formed into a _round Figure_; so likewise any small Parcel of any _fusible body_, if it be perfectly enclosed by the _Air_, will be driven into a _globular_ Form; and, when cold, will be found a _solid Ball_. This is plainly enough manifested to us by their way of making _shot_ with the _drops of Lead_; which being a very pretty curiosity, and known but to a very few, and having the liberty of publishing it granted me, by that _Eminent Virtuoso_ Sir _Robert Moray_, who brought in this Account of it to the _Royal Society_, I have here transcribed and inserted.

To make small shot of different sizes; Communicated by his Highness _P.R._

_Take Lead out of the Pig what quantity you please, melt it down, stir and clear it with an iron Ladle, gathering together the blackish parts that swim at top like scum, and when you see the colour of the clear Lead to be greenish, but no sooner, strew upon it _Auripigmentum_ powdered according to the quantity of Lead, about as much as will lye upon a half Crown piece will serve for eighteen or twenty pound weight of some sorts of Lead; others will require more, or less. After the _Auripigmentum_ is put in, stir the Lead well, and the _Auripigmentum_ will flame: when the flame is over, take out some of the Lead in a Ladle having a lip or notch in the brim for convenient pouring out of the Lead, and being well warmed amongst the melted Lead, and with a stick make some single drops of Lead trickle out of the Ladle into water in a Glass, which if they fall to be round and without tails, there is _Auripigmentum_ enough put in, and the temper of the heat is right, otherwise put in more. Then lay two bars of Iron (or some more proper Iron-tool made on purpose) upon a Pail of water, and place upon them a round Plate of Copper, of the size and figure of an ordinary large Pewter or Silver Trencher, the hollow whereof is to be about three inches over, the bottom lower then the brims about half an inch, pierced with thirty, forty, or more small holes; the smaller the holes are, the smaller the shot will be; and the brim is to be thicker then the bottom, to conserve the heat the better._

_The bottom of the Trencher being some four inches distant from the water in the Pail, lay upon it some burning Coles, to keep the Lead melted upon it. Then with the hot Ladle take Lead off the Pot where it stands melted, and pour it softly upon the burning Coles over the bottom of the Trencher, and it will immediately run through the holes into the water in small round drops. Thus pour on new Lead still as fast as it runs through the Trencher till all be done; blowing now and then the Coles with hand-Bellows, when the Lead in the Trencher cools so as to stop from running._

_Whilst one pours on the Lead, another must, with another Ladle, thrusted four or five inches under water in the Pail, catch from time to time some of the shot, as it drops down, to see the size of it, and whether there be any faults in it. The greatest care is to keep the Lead upon the Trencher in the right degree of heat; if it be too cool, it will not run through the Trencher, though it stand melted upon it; and this is to be helped by blowing the Coals a little, or pouring on new Lead that is hotter: but the cooler the Lead, the larger the Shot; and the hotter, the smaller; when it is too hot, the drops will crack and fly; then you must stop pouring on new Lead, and let it cool; and so long as you observe the right temper of the heat, the Lead will constantly drop into very round Shot, without so much as one with a tail in many pounds._

_When all is done, take your Shot out of the Pail of water, and put it in a Frying-pan over the fire to dry them, which must be done warily, still shaking them that they melt not; and when they are dry you may separate the small from the great, in Pearl Sives made of Copper or Lattin let into one another, into as many sizes at you please. But if you would have your Shot larger then the Trencher makes them, you may do it with a Stick, making them trickle out of the Ladle, as hath been said._

_If the Trencher be but toucht a very little when the Lead stops from going through it, and be not too cool, it will drop again, but it is better not to touch it at all. At the melting of the Lead take care that there be no kind of Oyl, Grease, or the like, upon the Pots, or Ladles, or Trencher._

_The Chief cause of this Globular Figure of the Shot, seems to be the _Auripigmentum_; for, as soon as it is put in among the melted Lead, it loses its shining brightness, contracting instantly a grayish film or skin upon it, when you scum it to make it clean with the Ladle. So that when the Air comes at the falling drop of the melted Lead, that skin constricts them every where equally: but upon what account, and whether this be the true cause, is left to further disquisition._

Much after this same manner, when the Air is exceeding cold through which it passes; do we find the drops of Rain, falling from the Clouds, congealed into round Hail-stones by the freezing Ambient.

To which may be added this other known Experiment, That if you gently let fall a drop of _water_ upon small _sand_ or _dust_, you shall find, as it were, an artificial _round stone_ quickly generated. I cannot upon this occasion omit the mentioning of the strange kind of _Grain_, which I have observed in a _stone_ brought from _Kettering_ in _Northamptonshire_, and therefore called by Masons _Kettering-Stone_, of which see the Description. Which brings into my mind what I long since observed in the fiery Sparks that are struck out of a Steel. For having a great desire to see what was left behind, after the Spark was gone out, I purposely struck fire over a very white piece of Paper, and observing diligently where some conspicuous sparks went out, I found a very little black spot no bigger then the point of a Pin, which through a _Microscope_ appeared to be a perfectly round Ball, looking much like a polisht ball of Steel, insomuch that I was able to see the Image of the window reflected from it. I cannot here stay (having done it more fully in another place) to examine the particular Reasons of it, but shall only hint, that I imagine it to be some small parcel of the Steel, which by the violence of the motion of the stroke (most of which seems to be imprest upon those small parcels) is made so glowing hot, that it is melted into a _Vitrum_, which by the ambient Air is thrust into the form of a Ball.

A Fifth thing which I thought worth Examination was, Whether the motion of all kind of Springs, might not be reduced to the Principle whereby the included _heterogeneous fluid_ seems to be moved; or to that whereby two Solids, as Marbles, or the like, are thrust and kept together by the _ambient fluid_.

A Sixth thing was, Whether the Rising and Ebullition of the Water out of Springs and Fountains (which lie much higher from the Center of the Earth then the Superficies of the Sea, from whence it seems to be derived) may not be explicated by the rising of Water in a smaller Pipe: For the Sea-water being strained through the Pores or Crannies of the Earth, is, as it were, included in little Pipes, where the pressure of the Air has not so great a power to resist its rising: But examining this way, and finding in it several difficulties almost irremovable, I thought upon a way that would much more naturally and conceivably explain it, which was by this following Experiment: I took a Glass-Tube, of the form of that described in the sixth Figure, and chusing two _heterogeneous fluids_, such as Water and Oyl, I poured in as much Water as filled up the Pipes as high as AB, then putting in some Oyl into the Tube AC, I deprest the superficies A of the Water to F, and B I raised to G, which was not so high perpendicularly as the superficies of the Oyl F, by the space FI, wherefore the proportion of the gravity of these two Liquors was as GH to FE.

This Experiment I tried with several other Liquors, and particularly with fresh Water and Salt (which I made by dissolving Salt in warm Water) which two though they are nothing heterogeneous, yet before they would perfectly mix one with another, I made trial of the Experiment: Nay, letting the Tube wherein I tried the Experiment remain for many dayes, I observed them not to mix; but the superficies of the fresh was rather more then less elevated above that of the Salt. Now the proportion of the gravity of Sea-water, to that of River-water, according to _Stevinus_ and _Varenius_, and as I have since found pretty true by making trial my self, is as 46. to 45. that is, 46. Ounces of the salt Water will take up no more room then 45. of the fresh. Or reciprocally 45 pints of salt-water weigh as much as 46 of fresh.

But I found the proportion of Brine to fresh Water to be near 13 to 12: Supposing therefore GHM to represent the Sea, and FI the height of the Mountain above the Superficies of the Sea, FM a Cavern in the Earth, beginning at the bottom of the Sea, and terminated at the top of the Mountain, LM the Sand at the bottom, through which the Water is as it were strained, so as that the fresher parts are only permitted to transude, and the saline kept back; if therefore the proportion of G M to FM be as 45 to 46, then may the Cylinder of Salt-water GM make the Cylinder of Fresh-water to rise as high as E, and to run over at N. I cannot here stand to examine or confute their Opinion, who make the depth of the Sea, below its Superficies, to be no more perpendicularly measured then the height of the Mountains above it: ’Tis enough for me to say, there is no one of those that have asserted it, have experimentally known the perpendicular of either; nor shall I here determine, whether there may not be many other causes of the separation of the fresh water from the salt, as perhaps some parts of the Earth through which it is to pass, may contain a Salt, that mixing and uniting with the Sea-salt, may precipitate it; much after the same manner as the _Alcalizate_ and _Acid Salts_ mix and precipitate each other in the preparation of _Tartarum Vitriolatum._ I know not also whether the exceeding cold (that must necessarily be) at the bottom of the Water, may not help towards this separation, for we find, that warm Water is able to dissolve and contain more Salt, then the same cold; insomuch that Brines strongly impregnated by heat, if let cool, do suffer much of their Salt to subside and crystallize about the bottom and sides. I know not also whether the exceeding pressure of the parts of the Water one against another, may not keep the Salt from descending to the very bottom, as finding little or no room to insert it self between those parts, protruded so violently together, or else squeeze it upwards into the superiour parts of the Sea, where it may more easily obtain room for it self, amongst the parts of the Water, by reason that there is more heat and less pressure. To this Opinion I was somewhat the more induced by the relations I have met with in _Geographical Writers_, of drawing fresh Water from the bottom of the Sea, which is salt above. I cannot now stand to examine, whether this natural perpetual motion may not artificially be imitated: Nor can I stand to answer the Objections which may be made against this my Supposition: As, First, How it comes to pass, that there are sometimes salt Springs much higher then the Superficies of the Water? And, Secondly, Why Springs do not run faster and slower, according to the varying height made of the Cylinder of Sea-water, by the ebbing and flowing of the Sea?

As to the First, In short, I say, the fresh Water may receive again a saline Tincture near the Superficies of the Earth, by passing through some salt _Mines_, or else many of the saline parts of the Sea may be kept back, though not all.

And as to the Second, The same _Spring_ may be fed and supplyed by divers _Caverns_, coming from very far distant parts of the _Sea_, so as that it may in one place be _high_, in another _low water_; and so by that means the _Spring_ may be equally supply’d at all times. Or else the _Cavern_ may be so straight and narrow, that the water not having so ready and free passage through it, cannot upon so short and quick mutations of pressure, be able to produce any sensible effect at such a distance. Besides that, to confirm this _hypothesis_, there are many _Examples_ found in _Natural Historians_, of _Springs_ that do ebb and flow like the Sea: As particularly, those recorded by the Learned _Camden_, and after him by _Speed_, to be found in this _Island_: One of which, they relate to be on the Top of a Mountain, by the small Village _Kilken_ in _Flintshire_, _Maris æmulus qui statis temporibus suas evomit & resorbet Aquas_; Which at certain times riseth and falleth after the manner of the Sea. A Second in _Caermardenshire_, near _Caermarden_, at a place called _Cantred Bichan_; _Qui (ut scribit Giraldus) naturali die bis undis deficiens, & toties exuberans, marinas imitatur instabilitates_; That twice in four and twenty hours ebbing and flowing; resembleth the unstable motions of the Sea. The _Phænomena_ of which two may be easily made out, by supposing the _Cavern_, by which they are fed, to arise from the bottom of the next Sea. A Third, is a Well upon the River _Ogmore_ in _Glamorganshire_, and near unto _Newton_, of which _Camden_ relates himself to be certified, by a Letter from a Learned Friend of his that observed it, _Fons abest hinc, &c._ The Letter is a little too long to be inserted, but the substance is this; That this Well ebbs and flows quite contrary to the flowing and ebbing of the Sea in those parts: for ’tis almost empty at Full Sea, but full at Low water. This may happen from the Channel by which it is supplied, which may come from the bottom of a Sea very remote from those parts, and where the Tides are much differing from those of the approximate shores. A Fourth, lies in _Westmorland_, near the River _Leder_; _Qui instar Euripi sæpius in die reciprocantibus undis fluit & refluit_, which ebbs and flows many times a day. This may proceed from its being supplyed from many Channels, coming from several parts of the Sea, lying sufficiently distant asunder to have the times of High water differing enough one from the other; so as that whensoever it shall be High water over any of those places, where these Channels begin, it shall likewise be so in the Well; but this is but a supposition.

A Seventh _Query_ was, Whether the _dissolution_ or mixing of several bodies, whether fluid or solid, with saline or other Liquors, might not partly be attributed to this Principle of the congruity of those bodies and their dissolvents? As of Salt in Water, Metals in several _Menstruums_, Unctuous Gums in Oyls, the mixing of Wine and Water, &c. And whether _precipitation_ be not partly made from the same Principle of Incongruity? I say _partly_, because there are in some Dissolutions, some other Causes concurrent.

I shall lastly make a much more seemingly strange and unlikely _Query_; and that is, Whether this Principle, well examined and explained, may not be found a _coefficient_ in the most considerable Operations of Nature? As in those of _Heat_, and _Light_, and consequently of _Rarefaction_ and _Condensation_, _Hardness_, and _Fluidness_, _Perspicuity_ and _Opacousness_, _Refractions_ and _Colours. &c._ Nay, I know not whether there may be many things done in Nature, in which this may not (be said to) have a Finger? This I have in some other passages of this Treatise further enquired into and shewn, that as well _Light_ as _Heat_ may be caused by _corrosion_, which is applicable to _congruity_, and consequently all the rest will be but _subsequents_: In the mean time I would not willingly be guilty of that _Error_, which the thrice Noble and Learned _Verulam_ justly takes notice of, as such, and calls _Philosophiæ Genus Empiricum, quod in paucorum Experimentorum Angustiis & Obscuritate fundatum est_. For I neither conclude from one single Experiment, nor are the Experiments I make use of all made upon one Subject: Nor wrest I any Experiment to make it _quadrare_ with any preconceiv’d Notion. But on the contrary, I endeavour to be conversant in divers kinds of Experiments, and all and every one of those Trials, I make the Standards or Touchstones, by which I try all my former Notions, whether they hold out in weight, and measure, and touch, &c. For as that Body is no other then a Counterfeit Gold, which wants any one of the Proprieties of Gold, (such as are the Malleableness, Weight, Colour, Fixtness in the Fire, Indissolubleness in _Aqua fortis_, and the like) though it has all the other; so will all those Notions be found to be false and deceitful, that will not undergo all the Trials and Tests made of them by Experiments. And therefore such as will not come up to the desired _Apex_ of Perfection, I rather wholly reject and take new, then by piecing and patching, endeavour to retain the old, as knowing such things at best to be but lame and imperfect. And this course I learned from Nature; whom we find neglectful of the old Body, and suffering its Decaies and Infirmities to remain without repair, and altogether sollicitous and careful of perpetuating the _Species_ by new _Individuals_. And it is certainly the most likely way to erect a glorious Structure and Temple to _Nature_, such as she will be found (by any _zealous Votary_) to reside in; to begin to build a new upon a sure Foundation of Experiments.

But to digress no further from the consideration of the _Phænomena_, more immediately explicable by this Experiment, we shall proceed to shew, That, as to the rising of Water in a _Filtre_, the reason of it will be manifest to him, that does take notice, that a _Filtre_ is constituted of a great number of small long solid bodies, which lie so close together, that the Air in its getting in between them, doth lose of its pressure that it has against the _Fluid_ without them, by which means the Water or Liquor not finding so strong a resistance between them as is able to counter-ballance the pressure on its superficies without, is raised upward, till it meet with a pressure of the Air which is able to hinder it. And as to the Rising of Oyl, melted Tallow, Spirit of Wine, &c. in the Week of a Candle or Lamp, it is evident, that it differs in nothing from the former, save only in this, that in a _Filtre_ the Liquor descends and runs away by another part; and in the Week the Liquor is dispersed and carried away by the Flame; something there is ascribable to the Heat, for that it may rarifie the more volatil and spirituous parts of those combustible Liquors, and so being made lighter then the Air, it may be protruded upwards by that more ponderous fluid body in the Form of Vapours; but this can be ascribed to the ascension of but a very little, and most likely of that only which ascends without the Week. As for the Rising of it in a Spunge, Bread, Cotton, &c. above the superficies of the subjacent Liquor, what has been said about the _Filtre_ (if considered) will easily suggest a reason, considering that all these bodies abound with small holes or pores.

From this same Principle also (_viz. the unequal pressure of the Air against the unequal superficies of the water_) proceeds the cause of the accession or incursion of any floating body against the sides of the containing Vessel; or the _appropinquation_ of two floating bodies, as _Bubbles_, _Corks_, _Sticks_, _Straws_, &c. one towards another. As for instance, Take a Glass jar, such as AB in the seventh _Figure_, and filling it pretty near the top with water, throw into it a small round piece of Cork, as C, and plunge it all over in water, that it be wet, so as that the water may rise up by the sides of it, then placing it any where upon the superficies, about an inch, or one inch and a quarter from any side, and you shall perceive it by degrees to make _perpendicularly_ toward the nearest part of the side, and the nearer it approaches, the faster to be moved, the reason of which _Phænomenon_ will be found no other then this, that the Air has a greater pressure against the middle of the _superficies_, then it has against those parts that approach nearer, and are _contiguous_ to the sides. Now that the pressure is greater, may (as I shewed before in the explication of the third _Figure_) be evinced from the flatting of the water in the middle, which arises from the gravity of the under _fluid_: for since, as I shewed before, if there were no gravity in the under _fluid_, or that it were equal to that of the upper, the terminating Surface would be _Spherical_, and since it is the additional pressure of the gravity of water that makes it so flat, it follows, that the pressure upon the middle must be greater then towards the sides. Hence the Ball having a stronger pressure against that side of it which respects the middle of the _superficies_, then against that which respects the _approximate_ side, must necessarily move towards that part, from whence it finds least resistance, and so be _accelerated_, as the resistance decrease. Hence the more the water is raised under that part of its way it is passing above the middle, the faster it is moved: And therefore you will find it to move faster in E then in D, and in D then in C. Neither could I find the floating substance to be moved at all, until it were placed upon some part of the _Superficies_ that was sensibly elevated above the height of the middle part. Now that this may be the true cause, you may try with a blown Bladder, and an exactly round Ball upon a very smooth side of some pliable body, as _Horn_ or _Quicksilver._ For if the Ball be placed under a part of the Bladder which is upon one side of the middle of its pressure, and you press strongly against the Bladder, you shall find the Ball moved from the middle towards the sides.

Having therefore shewn the reason of the motion of any float towards the sides, the reason of the incursion of any two floating bodies will easily appear: For the rising of the water against the sides of either of them, is an Argument sufficient, to shew the pressure of the Air to be there less, then it is further from it, where it is not so much elevated; and therefore the reason of the motion of the other toward it, will be the same as towards the side of the Glass, only here from the same reason, they are mutually moved toward each other, whereas the side of the Glass in the former remains fixt. If also you gently fill the Jar so full with water, that the water is _protuberant_ above the sides, the same piece of Cork that before did hasten towards the sides, does now fly from it as fast towards the middle of the Superficies; the reason of which will be found no other then this, that the pressure of the Air is stronger against the sides of the Superficies G and H, then against the middle I; for since, as I shewed before, the Principle of congruity would make the terminating Surface Spherical, and that the flatting of the Surface in the middle is from the abatement of the waters pressure outwards, by the contrary indeavour of its gravity; it follows that the pressure in the middle must be less then on the sides; and therefore the consecution will be the same as in the former. It is very odd to one that considers not the reason of it, to see two floating bodies of wood to approach each other, as though they were indued with some magnetical vigour; which brings into my mind what I formerly tried with a piece of Cork or such like body, which I so ordered, that by putting a little stick into the same water, one part of the said Cork would approach and make toward the stick, whereas another would discede and fly away, nay it would have a kind of verticity, so as that if the _Æquator_ (as I may so speak) of the Cork were placed towards the stick, if let alone, it would instantly turn its appropriate Pole toward it, and then run a-tilt at it: and this was done only by taking a dry Cork, and wetting one side of it with one small stroak; for by this means gently putting it upon the water, it would depress the superficies on every side of it that was dry, and therefore the greatest pressure of the Air, being near those sides, caused it either to chase away, or else to fly off from any other floating body, whereas that side only, against which the water ascended, was thereby able to attract.

It remains only, that I should determine how high the Water or other Liquor may by this means be raised in a smaller Pipe above the Superficies of that without it, and at what height it may be sustained: But to determine this, will be exceeding difficult, unless I could certainly know how much of the Airs pressure is taken off by the smalness of such and such a Pipe, and whether it may be wholly taken off, that is, whether there can be a hole or pore so small, into which Air could not at all enter, though water might with its whole force, for were there such, ’tis manifest, that the water might rise in it to some five or six and thirty English Foot high. I know not whether the capillary Pipes in the bodies of small Trees, which we call their _Microscopical pores_, may not be such; and whether the congruity of the sides of the Pore may not yet draw the juyce even higher then the Air was able by its bare pressure to raise it: For, Congruity is a principle that not only unites and holds a body joyned to it, but, which is more, attracts and draws a body that is very near it, and holds it above its usual height.

And this is obvious even in a drop of water suspended under any Similar or Congruous body: For, besides the ambient pressure that helps to keep it sustein’d, there is the Congruity of the bodies that are contiguous. This is yet more evident in Tenacious and Glutinous bodies; such as Gummous Liquors, Syrups, Pitch, and Rosin melted, &c. Tar, Turpentine, Balsom, Birdlime, &c. for there it is evident, that the Parts of the tenacious body, as I may so call it, do stick and adhere so closely together, that though drawn out into long and very slender Cylinders, yet they will not easily relinquish one another; and this, though the bodies be _aliquatenus_ fluid, and in motion by one another, which, to such as consider a fluid body only as its parts are in a confused irregular motion, without taking in also the congruity of the parts one among another, and incongruity to some other bodies, does appear not a little strange. So that besides the incongruity of the ambient fluid to it, we are to consider also the congruity of the parts of the contein’d fluid one with another.

And this Congruity (that I may here a little further explain it) is both a Tenacious and an Attractive power; for the Congruity, in the Vibrative motions, may be the cause of all kind of attraction, not only Electrical, but Magnetical also, and therefore it may be also of Tenacity and Glutinousness. For, from a perfect congruity of the motions of two distant bodies, the intermediate fluid particles are separated and droven away from between them, and thereby those congruous bodies are, by the incompassing mediums, compell’d and forced neerer together; wherefore that attractiveness must needs be stronger, when, by an immediate contact, they are forc’d to be exactly the same: As I shew more at large in my _Theory_ of the _Magnet_. And this hints to me the reason of the suspension of the _Mercury_ many inches, nay many feet, above the usual station of 30 inches. For the parts of _Quick-Silver_, being so very similar and congruous to each other, if once united, will not easily suffer a divulsion: And the parts of water, that were any wayes _heterogeneous_, being by _exantlation_ or rarefaction exhausted, the remaining parts being also very similar, will not easily part neither. And the parts of the Glass being solid, are more difficultly disjoyn’d; and the water, being somewhat similar to both, is, as it were, a medium to unite both the _Glass_ and the _Mercury_ together. So that all three being united, and not very dissimilar, by means of this contact, if care be taken that the Tube in erecting be not shogged, the _Quicksilver_ will remain suspended, notwithstanding its contrary indeavour of Gravity, a great height above its ordinary Station; but if this immediate Contact be removed, either by a meer separation of them one from another by the force of a shog, whereby the other becomes imbodied between them, and licks up from the surface some agil parts, and so hurling them makes them air, or else by some small heterogeneous agil part of the Water, or Air, or Quicksilver, which appears like a bubble, and by its jumbling to and fro there is made way for the _heterogeneous Æther_ to obtrude it self between the Glass and either of the other Fluids, the Gravity of _Mercury precipitates_ it downward with very great violence; and if the Vessel that holds the restagnating _Mercury_ be convenient, the _Mercury_ will for a time _vibrate_ to and fro with very large _reciprocations_, and at last will remain kept up by the pressure of the external Air at the height of neer thirty inches. And whereas it may be objected, that it cannot be, that the meer imbodying of the _Æther_ between these bodies can be the cause, since the _Æther_ having a free passage alwayes, both through the Pores of the Glass, and through those of the Fluids, there is no reason why it should not make a separation at all times whilst it remains suspended, as when it is violently disjoyned by a shog. To this I answer, That though the _Æther_ passes between the Particles, that is, through the Pores of bodies, so as that any chasm or separation being made, it has infinite passages to admit its entry into it, yet such is the tenacity or attractive virtue of Congruity, that till it be overcome by the meer strength of Gravity, or by a shog assisting that Conatus of Gravity, or by an agil Particle, that is like a leaver agitated by the _Æther_; and thereby the parts of the congruous substances are separated so far asunder, that the strength of congruity is so far weakened, as not to be able to reunite them, the parts to be taken hold of being removed out of the attractive Sphere, as I may so speak, of the congruity; such, I say, is the tenacity of congruity, that it retains and holds the almost contiguous Particles of the Fluid, and suffers them not to be separated, till by meer force that attractive or retentive faculty be overcome: But the separation being once made beyond the Sphere of the attractive activity of congruity, that virtue becomes of no effect at all, but the _Mercury_ freely falls downwards till it meet with a resistance from the pressure of the _ambient_ Air, able to resist its gravity, and keep it forced up in the Pipe to the height of about thirty inches.

Thus have I gently raised a Steel _pendulum_ by a Loadstone to a great Angle, till by the shaking of my hand I have chanced to make a separation between them, which is no sooner made, but as if the Loadstone had retained no attractive virtue, the _Pendulum_ moves freely from it towards the other side. So vast a difference is there between the attractive virtue of the _Magnet_ when it acts upon a contiguous and upon a disjoyned body: and much more must there be between the attractive virtues of congruity upon a contiguous and disjoyned body; and in truth the attractive virtue is so little upon a body disjoyned, that though I have with a _Microscope_ observed very diligently, whether there were any extraordinary _protuberance_ on the side of a drop of water that was exceeding neer to the end of a green stick, but did not touch it, I could not perceive the least; though I found, that as soon as ever it toucht it the whole drop would presently unite it self with it; so that it seems an absolute contact is requisite to the exercising of the tenacious faculty of congruity.

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Observ. VII. _Of some _Phænomena_ of Glass drops._

These _Glass Drops_ are small parcels of coarse green Glass taken out of the Pots that contain the _Metal_ (as they call it) in fusion, upon the end of an Iron Pipe; and being exceeding hot, and thereby of a kind of sluggish fluid Consistence, are suffered to drop from thence into a Bucket of cold Water, and in it to lye till they be grown sensibly cold.

Some of these I broke in the open air, by snapping off a little of the small stem with my fingers, others by crushing it with a small pair of Plyers; which I had no sooner done, then the whole bulk of the drop flew violently, with a very brisk noise, into multitudes of small pieces, some of which were as small as dust, though in some there were remaining pieces pretty large, without any flaw at all, and others very much flaw’d, which by rubbing between ones fingers was easily reduced to dust; these dispersed every way so violently, that some of them pierced my skin. I could not find, either with my naked Eye, or a _Microscope_, that any of the broken pieces were of a regular figure, nor any one like another, but for the most part those that flaw’d off in large pieces were prettily branched.

The ends of others of these drops I nipt off whilst all the bodies and ends of them lay buried under the water, which, like the former, flew all to pieces with as brisk a noise, and as strong a motion.

Others of these I tried to break, by grinding away the blunt end, and though I took a seemingly good one, and had ground away neer two thirds of the Ball, yet would it not fly to pieces, but now and then some small rings of it would snap and fly off, not without a brisk noise and quick motion, leaving the Surface of the drop whence it flew very prettily branched or creased, which was easily discoverable by the _Microscope_. This drop, after I had thus ground it, without at all impairing the remnant that was not ground away, I caused to fly immediately all into sand upon the nipping off the very tip of its slender end.

Another of these drops I began to grind away at the smaller end, but had not worn away on the stone above a quarter of an inch before the whole drop flew with a brisk crack into sand or small dust; nor would it have held so long, had there not been a little flaw in the piece that I ground away, as I afterwards found.

Several others of these drops I covered over with a thin but very tuff skin of _Icthyocolla_, which being very tough and very transparent, was the most convenient substance for these tryals that I could imagine, having dipt, I say, several of these drops in this transparent Glue whilst hot, and suffering them to hang by a string tied about the end of them till they were cold, and the skin pretty tough; then wrapping all the body of the drop (leaving out only the very tip) in fine supple Kids-leather very closely, I nipped off the small top, and found, as I expected, that notwithstanding this skin of Glue, and the close wrapping up in Leather, upon the breaking of the top, the drop gave a crack like the rest, and gave my hand a pretty brisk impulse: but yet the skin and leather was so strong as to keep the parts from flying out of their former posture; and, the skin being transparent, I found that the drop retained exactly its former figure and polish, but was grown perfectly opacous and all over flaw’d, all those flaws lying in the manner of rings, from the bottom or blunt end, to the very top or small point. And by several examinations with a _Microscope_, of several thus broken, I found the flaws, both within the body of the drop, and on the outward surface, to lye much in this order.

Let AB in the Figure X of the fourth Scheme represent the drop cased over with _Icthyocolla_ or _Isinglass_, (by being ordered as is before prescribed) crazed or flawed into pieces, but by the skin or case kept in its former figure, and each of its flawed parts preserved exactly in its due posture; the outward appearance of it somewhat plainly to the naked eye, but much more conspicuous if viewed with a small lens appeared much after this shape. That is, the blunt end B for a pretty breadth, namely, as far as the Ring CCC seemed irregularly flawed with divers clefts, which all seemed to tend towards the Center of it, being, as I afterwards found, and shall anon shew in the description of the figure Y, the Basis, as it were, of a Cone, which was terminated a little above the middle of the drop, all the rest of the Surface from CCC to A was flawed with an infinite number of small and parallel Rings, which as they were for the most part very round, so were they very thick and close together, but were not so exactly flaw’d as to make a perfect Ring, but each circular part was by irregular cracks flawed likewise into multitudes of irregular flakes or tiles; and this order was observed likewise the whole length of the neck.

Now though I could not so exactly cut this _conical Body_ through the _Axis_, as is represented by the figure Y; yet by _anatomizing_, as it were, of several, and taking notice of divers particular circumstances, I was informed, that could I have artificially divided a flaw’d drop through the _Axis_ or _Center_, I should with a _Microscope_ have found it to appear much of this form, where A signifies the _Apex_, and B the blunt end, CC the Cone of the Basis, which is terminated at T the top or end of it, which seems to be the very middle of the blunt end in which, not only the conical body of the Basis CC is terminated, but as many of the parts of the drop as reach as high as DD.

And it seemed to be the head or beginning of a Pith, as it were, or a part of the body which seemed more spungy then the rest, and much more irregularly flawed, which from T ascended by EE, though less visible, into the small neck towards A. The Grain, as it were, of all the flaws, that proceeds from all the outward Surface ADCCDA, was much the same, as is represented by the black strokes that meet in the middle DT, DT, DE, DE, &c.

Nor is this kind of Grain, as I may call it, peculiar to Glass drops thus quenched; for (not to mention _Coperas-stones_, and divers other _Marchasites_ and _Minerals_, which I have often taken notice of to be in the very same manner flaked or grained, with a kind of Pith in the middle) I have observed the same in all manner of cast Iron, especially the coarser sort, such as Stoves, and Furnaces, and Backs, and Pots are made of: For upon the breaking of any of those Substances it is obvious to observe, how from the out-sides towards the middle, there is a kind of Radiation or Grain much resembling this of the Glass-drop; but this Grain is most conspicuous in Iron-bullets, if they be broken: the same _Phænomena_ may be produced by casting _regulus_ of _Antimony_ into a Bullet-mold, as also with _Glass of Antimony_, or with almost any such kind of _Vitrified substance_, either cast into a cold Mold or poured into Water.

Others of these Drops I heat red hot in the fire, and then suffered them to cool by degrees. And these I found to have quite lost all their _fulminating_ or flying quality, as also their hard, brittle and springy texture; and to emerge of a much softer temper, and much easier to be broken or snapt with ones finger; but its strong and brittle quality was quite destroyed, and it seemed much of the same consistence with other green Glass well nealed in the Oven.

The Figure and bigness of these for the most part was the same with that of the Figure Z; that is, all the surface of them was very smooth and polisht, and for the most part round, but very rugged or knobbed about D, and all the length of the stem was here and there pitted or flatted. About D, which is at the upper part of the drop under that side of the stem which is concave, there usually was made some one or more little Hillocks or Prominences. The drop it self, before it be broken, appears very transparent, and towards the middle of it, to be very full of small Bubbles, of some kind of aerial substance, which by the refraction of the outward surface appear much bigger then really they are; and this may be in good part removed, by putting the drop under the surface of clear Water, for by that means most part of the refraction of the convex Surface of the drop is destroyed, and the bubbles will appear much smaller. And this, by the by, minds me of the appearing magnitude of the _aperture_ of the _iris_, or _pupil_ of the eye, which though it appear, and be therefore judged very large, is yet not above a quarter of the bigness it appears of, by the _lenticular_ refraction of the _Cornea_.

The cause of all which _Phænomena_ I imagine to be no other then this, That the Parts of the Glass being by the excessive heat of the fire kept off and separated one from another, and thereby put into a kind of sluggish fluid Consistence, are suffered to drop off with that heat or agitation remaining in them, into cold Water; by which means the outsides of the drop are presently cool’d and _crusted_, and are thereby made of a loose texture, because the parts of it have not time to settle themselves leisurely together, and so to lie very close together: And the innermost parts of the drop, retaining still much of their former heat and agitations, remain of a loose texture also, and, according as the cold strikes inwards from the bottom and sides, are quenched, as it were, and made rigid in that very posture wherein the cold finds them. For the parts of the _crust_ being already hardened, will not suffer the parts to shrink any more from the outward Surface inward; and though it shrink a little by reason of the small parcels of some Aerial substances dispersed through the matter of the Glass, yet that is not neer so much as it appears (as I just now hinted;) nor if it were, would it be sufficient for to consolidate and condense the body of Glass into a _tuff_ and close _texture_, after it had been so excessively rarified by the heat of the glass-Furnace.

But that there may be such an expansion of the aerial substance contained in those little _blebbs_ or bubbles in the body of the drop, this following Experiment will make more evident.

* * * * *

Take a small Glass-Cane about a foot long, seal up one end of it _hermetically_, then put in a very small bubble of Glass, almost of the shape of an Essence-viol with the open mouth towards the sealed end, then draw out the other end of the Pipe very small, and fill the whole Cylinder with water, then set this Tube by the Fire till the Water begin to boyl, and the Air in the bubble be in good part rarified and driven out, then by sucking at the smalling Pipe, more of the Air or vapours in the bubble may be suck’d out, so that it may sink to the bottom; when it is sunk to the bottom, in the flame of a Candle, or Lamp, nip up the slender Pipe and let it cool: whereupon it is obvious to observe, first, that the Water by degrees will subside and shrink into much less room: Next, that the Air or vapours in the Glass will expand themselves so, as to buoy up the little Glass: Thirdly, that all about the inside of the Glass-pipe there will appear an infinite number of small bubbles, which as the Water grows colder and colder will swell bigger and bigger, and many of them buoy themselves up and break at the top.

From this _Disceding_ of the heat in Glass drops, that is, by the quenching or cooling Irradiations propagated from the Surface upwards and inwards, by the lines CT, CT, DT, DE, &c. the bubbles in the drop have room to expand themselves a little, and the parts of the Glass contract themselves; but this operation being too quick for the sluggish parts of the Glass, the contraction is performed very unequally and irregularly, and thereby the Particles of the Glass are bent, some one way, and some another, yet so as that most of them draw towards the Pith or middle TEEE, or rather from that outward: so that they cannot _extricate_ or unbend themselves, till some part of TEEE be broken and loosened, for all the parts about that are placed in the manner of an Arch, and so till their hold at TEEE be loosened they cannot fly asunder, but uphold, and shelter, and fix each other much like the stones in a Vault, where each stone does concurre to the stability of the whole Fabrick, and no one stone can be taken away but the whole Arch falls. And wheresoever any of those radiating wedges DTD, &c. are removed, which are the component parts of this Arch, the whole Fabrick presently falls to pieces; for all the Springs of the several parts are set at liberty, which immediately extricate themselves and fly asunder every way; each part by its spring contributing to the darting of it self and some other contiguous part. But if this drop be heat so hot as that the parts by degrees can unbend themselves, and be settled and annealed in that posture, and be then suffered gently to subside and cool; The parts by this nealing losing their springiness, constitute a drop of a more soft but less brittle texture, and the parts being not at all under a flexure, though any part of the middle or Pith TEEE be broken, yet will not the drop at all fly to pieces as before.

This Conjecture of mine I shall indeavour to make out by explaining each particular Assertion with _analogous_ Experiments: The Assertions are these.

First, That the parts of the Glass, whilst in a fluid Consistence and hot, are more rarified, or take up more room, then when hard and cold.

Secondly, That the parts of the drop do suffer a two-fold contraction.

Thirdly, That the dropping or quenching the glowing metal in the Water makes it of a hard, springing, and rarified texture.

Fourthly, That there is a flexion or force remaining upon the parts of the Glass thus quenched, from which they indeavour to extricate themselves.

Fifthly, That the Fabrick of the drop, that is able to hinder the parts from extricating themselves, is _analogus_ to that of an Arch.

Sixthly, That the sudden flying asunder of the parts proceeds from their springiness.

Seventhly, That a gradual heating and cooling does anneal or reduce the parts of Glass to a texture that is more loose, and easilier to be broken, but not so brittle.

That the first of these is true may be gathered from this, That _Heat is a property of a body arising from the motion or agitation of its parts_; and therefore whatever body is thereby toucht must necessarily receive some part of that motion, whereby its parts will be shaken and agitated, and so by degrees free and extricate themselves from one another, and each part so moved does by that motion _exert_ a _conatus_ of _protruding_ and displacing all the adjacent Particles. Thus Air included in a vessel, by being heated will burst it to pieces. Thus have I broke a Bladder held over the fire in my hand, with such a violence and noise, that it almost made me deaf for the present, and much surpassed the noise of a Musket: The like have I done by throwing into the fire small glass Bubbles hermetically sealed, with a little drop of Water included in them. Thus Water also, or any other Liquor, included in a convenient vessel, by being warmed, manifestly expands it self with a very great violence, so as to break the strongest vessel, if when heated it be narrowly imprisoned in it. This is very manifest by the _Sealed Thermometers_, which I have, by several tryals, at last brought to a great certainty and tenderness: for I have made some with stems above four foot long, in which the expanding Liquor would so far vary, as to be very neer the very top in the heat of Summer, and prety neer the bottom at the coldest time of the Winter. The Stems I use for them are very thick, straight, and even Pipes of Glass, with a very small _perforation_, and both the head and body I have made on purpose at the Glass-house, of the same metal whereof the Pipes are drawn: these I can easily in the flame of a Lamp, urged with the blast of a pair of Bellows, seal and close together, so as to remain very firm, close and even; by this means I joyn on the body first, and then fill both it and a part of the stem, proportionate to the length of the stem and the warmth of the season I fill it in with the best rectified _Spirit of Wine_ highly _ting’d_ with the lovely colour of _Cocheneel_, which I deepen the more by pouring some drops of common _Spirit of Urine_, which must not be too well rectified, because it will be apt to make the Liquor to curdle and stick in the small perforation of the stem. This Liquor I have upon tryal found the most tender of any spirituous Liquor, and those are much more sensibly affected with the variations of heat and cold then other more flegmatick and ponderous Liquors, and as capable of receiving a deep tincture, and keeping it, as any Liquor whatsoever; and (which makes it yet more acceptable) is not subject to be frozen by any cold yet known. When I have thus filled it, I can very easily in the forementioned flame of a Lamp seal and joyn on the head of it.

Then, for graduating the stem, I fix that for the beginning of my division where the surface of the liquor in the stem remains when the ball is placed in common distilled water, that is so cold that it just begins to freeze and shoot into flakes; and that mark I fix at a convenient place of the stem, to make it capable of exhibiting very many degrees of cold, below that which is requisite to freeze water: the rest of my divisions, both above and below this (which I mark with a [0] or nought) I place according to the Degrees of _Expansion_, or _Contraction_ of the Liquor in proportion to the bulk it had when it indur’d the newly mention’d freezing cold. And this may be very easily and accurately enough done by this following way; Prepare a Cylindrical vessel of very thin plate Brass or Silver, ABCD of the figure Z; the Diameter AB of whose cavity let be about two inches, and the depth BC the same; let each end be cover’d with a flat and smooth plate of the same substance, closely soder’d on, and in the midst of the upper cover make a pretty large hole EF, about the bigness of a fifth part of the Diameter of the other; into this fasten very well with cement a straight and even Cylindrical pipe of Glass, EFGH, the Diameter of whose cavity let be exactly one tenth of the Diameter of the greater Cylinder. Let this pipe be mark’d at GH with a Diamant, so that G from E may be distant just two inches, or the same height with that of the cavity of the greater Cylinder, then divide the length EG exactly into 10 parts, so the capacity of the hollow of each of these divisions will be ¹⁄₁₀₀₀ part of the capacity of the greater Cylinder. This vessel being thus prepared, the way of marking and graduating the _Thermometers_ may be very easily thus performed:

Fill this Cylindrical vessel with the same liquor wherewith the _Thermometers_ are fill’d, then place both it and the _Thermometer_ you are to _graduate_, in water that is ready to be frozen, and bring the surface of the liquor in the _Thermometer_ to the first marke or [0]; then so proportion the liquor in the Cylindrical vessel, that the surface of it may just be at the lower end of the small glass-Cylinder; then very gently and gradually warm the water in which both the _Thermometer_ and this Cylindrical vessel stand, and as you perceive the ting’d liquor to rise in both stems, with the point of a Diamond give several marks on the stem of the _Thermometer_ at those places, which by comparing the expansion in both Stems, are found to correspond to the divisions of the cylindrical vessel, and having by this means marked some few of these divisions on the Stem, it will be very easie by these to mark all the rest of the Stem, and accordingly to assign to every division a proper character.

A _Thermometer_, thus marked and prepared, will be the fittest Instrument to make a Standard of heat and cold that can be imagined. For being sealed up, it is not at all subject to variation or wasting, nor is it liable to be changed by the varying pressure of the Air, which all other kind of _Thermometers_ that are open to the Air are liable to. But to proceed.

This property of Expansion with Heat, and Contraction with Cold, is not peculiar to Liquors only, but to all kind of solid Bodies also, especially Metals, which will more manifestly appear by this Experiment.

Take the Barrel of a Stopcock of Brass, and let the Key, which is well fitted to it, be riveted into it, so that it may slip, and be easily turned round, then heat this Cock in the fire, and you will find the Key so swollen, that you will not be able to turn it round in the Barrel; but if it be suffered to cool again, as soon as it is cold it will be as movable, and as easie to be turned as before.

This Quality is also very observable in _Lead_, _Tin_, _Silver_, _Antimony_, _Pitch_, _Rosin_, _Bees-wax_, _Butter_, and the like; all which, if after they be melted you suffer gently to cool, you shall find the parts of the upper Surface to subside and fall inwards, losing that plumpness and smoothness it had whilst in fusion. The like I have also observed in the cooling of _Glass of Antimony_, which does very neer approach the nature of Glass,

But because these are all Examples taken from other materials then Glass, and argue only, that possibly there may be the like property also in Glass, not that really there is; we shall by three or four Experiments indeavour to manifest that also.

And the First is an Observation that is very obvious even in these very drops, to wit, that they are all of them terminated with an unequal or irregular Surface, especially about the smaller part of the drop, and the whole length of the stem; as about D, and from thence to A, the whole Surface, which would have been round if the drop had cool’d leisurely, is, by being quenched hastily, very irregularly flatted and pitted; which I suppose proceeds partly from the Waters unequally cooling and pressing the parts of the drop, and partly from the self-contracting or subsiding quality of the substance of the Glass: For the vehemency of the heat of the drop causes such hidden motions and bubbles in the cold Water, that some parts of the Water bear more forcibly against one part then against another, and consequently do more suddenly cool those parts to which they are contiguous.

A Second Argument may be drawn from the Experiment of cutting Glasses with a hot Iron. For in that Experiment the top of the Iron heats, and thereby rarifies the parts of the Glass that lie just before the crack, whence each of those agitated parts indeavouring to expand its self and get elbow-room, thrusts off all the rest of the contiguous parts, and consequently promotes the crack that was before begun.

A Third Argument may be drawn from the way of producing a crack in a sound piece or plate of Glass, which is done two wayes, either First, by suddenly heating a piece of Glass in one place more then in another. And by this means _chymists_ usually cut off the necks of Glass-bodies, by two kinds of Instruments, either by a glowing hot round Iron-Ring, which just incompasses the place that is to be cut, or else by a _Sulphur’d_ Threed, which is often wound about the place where the separation is to be made, and then fired. Or Secondly, A Glass may be cracked by cooling it suddenly in any place with Water, or the like, after it has been all leisurely and gradually heated very hot. Both which _Phænomena_ seem manifestly to proceed from the _expansion_ and contraction of the parts of the Glass, which is also made more probable by this circumstance which I have observed, that a piece of common window-glass being heated in the middle very suddenly with a live Coal or hot Iron, does usually at the first crack fall into pieces, whereas if the Plate has been gradually heated very hot, and a drop of cold Water and the like be put on the middle of it, it only flaws it, but does not break it asunder immediately.

A Fourth Argument may be drawn from this Experiment; Take a Glass-pipe, and fit into a solid stick of Glass, so as it will but just be moved in it. Then by degrees heat them whilst they are one within another, and they will grow stiffer, but when they are again cold, they will be as easie to be turned as before. This Expansion of Glass is more manifest in this Experiment.

Take a stick of Glass of a considerable length, and fit it so between the two ends or screws of a Lath, that it may but just easily turn, and that the very ends of it may be just toucht and susteined thereby; then applying the flame of the Candle to the middle of it, and heating it hot, you will presently find the Glass to stick very fast on those points, and not without much difficulty to be convertible on them, before that by removing the flame for a while from it, it be suffered to cool, and when you will find it as easie to be turned round as at the first.

From all which Experiments it is very evident, that all those Bodies, and particularly Glass, suffers an Expansion by Heat, and that a very considerable one, whilst they are in a state of Fusion. For _Fluidity_, as I elsewhere mention, _being nothing but an effect of very strong and quick shaking motion, whereby the parts are, as it were, loosened from each other, and consequently leave an interjacent space or vacuity_; it follows, that all those shaken Particles must necessarily take up much more room then when they were at rest, and lay quietly upon each other. And this is further confirmed by a Pot of _boyling Alabaster_, which will manifestly rise a sixth or eighth part higher in the Pot, whilst it is boyling, then it will remain at, both before and after it be boyled. The reason of which odd _Phænomenon_ (to hint it here only by the way) is this, that there is in the curious powder of Alabaster, and other calcining Stones, a certain watery substance, which is so fixt and included with the solid Particles, that till the heat be very considerable they will not fly away; but after the heat is increased to such a degree, they break out every way in vapours, and thereby so shake and loosen the small corpuscles of the Powder from each other, that they become perfectly of the nature of a fluid body, and one may move a stick to and fro through it, and stir it as easily as water, and the vapours burst and break out in bubbles just as in boyling water, and the like; whereas, both before those watery parts are flying away, and after they are quite gone; that is, before and after it have done boyling, all those effects cease, and a stick is as difficultly moved to and fro in it as in sand, or the like. Which Explication I could easily prove, had I time; but this is not a fit place for it.

To proceed therefore, I say, that the dropping of this expanded Body into cold Water, does make the parts of the Glass suffer a double contraction: The first is, of those parts which are neer the Surface of the Drop. For Cold, as I said before, contracting Bodies, that is, _by the abatement of the agitating faculty the parts falling neerer together_; the parts next adjoyning to the Water must needs lose much of their motion, and impart it to the Ambient water (which the Ebullition and commotion of it manifests) and thereby become a solid and hard crust, whilst the innermost parts remain yet fluid and expanded; whence, as they grow cold also by degrees, their parts must necessarily be left at liberty to be condensed, but because of the hardness of the outward crust, the contraction cannot be admitted that way; but there being many very small, and before inconspicuous, bubbles in the substance of the Glass, upon the subsiding of the parts of the Glass, the agil substance contained in them has liberty of expanding it self a little, and thereby those bubbles grow much bigger, which is the second Contraction. And both these are confirmed from the appearance of the Drop it self: for as for the outward parts, we see, first, that it is irregular and shrunk, as it were, which is caused by the yielding a little of the hardened Skin to a Contraction, after the very outmost Surface is settled; and as for the internal parts, one may with ones naked Eye perceive abundance of very conspicuous bubbles, and with the _Microscope_ many more.

The Consideration of which Particulars will easily make the Third Position probable, that is, that the parts of the drop will be of a very hard, though of a rarified Texture; for if the outward parts of the Drop, by reason of its hard crust, will indure very little Contraction, and the agil Particles, included in those bubbles, by the losing of their agitation, by the decrease of the Heat, lose also most part of their Spring and Expansive power; it follows (the withdrawing of the heat being very sudden) that the parts must be left in a very loose Texture, and by reason of the implication of the parts one about another, which from their sluggishnes and glutinousness I suppose to be much after the manner of the sticks in a Thorn-bush, or a Lock of Wool; it will follow, I say, that the parts will hold each other very strongly together, and indeavour to draw each other neerer together, and consequently their Texture must be very hard and stiff, but very much rarified.

And this will make probable my next Position, That _the parts of the Glass are under a kind of tension or flexure, out of which they indeavour to extricate and free themselves_, and thereby all the parts draw towards the Center or middle, and would, if the outward parts would give way, as they do when the outward parts cool leisurely (as in baking of Glasses) contract the bulk of the drop into a much less compass. For since, as I proved before, the Internal parts of the drop, when fluid, were of a very rarified Texture, and, as it were, tos’d open like a Lock of Wool, and if they were suffered leisurely to cool, would be again prest, as it were, close together: And since that the heat, which kept them bended and open, is removed, and yet the parts not suffered to get as neer together as they naturally would; It follows, that the Particles remain under a kind of _tension_ and _flexure_, and consequently have an indeavour to free themselves from that _bending_ and _distension_, which they do, as soon as either the tip be broken, or as soon as by a leisurely heating and cooling, the parts are nealed into another posture.

And this will make my next Position probable, that _the parts of the Glass drops are contignated together in the form of an Arch_, cannot any where yield or be drawn inwards, till by the removing of some one part of it (as it happens in the removing one of the stones of an Arch) the whole Fabrick is shatter’d, and falls to pieces, and each of the Springs is left at liberty, suddenly to extricate it self: for since I have made it probable, that the internal parts of the Glass have a contractive power inwards, and the external parts are incapable of such a Contraction, and the figure of it being spherical; it follows, that the superficial parts must bear against each other, and keep one another from being condens’d into a less room, in the same manner as the stones of an Arch conduce to the upholding each other in that Figure. And this is made more probable by another Experiment which was communicated to me by an excellent Person, whose extraordinary Abilities in all kind of Knowledg, especially in that of Natural things, and his generous Disposition in communicating, incouraged me to have recourse to him on many occasions. The Experiment was this: Small Glass-balls (about the bigness of that represented in the _Figure &._) would, upon rubbing or scratching the inward Surface, fly all insunder, with a pretty brisk noise; whereas neither before nor after the inner Surface had been thus scratcht, did there appear any flaw or crack. And putting the pieces of one of those broken ones together again, the flaws appeared much after the manner of the black lines on the Figure, _&._ These Balls were small, but exceeding thick bubbles of Glass, which being crack’d off from the _Puntilion_ whilst very hot, and so suffered to cool without nealing them in the Oven over the Furnace, do thereby (being made of white Glass, which cools much quicker then green Glass, and is thereby made much brittler) acquire a very _porous_ and very brittle _texture_: so that if with the point of a Needle or Bodkin, the inside of any of them be rubbed prety hard, and then laid on a Table, it will, within a very little while, break into many pieces with a brisk noise, and throw the parts above a span asunder on the Table: Now though the pieces are not so small as those of a _fulminating_ drop, yet they as plainly shew, that the outward parts of the Glass have a great _Conatus_ to fly asunder, were they not held together by the _tenacity_ of the parts of the inward Surface: for we see as soon as those parts are crazed by hard rubbing, and thereby their tenacity spoiled, the springiness of the more outward parts quickly makes a divulsion, and the broken pieces will, if the concave Surface of them be further scratcht with a Diamond, fly again into smaller pieces.

From which preceding considerations it will follow Sixthly, That the sudden flying asunder of the parts as soon as this Arch is any where disordered or broken, proceeds from the springing of the parts; which, indeavouring to _extricate_ themselves as soon as they get the liberty, they perform it with such a quickness, that they throw one another away with very great violence: for the Particles that compose the Crust have a _Conatus_ to lye further from one another, and therefore as soon as the external parts are loosened they dart themselves outward with great violence, just as so many Springs would do, if they were detained and fastened to the body, as soon as they should be suddenly loosened; and the internal parts drawing inward, they contract so violently; that they rebound back again and fly into multitude of small shivers or sands. Now though they appear not, either to the naked Eye, or the _Microscope_, yet I am very apt to think there may be abundance of small flaws or cracks, which, by reason the strong reflecting Air is not got between the _contiguous_ parts, appear not. And that this may be so, I argue from this, that I have very often been able to make a crack or flaw, in some convenient pieces of Glass, to appear and disappear at pleasure, according as by pressing together, or pulling asunder the contiguous parts, I excluded or admitted the strong reflecting Air between the parts: And it is very probable, that there may be some Body, that is either very rarified Air, or something _analogous_ to it, which fills the bubbles of these drops; which I argue, first, from the roundness of them, and next, from the vivid reflection of Light which they exhibite: Now though I doubt not, but that the Air in them is very much rarified, yet that there is some in them, to such as well consider this Experiment of the disappearing of a crack upon the _extruding_ of the Air, I suppose it will seem more then probable.

The Seventh and last therefore that I shall prove, is, _That the gradual heating and cooling of these so extended bodies does reduce the parts of the Glass to a looser and softer temper_. And this I found by heating them, and keeping them for a prety while very red hot in a fire; for thereby I found them to grow a little lighter, and the small Stems to be very easily broken and snapt any where, without at all making the drop fly; whereas before they were so exceeding hard, that they could not be broken without much difficulty; and upon their breaking the whole drop would fly in pieces with very great violence. The Reason of which last seems to be, that the leisurely heating and cooling of the parts does not only wast some part of the Glass it self, but ranges all the parts into a better order, and gives each Particle an opportunity of _relaxing_ its self, and consequently neither will the parts hold so strongly together as before, nor be so difficult to be broken: The parts now more easily yielding, nor will the other parts fly in pieces, because the parts have no bended Springs. The _relaxation_ also in the temper of hardned Steel, and hammered Metals, by nealing them in the fire, seems to proceed from much the same cause. For both by quenching suddenly such Metals as have _vitrified_ parts interspers’d, as Steel has, and by hammering of other kinds that do not so much abound with them, as Silver, Brass, &c. the parts are put into and detained in a bended posture, which by the agitation of Heat are shaken, and loosened, and suffered to unbend themselves.

* * * * *

Observ. VIII. _Of the fiery Sparks struck from a Flint or Steel._

It is a very common Experiment, by striking with a Flint against a Steel, to make certain fiery and shining Sparks to fly out from between those two compressing Bodies. About eight years since, upon casually reading the Explication of this odd _Phænomenon_, by the most Ingenious _Des Cartes_, I had a great desire to be satisfied, what that Substance was that gave such a shining and bright Light: And to that end I spread a sheet of white Paper, and on it, observing the place where several of these Sparks seemed to vanish, I found certain very small, black, but glistering Spots of a movable Substance, each of which examining with my _Microscope_, I found to be a small round _Globule_; some of which, as they looked prety small, so did they from their Surface yield a very bright and strong reflection on that side which was next the Light; and each look’d almost like a prety bright Iron-Ball, whose Surface was prety regular, such as is represented by the Figure A. In this I could perceive the Image of the Window prety well, or of a Stick, which I moved up and down between the Light and it. Others I found, which were, as to the bulk of the Ball, prety regularly round, but the Surface of them, as it was not very smooth, but rough, and more irregular, so was the reflection from it more faint and confused. Such were the Surfaces of B. C. D. and E. Some of these I found cleft or cracked, as C, others quite broken in two and hollow, as D. which seemed to be half the hollow shell of a Granado, broken irregularly in pieces. Several others I found of other shapes; but that which is represented by E, I observed to be a very big Spark of fire, which went out upon one side of the Flint that I struck fire withall, to which it stuck by the root F, at the end of which small Stem was fastened-on a _Hemisphere_, or half a hollow Ball, with the mouth of it open from the stemwards, so that it looked much like a Funnel, or an old fashioned Bowl without a foot. This night, making many tryals and observations of this Experiment, I met, among a multitude of the Globular ones which I had observed, a couple of Instances, which are very remarkable to the confirmation of my _Hypothesis_.

And the First was of a pretty big Ball fastened on to the end of a small sliver of Iron, which _Compositum_ seemed to be nothing else but a long thin chip of Iron, one of whose ends was melted into a small round Globul; the other end remaining unmelted and irregular, and perfectly Iron.

The Second Instance was not less remarkable then the First; for I found, when a Spark went out, nothing but a very small thin long sliver of Iron or Steel, unmelted at either end. So that it seems, that some of these Sparks are the slivers or chips of the Iron _vitrified_, Others are only the slivers melted into Balls without vitrification, And the third kind are only small slivers of the Iron, made red-hot with the violence of the stroke given on the Steel by the Flint.

He that shall diligently examine the _Phænomena_ of this Experiment, will, I doubt not, find cause to believe, that the reason I have heretofore given of it, is the true and genuine cause of it, namely, That _the Spark, appearing so bright in the falling, is nothing else but a small piece of the Steel or Flint, but most commonly of the Steel, which by the violence of the stroke is at the same time sever’d and heat red-hot, and that sometimes to such a degree, as to make it melt together into a small Globule of Steel; and sometimes also is that heat so very intense, as further to melt it and vitrifie it; but many times the heat is so gentle, as to be able to make the sliver only red hot, which notwithstanding falling upon the tinder_ (that is only a very curious small Coal made of the small threads of Linnen burnt to coals and char’d) _it easily sets it on fire_. Nor will any part of this _Hypothesis_ seem strange to him that considers, First, that either hammering, or filing or otherwise violently rubbing of Steel, will presently make it so hot as to be able to burn ones fingers. Next, that the whole force of the stroke is exerted upon that small part where the Flint and Steel first touch: For the Bodies being each of them so very hard, the puls cannot be far communicated, that is, the parts of each can yield but very little, and therefore the violence of the concussion will be _exerted_ on that piece of Steel which is cut off by the Flint. Thirdly, that the filings or small parts of Steel are very apt, as it were, to take fire, and are presently red hot, that is, there seems to be a very _combustible sulphureous_ Body in Iron or Steel, which the Air very readily preys upon, as soon as the body is a little violently heated.

And this is obvious in the filings of Steel or Iron cast through the flame of a Candle; for even by that sudden _transitus_ of the small chips of Iron, they are heat red hot, and that _combustible sulphureous_ Body is presently prey’d upon and devoured by the _aereal_ incompassing _Menstruum_, whose office in this Particular I have shewn in the Explication of Charcole.

And in prosecution of this Experiment, having taken the filings of Iron and Steel, and with the point of a Knife cast them through the flame of a Candle, I observed where some conspicuous shining Particles fell, and looking on them with my _Microscope_, I found them to be nothing else but such round Globules, as I formerly found the Sparks struck from the Steel by a stroke to be, only a little bigger; and shaking together all the filings that had fallen upon the sheet of Paper underneath and observing them with the _Microscope_, I found a great number of small Globules, such as the former, though there were also many of the parts that had remained untoucht and rough filings or chips of Iron. So that, it seems, Iron does contain a very _combustible sulphureous_ Body, which is, in all likelihood, one of the causes of this _Phænomenon_, and which may be perhaps very much concerned in the business of its hardening and tempering: of which somewhat is said in the Description of _Muscovy-glass_.

So that, these things considered, we need not trouble our selves to find out what kind of Pores they are, both in the Flint and Steel, that contain the _Atoms of fire_, nor how those _Atoms_ come to be hindred from running all out, when a dore or passage in their Pores is made by the concussion: nor need we trouble our selves to examine by what _Prometheus_ the Element of Fire comes to be fetcht down from above the Regions of the Air, in what Cells or Boxes it is kept, and what _Epimetheus_ lets it go: Nor to consider what it is that causes so great a conflux of the atomical Particles of Fire, which are said to fly to a flaming Body, like Vultures or Eagles to a putrifying Carcass, and there to make a very great pudder. Since we have nothing more difficult in this _Hypothesis_ to conceive, first, as to the kindling of Tinder, then how a large Iron-bullet, let fall red or glowing hot upon a heap of Small-coal, should set fire to those that are next to it first: Nor secondly, is this last more difficult to be explicated, then that a Body, as Silver for Instance, put into a weak _Menstruum_, as unrectified _Aqua fortis_ should, when it is put in a great heat, be there dissolved by it, and not before; which _Hypothesis_ is more largely explicated in the Description of Charcoal. To conclude, we see by this Instance, how much Experiments may conduce to the regulating of _Philosophical notions_. For if the most Acute _Des Cartes_ had applied himself experimentally to have examined what substance it was that caused that shining of the falling Sparks struck from a Flint and a Steel, he would certainly have a little altered his _Hypothesis_, and we should have found, that his Ingenious Principles would have admitted a very plausible Explication of this _Phænomenon_; whereas by not examining so far as he might, he has set down an Explication which Experiment do’s contradict.

But before I leave this Description, I must not forget to take notice of the Globular form into which each of these is most curiously formed. And this _Phænomenon_, as I have elsewhere more largely shewn, proceeds from a propriety which belongs to all kinds of fluid Bodies more or less, and is caused by the Incongruity of the Ambient and included Fluid, which so acts and modulates each other, that they acquire, as neer as is possible, a _spherical_ or _globular_ form, which propriety and several of the _Phænomena_ that proceed from it, I have more fully explicated in the sixth Observation.

One Experiment, which does very much illustrate my present Explication, and is in it self exceeding pretty, I must not pass by: And that is a way of making small _Globules_ or _Balls_ of Lead, or Tin, as small almost as these of Iron or Steel, and that exceeding easily and quickly, by turning the filings or chips of those Metals also into perfectly round _Globules_. The way, in short, as I received it from the _Learned Physitian Doctor_ I.G. is this;

Reduce the Metal you would thus shape, into exceeding fine filings, the finer the filings are, the finer will the Balls be: _Stratifie_ these filings with the fine and well dryed powder of quick Lime in a _Crucible_ proportioned to the quantity you intend to make: When you have thus filled your _Crucible_, by continual _stratifications_ of the filings and powder, so that, as neer as may be, no one of the filings may touch another, place the _Crucible_ in a _gradual fire_, and by degrees let it be brought to a heat big enough to make all the filings, that are mixt with the quick Lime, to melt, and no more; for if the fire be too hot, many of these filings will joyn and run together; whereas if the heat be proportioned, upon washing the Lime-dust in fair Water, all those small filings of the Metal will subside to the bottom in a most curious powder, consisting all of exactly round _Globules_, which, if it be very fine, is very excellent to make Hour-glasses of.

Now though quick Lime be the powder that this direction makes choice of, yet I doubt not, but that there may be much more convenient ones found out, one of which I have made tryal of, and found very effectual; and were it not for discovering, by the mentioning of it, another Secret, which I am not free to impart, I should have here inserted it.

* * * * *

Observ. IX. _Of the Colours observable in Muscovy-Glass, and other thin Bodies_.

Moscovy-glass, or _Lapis specularis_, is a Body that seems to have as many Curiosities in its Fabrick as any common Mineral I have met with: for first, It is transparent to a great thickness: Next, it is compounded of an infinite number of thin flakes joyned or generated one upon another so close & smooth, as with many hundreds of them to make one smooth and thin Plate of a transparent flexible substance, which with care and diligence may be slit into pieces so exceedingly thin as to be hardly perceivable by the eye, and yet even those, which I have thought the thinnest, I have with a good _Microscope_ found to be made up of many other Plates, yet thinner; and it is probable, that, were our _Microscopes_ much better, we might much further discover its divisibility. Nor are these flakes only regular as to the smoothness of their Surfaces, but thirdly, In many Plates they may be perceived to be terminated naturally with edges of the figure of a _Rhomboeid_. This Figure is much more conspicuous in our English talk, much whereof is found in the Lead Mines, and is commonly called _Spar_, and _Kauck_, which is of the same kind of substance with the _Selenitis_, but is seldom found in so large flakes as that is, nor is it altogether so tuff, but is much more clear and transparent, and much more curiously shaped, and yet may be cleft and flak’d like the other _Selenitis_. But fourthly, this stone has a property, which in respect of the _Microscope_, is more notable, and that is, that it exhibits several appearances of Colours, both to the naked Eye, but much more conspicuously to the _Microscope_; for the exhibiting of which, I took a piece of _Muscovy-glass_, and splitting or cleaving it into thin Plates, I found that up and down in several parts of them I could plainly perceive several white specks or flaws, and others diversly coloured with all the Colours of the _Rainbow_; and with the _Microscope_ I could perceive, that these Colours were ranged in rings that incompassed the white speck or flaw, and were round or irregular, according to the shape of the spot which they terminated; and the position of Colours, in respect of one another, was the very same as in the _Rainbow_. The consecution of those Colours from the middle of the spot outward being Blew, Purple, Scarlet, Yellow, Green; Blew, Purple, Scarlet, and so onwards, sometimes half a score times repeated, that is, there appeared six, seven, eight, nine or ten several coloured rings or lines, each incircling the other, in the same manner as I have often seen a very _vivid Rainbow_ to have four or five several Rings of Colours, that is, accounting all the Gradations between Red and Blew for one: But the order of the Colours in these Rings was quite contrary to the primary or innermost _Rainbow_, and the same with those of the secondary or outermost Rainbow; these coloured Lines or _Irises_, as I may so call them, were some of them much brighter then others, and some of them also very much broader, they being some of them ten, twenty, nay, I believe, neer a hundred times broader then others; and those usually were broadest which were neerest the center or middle of the flaw. And oftentimes I found, that these Colours reacht to the very middle of the flaw, and then there appeared in the middle a very large spot, for the most part, all of one colour, which was very vivid, and all the other Colours incompassing it, gradually ascending, and growing narrower towards the edges, keeping the same order, as in the _secundary Rainbow_, that is, if the middle were Blew, the next incompassing it would be a Purple, the third a Red, the fourth a Yellow, &c. as above; if the middle were a Red, the next without it would be a Yellow, the third a Green, the fourth a Blew, and so onward. And this order it alwayes kept whatsoever were the middle Colour.

There was further observable in several other parts of this Body, many Lines or Threads, each of them of some one peculiar Colour, and those so exceedingly bright and vivid, that it afforded a very pleasant object through the _Microscope_. Some of these _threads_ I have observed also to be pieced or made up of several short lengths of differently coloured _ends_ (as I may so call them) as a line appearing about two inches long through the _Microscope_, has been compounded of about half an inch of a Peach colour, ⅛ of a lovely Grass-green, ¾ of an inch more of a bright Scarlet, and the rest of the line of a Watchet blew. Others of them were much otherwise coloured; the variety being almost infinite. Another thing which is very observable, is, that if you find any place where the colours are very broad and conspicuous to the naked eye, you may, by pressing that place with your finger, make the colours change places, and go from one part to another.

There is one _Phænomenon_ more, which may, if care be used, exhibit to the beholder, as it has divers times to me, an exceeding pleasant, and not less instructive Spectacle; And that is, if curiosity and diligence be used, you may so split this admirable Substance, that you may have pretty large Plates (in companion of those smaller ones which you may observe in the Rings) that are perhaps an ⅛ or a ⅙ part of an inch over, each of them appearing through the _Microscope_ most curiously, intirely, and uniformly adorned with some one vivid colour: this, if examined with the _Microscope_, may be plainly perceived to be in all parts of it equally thick. Two, three, or more of these lying one upon another, exhibit oftentimes curious compounded colours, which produce such a _Compositum_, as one would scarce imagine should be the result of such _ingredients_: As perhaps a _faint yellow_ and a _blew_ may produce a very _deep purple_. But when anon we come to the more strict examination of these _Phænomena_, and to inquire into the causes and reasons of these productions, we shall, I hope, make it more conceivable how they are produced, and shew them to be no other then the natural and necessary effects arising from the peculiar union of concurrent causes.

These _Phænomena_, being so various, and so truly admirable, it will certainly be very well worth our inquiry, to examine the causes and reasons of them, and to consider, whether from these causes demonstratively evidenced, may not be deduced the true causes of the production of all kind of Colours. And I the rather now do it, instead of an Appendix or Digression to this History, then upon the occasion of examining the Colours in Peacocks, or other Feathers, because this Subject, as it does afford more variety of particular Colours, so does it afford much better wayes of examining each circumstance. And this will be made manifest to him that considers, first, that this laminated body is more simple and regular then the parts of Peacocks feathers, this consisting only of an indefinite number of plain and smooth Plates, heaped up, or _incumbent_ on each other. Next, that the parts of this body are much more manageable, to be divided or joyned, then the parts of a Peacocks feather, or any other substance that I know. And thirdly, because that in this we are able from a colourless body to produce several coloured bodies, affording all the variety of Colours imaginable: And several others, which the subsequent Inquiry will make manifest.

To begin therefore, it is manifest from several circumstances, that the material cause of the _apparition_ of these several Colours, is some _Lamina_ or Plate of a transparent or pellucid body of a thickness very determinate and proportioned according to the greater or less refractive power of the _pellucid_ body. And that this is so, abundance of Instances and particular Circumstances will make manifest.

As _first_, if you take any small piece of the _Muscovy-glass_, and with a Needle, or some other convenient Instrument, cleave it oftentimes into thinner and thinner _Laminæ_, you shall find, that till you come to a determinate thinness of them, they shall all appear transparent and colourless, but if you continue to split and divide them further, you shall find at last, that each Plate, after it comes to such a determinate thickness, shall appear most lovely ting’d or imbued with a determinate colour. If _further_, by any means you so flaw a pretty thick piece, that one part does begin to cleave a little from the other, and between those two there be by any means gotten some pellucid _medium_, those _laminated_ pellucid bodies that fill that space, shall exhibit several Rainbows or coloured Lines, the colours of which will be disposed and ranged according to the various thicknesses of the several parts of that Plate. That this is so, is yet _further_ confirmed by this Experiment.

Take two small pieces of ground and polisht Looking-glass-plate, each about the bigness of a shilling, take these two dry, and with your fore-fingers and thumbs press them very hard and close together, and you shall find, that when they approach each other very near, there will appear several _Irises_ or coloured Lines, in the same manner almost as in the _Muscovy-glass_; and you may very easily change any of the Colours of any part of the interposed body, by pressing the Plates closer and harder together, or leaving them more lax; that is, a part which appeared coloured with a red, may be presently ting’d with a yellow, blew, green, purple, or the like, by altering the appropinquation of the terminating Plates. Now that air is not necessary to be the interposed body, but that any other transparent fluid will do much the same, may be tryed by wetting those approximated Surfaces with Water, or any other transparent Liquor, and proceeding with it in the same manner as you did with the Air; and you will find much the like effect, only with this difference, that those comprest bodies, which differ most, in their refractive quality, from the compressing bodies, exhibit the most strong and vivid tinctures. Nor is it necessary, that this _laminated_ and _ting’d_ body should be of a fluid substance, any other substance, provided it be thin enough and transparent, doing the same thing: this the _Laminæ_ of our _Muscovy-glass_ hint; but it may be confirm’d by multitudes of other Instances.

And first, we shall find, that even Glass it self may, by the help of a Lamp, be blown thin enough to produce these _Phænomena_ of Colours: which _Phænomena_ accidentally happening, as I have been attempting to frame small Glasses with a Lamp, did not a little surprize me at first, having never heard or seen any thing of it before; though afterwards comparing it with the _Phænomena_, I had often observed in those Bubbles which Children use to make with Soap-water, I did the less wonder; especially when upon Experiment I found, I was able to produce the same _Phænomena_ in thin Bubbles made with any other transparent Substance. Thus have I produced them with Bubbles of _Pitch_, _Rosin_, _Colophony_, _Turpentine_, _Solutions_ of several _Gums_, as _Gum-Arabick_ in water; any _glutinous_ Liquor, as _Wort_, _Wine_, _Spirit of Wine_, _Oyl of Turpentine_, _Glare of Snails_, &c.

It would be needless to enumerate the several Instances, these being enough to shew the generality or universality of this propriety. Only I must not omit, that we have instances also of this kind even in metalline Bodies and animal; for those several Colours which are observed to follow each other upon the polisht surface of hardned Steel, when it is by a sufficient degree of heat gradually tempered or softened, are produced, from nothing else but a certain thin _Lamina_ of a _vitrum_ or _vitrified_ part of the Metal, which by that degree of heat, and the concurring action of the ambient Air, is driven out and fixed on the surface of the Steel.

And this hints to me a very probable (at least, if not the true) cause of the hardning and tempering of Steel, which has not, I think, been yet given, nor, that I know of been so much as thought of by any. And that is this, that the hardness of it arises from a greater proportion of a vitrified Substance interspersed through the pores of the Steel. And that the tempering or softning of it arises from the proportionate or smaller parcels of it left within those pores. This will seem the more probable, if we consider these Particulars.

First, That the pure parts of Metals are of themselves very _flexible_ and _tuff_; that is, will indure bending and hammering, and yet retain their continuity.

Next, That the Parts of all vitrified Substances, as all kinds of Glass, the _Scoria_ of Metals, &c. are very hard, and also very brittle, being neither _flexible_ nor _malleable_, but may by hammering or beating be broken into small parts or powders.

Thirdly, That all Metals (excepting Gold and Silver, which do not so much with the bare fire, unless assisted by other saline Bodies) do more or less _vitrifie_ by the strength of fire, that is, are corroded by a Saline Substance, which I elsewhere shew to be the true cause of fire; and are thereby, as by several other _Menstruums_ converted into _Scoria_; And this is called, _calcining_ of them, by Chimists. Thus Iron and Copper by heating and quenching do turn all of them by degrees into _Scoria_, which are evidently _vitrified_ Substances, and unite with Glass, and are easily _fusible_; and when cold, very hard, and very brittle.

Fourthly, That most kind of _Vitrifications_ or _Calcinations_ are made by Salts, uniting and incorporating with the metalline Particles. Nor do I know any one _calcination_ wherein a _Saline_ body may not, with very great probability, be said to be an agent or coadjutor.

Fifthly, That Iron is converted into Steel by means of the incorporation of certain salts, with which it is kept a certain time in the fire.

Sixthly, That any Iron may, in a very little time, be _case hardned_, as the Tradesmen call it, by casing the iron to be hardned with clay, and putting between the clay and iron a good quantity of a mixture of _Urine_, _Soot_, _Sea-salt_, and _Horses hoofs_ (all which contein great quantities of Saline bodies) and then putting the case into a good strong fire, and keeping it in a considerable degree of heat for a good while, and afterwards heating, and quenching or cooling it suddenly in cold water.

Seventhly, That all kind of vitrify’d substances, by being suddenly cool’d, become very hard and brittle. And thence arises the pretty _Phænomena_ of the Glass Drops, which I have already further explained in its own place.

Eighthly, That those metals which are not so apt to vitrifie, do not acquire any hardness by quenching in water, as Silver, Gold, &c.

These considerations premis’d, will, I suppose, make way for the more easie reception of this following Explication of the _Phænomena_ of hardned and temper’d Steel. That Steel is a substance made out of Iron, by means of a certain proportionate _Vitrification_ of several parts, which are so curiously and proportionately mixt with the more tough and unalter’d parts of the Iron, that when by the great heat of the fire this vitrify’d substance is melted, and consequently rarify’d, and thereby the pores of the Iron are more open, if then by means of dipping it in cold water it be suddenly cold, and the parts hardned, that is, stay’d in that same degree of _Expansion_ they were in when hot, the parts become very hard and brittle, and that upon the same account almost as small parcels of glass quenched in water grow brittle, which we have already explicated. If after this the piece of Steel be held in some convenient heat, till by degrees certain colours appear upon the surface of the brightned metal, the very hard and brittle tone of the metal, by degrees relaxes and becomes much more tough and soft; namely, the action of the heat does by degrees loosen the parts of the Steel that were before streached or set _atilt_ as it were, and stayed open by each other, whereby they become relaxed and set at liberty, whence some of the more brittle interjacent parts are thrust out and melted into a thin skin on the surface of the Steel, which from no colour increases to a deep Purple, and so onward by these _gradations_ or consecutions, _White, Yellow, Orange, Minium, Scarlet, Purple, Blew, Watchet_, &c. and the parts within are more conveniently, and proportionately mixt; and so they gradually subside into a texture which is much better proportion’d and closer joyn’d, whence that rigidness of parts ceases, and the parts begin to acquire their former _ductilness_.

Now, that ’tis nothing but the vitrify’d metal that sticks upon the surface of the colour’d body, is evident from this, that if by any means it be scraped and rubb’d off, the metal underneath it is white and clear; and if it be kept longer in the fire, so as to increase to a considerable thickness, it may, by blows, be beaten off in flakes. This is further confirm’d by this observable, that that Iron or Steel will keep longer from rusting which is covered with this vitrify’d case: Thus also Lead will, by degrees, be all turn’d into a litharge; for that colour which covers the top being scum’d or shov’d aside, appears to be nothing else but a litharge or vitrify’d Lead.

This is observable also in some sort, on Brass, Copper, Silver, Gold, Tin, but is most conspicuous in Lead: all those Colours that cover the surface of the Metal being nothing else, but a very thin vitrifi’d part of the heated Metal.

The other Instance we have, is in Animal bodies, as in Pearls, Mother of Pearl-shels, Oyster-shels, and almost all other kinds of stony shels whatsoever. This have I also sometimes with pleasure observ’d even in Muscles and Tendons. Further, if you take any glutinous substance and run it exceedingly thin upon the surface of a smooth glass or a polisht metaline body, you shall find the like effects produced: and in general, wheresoever you meet with a transparent body thin enough, that is terminated by reflecting bodies of differing refractions from it, there will be a production of these pleasing and lovely colours.

Nor is it necessary, that the two _terminating_ Bodies should be both of the same kind, as may appear by the _vitrified Laminæ_ on _Steel_, _Lead_, and other Metals, one surface of which _Laminæ_ is contiguous to the surface of the Metal, the other to that of the Air.

Nor is it necessary, that these colour’d _Laminæ_ should be of an even thickness, that is, should have their edges and middles of equal thickness, as in a Looking-glass-plate, which circumstance is only requisite to make the Plate appear all of the same colour; but they may resemble a _Lens_, that is, have their middles thicker then their edges; or else a _double concave_, that is, be thinner in the middle then at the edges; in both which cases there will be various coloured rings or lines, with differing consecutions or orders of Colours; the order of the first from the middle outwards being Red, Yellow, Green, Blew, &c. And the latter quite contrary.

But further, it is altogether necessary, that the Plate, in the places where the Colours appear, should be of a determinate thickness: First, It must not be more then such a thickness, for when the Plate is increased to such a thickness, the Colours cease; and besides, I have seen in a thin piece of _Muscovy-glass_, where the two ends of two Plates, which appearing both single, exhibited two distinct and differing Colours; but in that place where they were united, and constituted one double Plate (as I may call it) they appeared transparent and colourless. Nor, Secondly, may the Plates be _thinner_ then such a determinate _cize_; for we alwayes find, that the very outmost Rim of these flaws is terminated in a white and colourless Ring.

Further, in this Production of Colours there is no need of a determinate Light of such a bigness and no more, nor of a determinate position of that Light, that it should be on this side, and not on that side; nor of a terminating shadow, as in the Prisme, and Rainbow, or Water-ball: for we find, that the Light in the open Air, either in or out of the Sun-beams, and within a Room, either from one or many Windows, produces much the same effect: only where the Light is brightest, there the Colours are most _vivid_. So does the light of a Candle, collected by a Glass-ball. And further, it is all one whatever side of the coloured Rings be towards the light; for the whole Ring keeps its proper Colours from the middle outwards in the same order as I before related, without varying at all, upon changing the position of the light.

But above all it is most observable, that here are all kind of Colours generated in a _pellucid_ body, where there is properly no such refraction as _Des Cartes_ supposes his _Globules_ to acquire a _verticity_ by: For in the plain and even Plates it is manifest, that the second refraction (according to _Des Cartes_ his Principles in the _fifth Section of the eighth Chapter of his Meteors_) does regulate and restore the supposed _turbinated Globules_ unto their former uniform motion. This Experiment therefore will prove such a one as our _thrice excellent Verulam_ calls _Experimentum Crucis_, serving as a Guide or Land-mark, by which to direct our course in the search after the true cause of Colours. Affording us this particular negative Information, that for the production of Colours there is not necessary either a great refraction, as in the Prisme; nor Secondly, a determination of Light and shadow, such as is both in the Prisme and Glass-ball. Now that we may see likewise what affirmative and positive Instruction it yields, it will be necessary, to examine it a little more particularly and strictly; which that we may the better do, it will be requisite to premise somewhat in general concerning the nature of Light and Refraction.

And first for Light it seems very manifest, that there is no luminous Body but has the parts of it in motion more or less.

First, That all kind of _fiery burning Bodies_ have their parts in motion, I think, will be very easily granted me. That the _spark_ struck from a Flint and Steel is in a rapid agitation, I have elsewhere made probable. And that the Parts of _rotten Wood_, _rotten Fish_ and the like, are also in motion, I think, will as easily be conceded by those, who consider, that those parts never begin to shine till the Bodies be in a state of putrefaction; and that is now generally granted by all, to be caused by the motion of the parts of putrifying bodies. That the _Bononian stone_ shines no longer then it is either warmed by the Sun-beams, or by the flame of a Fire or of a Candle, is the general report of those that write of it, and of others that have seen it. And that heat argues a motion of the internal parts is (as I said before) generally granted.

But there is one Instance more, which was first shewn to the _Royal Society_ by Mr. _Clayton_ a worthy Member thereof, which does make this Assertion more evident then all the rest: And that is, That a _Diamond_ being _rub’d_, _struck_ or _heated_ in the dark, shines for a pretty while after, so long as that motion, which is imparted by any of those Agents, remains (in the same manner as a Glass, rubb’d, struck, or (by a means which I shall elsewhere mention) heated, yields a sound which lasts as long as the vibrating motion of that _sonorous_ body) several Experiments made on which Stone, are since published in a Discourse of Colours, by the truly honourable Mr. _Boyle_. What may be said of those _Ignes fatui_ that appear in the night, I cannot so well affirm, having never had the opportunity to examine them my self, nor to be inform’d by any others that had observ’d them: And the relations of them in Authors are so imperfect, that nothing can be built on them. But I hope I shall be able in another place to make it at least very probable, that there is even in those also a Motion which causes this effect. That the shining of _Sea-water_ proceeds from the same cause, may be argued from this, That it shines not till either it be beaten against a Rock, or be some other wayes broken or agitated by Storms, or Oars, or other _percussing_ bodies. And that the Animal _Energies_ or Spirituous _agil_ parts are very active in _Cats eyes_ when they shine, seems evident enough, because their eyes never shine but when they look very intensly either to find their prey, or being hunted in a dark room, when they seek after their adversary, or to find a way to escape. And the like may be said of the shining _Bellies of Gloworms_; since ’tis evident they can at pleasure either increase or extinguish that Radiation.

It would be somewhat too long a work for this place _Zetetically_ to examine, and positively to prove, what particular kind of motion it is that must be the efficient of Light; for though it be a motion, yet ’tis not every motion that produces it, since we find there are many bodies very violently mov’d, which yet afford not such an effect; and there are other bodies, which to our other senses, seem not mov’d so much, which yet shine. Thus Water and quick-silver, and most other liquors heated, shine not; and several hard bodies, as Iron, Silver, Brass, Copper, Wood, &c. though very often struck with a hammer, shine not presently, though they will all of them grow exceeding hot; whereas rotten Wood, rotten Fish, Sea-water, Gloworms, &c. have nothing of tangible heat in them, and yet (where there is no stronger light to affect the Sensory) they shine some of them so Vividly, that one may make a shift to read by them.

It would be too long, I say, here to insert the discursive progress by which I inquir’d after the proprieties of the motion of Light, and therefore I shall only add the result.

And, First, I found it ought to be exceeding _quick_, such as those motions of _fermentation_ and _putrefaction_, whereby, certainly, the parts are exceeding nimbly and violently mov’d; and that, because we find those motions are able more minutely to shatter and divide the body, then the most violent heats or _menstruums_ we yet know. And that fire is nothing else but such a _dissolution_ of the Burning body, made by the most _universal menstruum_ of all _sulphureous bodies_, namely, the Air, we shall in an other place of this Tractate endeavour to make probable. And that, in all extreamly hot shining bodies, there is a very quick motion that causes Light, as well as a more robust that causes Heat, may be argued from the celerity wherewith the bodyes are dissolv’d.

Next, it must be a _Vibrative motion_. And for this the newly mention’d _Diamond_ affords us a good argument; since if the motion of the parts did not return, the Diamond must after many rubbings decay and be wasted: but we have no reason to suspect the latter, especially if we consider the exceeding difficulty that is found in cutting or wearing away a Diamond. And a Circular motion of the parts is much more improbable, since, if that were granted, and they be suppos’d irregular and Angular parts, I see not how the parts of the Diamond should hold so firmly together, or remain in the same sensible dimensions, which yet they do. Next, if they be _Globular_, and mov’d only with a _turbinated_ motion, I know not any cause that can impress that motion upon the _pellucid medium_, which yet is done. Thirdly, any other _irregular_ motion of the parts one amongst another, must necessarily make the body of a fluid consistence, from which it is far enough. It must therefore be a _Vibrating_ motion.

And Thirdly, That it is a very _short-vibrating motion_, I think the instances drawn from the shining of Diamonds will also make probable. For a Diamond being the hardest body we yet know in the World, and consequently the least apt to yield or bend, must consequently also have its _vibrations_ exceeding short.

And these, I think, are the three principal proprieties of a motion, requisite to produce the effect call’d Light in the Object.

The next thing we are to consider, is the way or manner of the _trajection_ of this motion through the interpos’d pellucid body to the eye: And here it will be easily granted,

First, That it must be a body _susceptible_ and _impartible_ of this motion that will deserve the name of a Transparent. And next, that the parts of such a body must be _Homogeneous_, or of the same kind. Thirdly, that the constitution and motion of the parts must be such, that the appulse of the luminous body may be communicated or propagated through it to the greatest imaginable distance in the least imaginable time, though I see no reason to affirm, that it must be in an instant: For I know not any one Experiment or observation that does prove it. And, whereas it may be objected, That we see the Sun risen at the very instant when it is above the sensible Horizon, and that we see a Star hidden by the body of the Moon at the same instant, when the Star, the Moon, and our Eye are all in the same line; and the like Observations, or rather suppositions, may be urg’d. I have this to answer, That I can as easily deny as they affirm; for I would fain know by what means any one can be assured any more of the Affirmative, then I of the Negative. If indeed the propagation were very slow, ’tis possible something might be discovered by Eclypses of the Moon; but though we should grant the progress of the light from the Earth to the Moon, and from the Moon back to the Earth again to be full two Minutes in performing, I know not any possible means to discover it; nay, there may be some instances perhaps of Horizontal Eclypses that may seem very much to favour this supposition of the slower progression of Light then most imagine. And the like may be said of the Eclypses of the Sun, &c. But of this only by the by. Fourthly, That the motion is propagated every way through an _Homogeneous medium_ by _direct_ or _straight_ lines extended every way like Rays from the center of a Sphere. Fifthly, in an _Homogeneous medium_ this motion is propagated every way with _equal velocity_, whence necessarily every _pulse_ or _vibration_ of the luminous body will generate a Sphere, which will continually increase, and grow bigger, just after the same manner (though indefinitely swifter) as the waves or rings on the surface of the water do swell into bigger and bigger circles about a point of it, where, by the sinking of a Stone the motion was begun, whence it necessarily follows, that all the parts of these Spheres undulated through an _Homogeneous medium_ cut the Rays at right angles.

But because all transparent _mediums_ are not _Homogeneous_ to one another, therefore we will next examine how this pulse or motion will be propagated through differingly transparent _mediums_. And here, according to the most acute and excellent Philosopher _Des Cartes_, I suppose the sign of the angle of inclination in the first _medium_ to be to the sign of refraction in the second, As the density of the first, to the density of the second. By density, I mean not the density in respect of gravity (with which the refractions or transparency of _mediums_ hold no proportion) but in respect onely to the _trajection_ of the Rays of light, in which respect they only differ in this; that the one propagates the pulse more easily and weakly, the other more slowly, but more strongly. But as for the pulses themselves, they will by the refraction acquire another propriety, which we shall now endeavour to explicate.

We will suppose therefore in the first Figure ACFD to be a physical Ray, or ABC and DEF to be two Mathematical Rays, _trajected_ from a very remote point of a luminous body through an _Homogeneous_ transparent _medium_ LLL, and DA, EB, FC, to be small portions of the orbicular impulses which must therefore cut the Rays at right angles; these Rays meeting with the plain surface NO of a _medium_ that yields an easier _transitus_ to the propagation of light, and falling _obliquely_ on it, they will in the _medium_ MMM be refracted towards the perpendicular of the surface. And because this _medium_ is more easily _trajected_ then the former by a third, therefore the point C of the orbicular pulse FC will be mov’d to H four spaces in the same time that F the other end of it is mov’d to G three spaces, therefore the whole refracted pulse GH shall be _oblique_ to the refracted Rays CHK and GI; and the angle GHC shall be an acute, and so much the more acute by how much the greater the refraction be, then which nothing is more evident, for the sign of the inclination is to the sign of refraction as GF to TC the distance between the point C and the perpendicular from G on CK, which being as four to three, HC being longer then GF is longer also then TC, therefore the angle GHC is less than GTC. So that henceforth the parts of the pulses GH and IK are mov’d ascew, or cut the Rays at _oblique_ angles.

It is not my business in this place to set down the reasons why this or that body should impede the Rays more, others less: as why Water should transmit the Rays more easily, though more weakly than air. Onely thus much in general I shall hint, that I suppose the _medium_ MMM to have less of the transparent undulating subtile matter, and that matter to be less implicated by it, whereas LLL I suppose to contain a greater quantity of the fluid undulating substance, and this to be more implicated with the particles of that _medium_.

But to proceed, the same kind of _obliquity_ of the Pulses and Rays will happen also when the refraction is made out of a more easie into a more difficult _mediu_; as by the calculations of GQ & CSR which are refracted from the perpendicular. In both which calculations ’tis _obvious_ to observe, that always that part of the Ray towards which the refraction is made has the end of the _orbicular pulse_ precedent to that of the other side. And always, the oftner the refraction is made the same way, Or the greater the single refraction is, the more is this unequal progress. So that having found this odd propriety to be an inseparable concomitant of a refracted Ray, not streightned by a contrary refraction, we will next examine the refractions of the Sun-beams, as they are suffer’d onely to pass through a small passage, _obliquely_ out of a more difficult, into a more easie _medium_.

Let us suppose therefore ABC in the second Figure to represent a large _Chemical Glass-body_ about two foot long, filled with very fair Water as high as AB, and inclin’d in a convenient posture with B towards the Sun: Let us further suppose the top of it to be cover’d with an _opacous_ body, all but the hole ab, through which the Sun-beams are suffer’d to pass into the Water, and are thereby refracted to cdef, against which part, if a Paper be expanded on the outside, there will appear all the colours of the Rainbow, that is, there will be generated the two principal colours, _Scarlet_ and _Blue_, and all the _intermediate_ ones which arise from the composition and dilutings of these two, that is, cd shall exhibit a _Scarlet_, which toward d is diluted into a _Yellow_; this is the refraction of the Ray, ik, which comes from the underside of the Sun; and the Ray ef shall appear of a deep _Blue_, which is gradually towards e diluted into a pale _Watchet-blue_. Between d and e the two _diluted_ colours. _Blue_ and _Yellow_ are mixt and compounded into a _Green_; and this I imagine to be the reason why _Green_ is so acceptable a colour to the eye, and that either of the two extremes are, if intense, rather a little offensive, namely, the being plac’d in the middle between the two extremes, and compounded out of both those, _diluted_ also, or somewhat qualifi’d, for the _composition_, arising from the mixture of the two extremes _undiluted_, makes a _Purple_, which though it be a lovely colour, and pretty acceptable to the eye, yet is it nothing comparable to the ravishing pleasure with which a curious and well tempered _Green_ affects the eye. If removing the Paper, the eye be plac’d against cd, it will perceive the lower side of the Sun (or a Candle at night which is much better, because it offends not the eye, and is more easily manageable) to be of a deep _Red_, and if against ef it will perceive the upper part of the luminous body to be of a deep _Blue_; and these colours will appear deeper and deeper, according as the Rays from the luminous body fall more _obliquely_ on the surface of the Water, and thereby suffer a greater refraction, and the more distinct, the further cdef is removed from the trajecting hole.

So that upon the whole, we shall find that the reason of the _Phænomena_ seems to depend upon the _obliquity_ of the _orbicular pulse_, to the Lines of Radiation, and in particular, that the Ray cd which constitutes the _Scarlet_ has its inner parts, namely those which are next to the middle of the luminous body, precedent to the outermost which are contiguous to the dark and _unradiating_ skie. And that the Ray ef which gives a _Blue_, has its outward part, namely, that which is contiguous to the dark skie precedent to the pulse from the innermost, which borders on the bright _area_ of the luminous body.

We may observe further, that the cause of the _diluting_ of the colours towards the middle, proceeds partly from the wideness of the hole through which the Rays pass, whereby the Rays from several parts of the luminous body, fall upon many of the same parts between c and f as is more manifest by the Figure: And partly also from the nature of the refraction it self, for the vividness or strength of the two terminating colours, arising chiefly as we have seen, from the very great difference that is betwixt the outsides of those _oblique undulations_ & the dark Rays circumambient, and that disparity betwixt the _approximate_ Rays, decaying gradually: the further inward toward the middle of the luminous body they are remov’d, the more must the colour approach to a white or an undisturbed light.

Upon the calculation of the refraction and reflection from a Ball of Water or Glass, we have much the same _Phænomena_, namely, an _obliquity_ of the undulation in the same manner as we have found it here. Which, because it is very much to our present purpose, and affords such an _Instancia crucis_, as no one that I know has hitherto taken notice of, I shall further examine. For it does very plainly and positively distinguish, and shew, which of the two _Hypotheses_, either the _Cartesian_ or this is to be followed, by affording a generation of all the colors in the Rainbow, where according to the _Cartesian Principles_ there should be none at all generated. And secondly, by affording an instance that does more closely confine the cause of these _Phænomena_ of colours to this present _Hypothesis_.

And first, for the _Cartesian_, we have this to object against it, That whereas he says (_Meteorum Cap. 8. Sect. 5._) _Sed judicabam unicam (refractione scilicet) ad minimum requiri, & quidem talem ut ejus effectus aliâ contrariâ (refractione) non destruatur: Nam experientia docet si superficies _NM_ & _NP_ (nempe refringentes) Parallelæ forent, radios tantundem per alteram iterum erectos quantum per unam frangerentur, nullos colores depicturos_; This Principle of his holds true indeed in a prisme where the refracting surfaces are plain, but is contradicted by the Ball or Cylinder, whether of Water or Glass, where the refracting surfaces are Orbicular or Cylindrical. For if we examine the passage of any _Globule_ or Ray of the primary _Iris_, we shall find it to pass out of the Ball or Cylinder again, with the same inclination and refraction that it enter’d in withall, and that that last refraction by means of the _intermediate_ reflection shall be the same as if without any reflection at all the Ray had been twice refracted by two Parallel surfaces.

And that this is true, not onely in one, but in every Ray that goes to the constitution of the Primary Iris; nay, in every Ray, that suffers only two refractions, and one reflection, by the surface of the round body, we shall presently see most evident, if we repeat the _Cartesian Scheme_, mentioned in the tenth _Section_ of the eighth _Chapter_ of his _Meteors_, where EFKNP in the third Figure[9] is one of the Rays of the Primary Iris, twice refracted at F and N, and once reflected at K by the surface of the Water-ball. For, first it is evident, that KF and KN are equal, because KN being the reflected part of KF they have both the same inclination on the surface K that is the angles FKT, and NKV made by the two Rays and the Tangent of K are equal, which is evident by the Laws of reflection; whence it will follow also, that KN has the same inclination on the surface N, or the Tangent of it XN that the Ray KF has to the surface F, or the Tangent of it FY, whence it must necessarily follow, that the refractions at F and N are equal, that is, KFE and KNP are equal. Now, that the surface N is by the reflection at K made parallel to the surface at F, is evident from the principles of reflection; for reflection being nothing but an inverting of the Rays, if we re-invert the Ray KNP, and make the same inclinations below the line TKV that it has above, it will be most evident, that KH the inverse of KN will be the continuation of the line FK, and that LHI the inverse of OX is parallel to FY. And HM the inverse of NP is Parallel to EF for the angle KHI is equal to KNO which is equal to KFY, and the angle KHM is equal to KNP which is equal to KFE which was to be prov’d.

So that according to the above mentioned _Cartesian_ principles there should be generated no colour at all in a Ball of Water or Glass by two refractions and one reflection, which does hold most true indeed, if the surfaces be plain, as may be experimented with any kind of prisme where the two refracting surfaces are equally inclin’d to the reflecting; but in this the _Phænomena_ are quite otherwise.

The cause therefore of the generation of colour must not be what _Des Cartes_ assigns, namely, a certain _rotation_ of the _Globuli ætherei_, which are the particles which he supposes to constitute the _Pellucid medium_, But somewhat else, perhaps what we have lately supposed, and shall by and by further prosecute and explain.

But, First I shall crave leave to propound some other difficulties of his, notwithstanding exceedingly ingenious _Hypothesis_, which I plainly confess to me seem such; and those are,

First, if that light be (as is affirmed, _Diopt._ cap. 1. §. 8.) not so properly a motion, as an action or propension to motion, I cannot conceive how the eye can come to be sensible of the _verticity_ of a _Globule_, which is generated in a drop of Rain, perhaps a mile off from it. For that _Globule_ is not carry’d to the eye according to his formerly recited Principle; and if not so, I cannot conceive how it can communicate its _rotation_, or circular motion to the line of the _Globules_ between the drop and the eye. It cannot be by means of every ones turning the next before him; for if so, then onely all the _Globules_ that are in the odd places must be turned the same way with the first, namely, the 3. 5. 7. 9. 11, &c. but all the _Globules_ interposited between them in the even places; namely, the 2. 4. 6. 8. 10. &c. must be the quite contrary, whence, according to the _Cartesian Hypothesis_, there must be no distinct colour generated, but a confusion. Next, since the _Cartesian Globuli_ are suppos’d (_Principiorum Philosoph._ Part. 3. §. 86.) to be each of them continually in motion about their centers, I cannot conceive how the eye is able to distinguish this new generated motion from their former inherent one, if I may so call that other wherewith they are mov’d or _turbinated_, from some other cause than refraction. And thirdly, I cannot conceive how these motions should not happen sometimes to oppose each other, and then, in stead of a _rotation_, there would be nothing but a direct motion generated, and consequently no colour. And fourthly, I cannot conceive, how by the _Cartesian Hypothesis_ it is possible to give any plausible reason of the nature of the Colours generated in the thin _laminæ_ of these our _Microscopical Observations_; for in many of these, the refracting and reflecting surfaces are parallel to each other, and consequently no _rotation_ can be generated, nor is there any necessity of a shadow or termination of the bright Rays, such as is suppos’d (_Chap._ 8. §. 5. _Et præterea observavi umbram quoque, aut limitationem luminis requiri:_ and _Chap._ 8. §. 9.) to be necessary to the generation of any distinct colours; Besides that, here is oftentimes one colour generated without any of the other appendant ones, which cannot be by the _Cartesian Hypothesis_.

There must be therefore some other propriety of refraction that causes colour. And upon the examination of the thing, I cannot conceive any one more general, inseparable, and sufficient, than that which I have before assign’d. That we may therefore see how exactly our _Hypothesis_ agrees also with the _Phænomena_ of the refracting round body, whether _Globe_ or _Cylinder_, we shall next subjoyn our _Calculation_ or _Examen_ of it.

And to this end, we will calculate any two Rays: as for instance;[10] let EF be a Ray cutting the _Radius_ CD (divided into 20. parts) in G 16. parts distant from C, and ef another Ray, which cuts the same _Radius_ in g 17. parts distant, these will be refracted to K and k, and from thence reflected to N and n, and from thence refracted toward P and p; therefore the Arch Ff will be 5.d 5′. The Arch FK 106.d 30′. the Arch fk 101.d 2′. The line FG 6000. and fg 5267. therefore hf. 733. therefore Fc 980, almost. The line FK 16024. and fk 15436. therefore Nd 196. and no 147 almost, the line Nn 1019 the Arch Nn 5.d 51′. therefore the Angle Nno is 34.d 43′. therefore the Angle Non is 139.d 56′. which is almost 50.d more than a right Angle.

It is evident therefore by this _Hypothesis_, that at the same time that ef touches f. EF is arrived at c. And by that time efkn is got to n, EFKN is got to d and when it touches N, the pulse of the other Ray is got to o. and no farther, which is very short of the place it should have arriv’d to, to make the Ray np to cut the _orbicular pulse_ No at right Angles: therefore the Angle Nop is an acute Angle, but the quite contrary of this will happen, if 17. and 18. be calculated in stead of 16. and 17. both which does most exactly agree with the _Phænomena_: For if the Sun, or a Candle (which is better) be placed about Ee, and the eye about Pp, the Rays EFef at 16. and 17. will paint the side of the luminous object toward np _Blue_, and towards NP _Red_. But the quite contrary will happen when EF is 17. and ef 18. for then towards NP shall be a _Blue_, and towards np a _Red_, exactly according to the calculation. And there appears the _Blue_ of the Rainbow, where the two _Blue_ sides of the two Images unite, and there the _Red_ where the two _Red_ sides unite, that is, where the two Images are just disappearing; which is, when the Rays EF and NP produc’d till they meet, make an Angle of about 41. and an half; the like union is there of the two Images in the Production of the _Secundary Iris_, and the same causes, as upon calculation may appear; onely with this difference, that it is somewhat more faint, by reason of the duplicate reflection, which does always weaken the impulse the oftner it is repeated.

Now, though the second refraction made at Nn be convenient, that is, do make the Rays glance the more, yet is it not altogether requisite; for it is plain from the calculation, that the pulse dn is sufficiently _oblique_ to the Rays KN and kn, as wel as the pulse fc is _oblique_ to the Rays FK & fk. And therefore if a piece of very fine Paper be held close against Nn and the eye look on it either through the Ball as from D, or from the other side, as from B. there shall appear a Rainbow, or colour’d line painted on it with the part toward X appearing _Red_, towards O, _Blue_; the same also shall happen, if the Paper be placed about Kk, for towards T shall appear a _Red_, and towards V a _Blue_, which does exactly agree with this my _Hypothesis_, as upon the calculation of the progress of the pulse will most easily appear.

Nor do these two observations of the colours appearing to the eye about p differing from what they appear on the Paper at N contradict each other; but rather confirm and exactly agree with one another, as will be evident to him that examines the reasons set down by the ingenious. _Des Cartes_ in the 12. _Sect._ of the 8. _Chapter of his Meteors_, where he gives the true reason why the colours appear of a quite contrary order to the eye, to what they appear’d on the Paper if the eye be plac’d in steed of the Paper: And as in the Prisme, so also in the Water-drop, or Globe the _Phænomena_, and reason are much the same.

Having therefore shewn that there is such a propriety in the _prisme_ and water _Globule_ whereby the pulse is made _oblique_ to the progressive, and that so much the more, by how much greater the refraction is, I shall in the next place consider, how this conduces to the production of colours, and what kind of impression it makes upon the bottom of the eye; and to this end it will be requisite to examine this _Hypothesis_ a little more particularly.

First therefore, if we consider the manner of the progress of the pulse, it will seem rational to conclude, that that part or end of the pulse which precedes the other, must necessarily be somwhat more _obtunded_, or _impeded_ by the resistance of the transparent _medium_, than the other part or end of it which is subsequent, whose way is, as it were, prepared by the other; especially if the adjacent _medium_ be not in the same manner enlightned or agitated. And therefore (in the fourth _Figure_ of the sixth _Iconism_) the Ray AAAHB will have its side HH more deadened by the resistance of the dark or quiet _medium_ PPP, Whence there will be a kind of deadness superinduc’d on the side HHH, which will continually increase from B, and strike deeper and deeper into the Ray by the line BR; Whence all the parts of the triangle, RBHO will be of a dead _Blue_ colour, and so much the deeper, by how much the nearer they lie to the line BHH, which is most deaded or impeded, and so much the more _dilute_, by how much the nearer it approaches the line BR. Next on the other side of the Ray AAN, the end A of the pulse AH will be promoted, or made stronger, having its passage already prepar’d as ’twere by the other parts preceding, and so its impression wil be stronger; And because of its _obliquity_ to the Ray, there will be propagated a kind of faint motion into QQ the adjacent dark or quiet _medium_, which faint motion will spread further and further into QQ as the Ray is propagated further and further from A, namely, as far as the line MA, whence all the triangle MAN will be ting’d with a _Red_, and that _Red_ will be the deeper the nearer it approaches the line MA, and the _paler_ or _yellower_ the nearer it is the line NA. And if the Ray be continued, so that the lines AN and BR (which are the bounds of the _Red_ and _Blue diluted_) do meet and cross each other, there will be beyond that intersection generated all kinds of _Greens_.

Now, these being the proprieties of every single refracted Ray of light, it will be easie enough to consider what must be the result of very many such Rays collateral: As if we suppose infinite such Rays _interjacent_ between AKSB and ANOB, which are the terminating: For in this case the Ray AKSB will have its _Red_ triangle intire, as lying next to the dark or quiet _medium_, but the other side of it BS will have no _Blue_, because the _medium adjacent_ to it SBO, is mov’d or enlightned, and consequently that light does destroy the colour. So likewise will the Ray ANOB lose its _Red_, because the _adjacent medium_ is mov’d or enlightned, but the other side of the Ray that is _adjacent_ to the dark, namely, AHO will preserve its _Blue_ entire, and these Rays must be so far produc’d as till AN and BR cut each other, before there will be any _Green_ produc’d. From these Proprieties well consider’d, may be deduc’d the reasons of all the _Phænomena_ of the _prisme_, and of the _Globules_ or drops of Water which conduce to the production of the Rainbow.

Next for the impression they make on the _Retina_, we will further examine this _Hypothesis_: Suppose therefore ABCDEF, in the fifth _Figure_, to represent the Ball of the eye: on the _Cornea_ of which ABC two Rays GACH and KCAI (which are the terminating Rays of a luminous body) falling, are by the refraction thereof collected or _converg’d_ into two points at the bottom of the eye. Now, because these terminating Rays, and all the _intermediate_ ones which come from any part of the luminous body, are suppos’d by some sufficient refraction before they enter the eye, to have their pulses made _oblique_ to their progression, and consequently each Ray to have potentially _superinduc’d_ two proprieties, or colours, viz. a _Red_ on the one side, and a _Blue_ on the other, which notwithstanding are never actually manifest, but when this or that Ray has the one or the other side of it bordering on a dark or unmov’d _medium_, therefore as soon as these Rays are entred into the eye and so have one side of each of them bordering on a dark part of the humours of the eye, they will each of them actually exhibit some colour; therefore ADC the production of GACH will exhibit a _Blue_, because the side CD is _adjacent_ to the dark _medium_ CQDC, but nothing of a _Red_, because its side AD is _adjacent_ to the enlightned _medium_ ADFA: And all the Rays that from the points of the luminous body are collected on the parts of the _Retina_ between D and F shall have their _Blue_ so much the more _diluted_ by how much the farther these points of collection are distant from D towards F; and the Ray AFC the production of KCAI, will exhibit a _Red_, because the side AF is adjacent to the dark or quiet _medium_ of the eye APFA, but nothing of a _Blue_, because its side CF is _adjacent_ to the enlightned _medium_ CFDC, and all the Rays from the intermediate parts of the luminous body that are collected between F and D shall have their _Red_ so much the more diluted, by how much the farther they are distant from F towards D.

Now, because by the refraction in the _Cornea_, and some other parts of the eye, the sides of each Ray, which before were almost parallel, are made to _converge_ and meet in a point at the bottom of the eye, therefore that side of the _pulse_ which preceded before these refractions, shall first touch the _Retina_, and the other side last. And therefore according as this or that side, or end of the pulse shall be impeded, accordingly will the _impressions_ on the _Retina_ be varied; therefore by the Ray GACH refracted by the _Cornea_ to D there shall be on that point a stroke or impression confus’d, whose weakest end, namely, that by the line CD shall precede, and the stronger, namely, that by the line AD shall follow. And by the Ray KCAI refracted to F, there shall be on that part a confus’d stroke or impression, whose strongest part, namely, that by the line CF shal precede, and whose weakest or impeded, namely, that by the line AF shall follow, and all the intermediate points between F and D will receive impressions from the _converg’d_ Rays so much the more like the impressions on F and D by how much the nearer they approach that or this.

From the consideration of the proprieties of which impressions, we may collect these short definitions of Colours: That _Blue is an impression on the Retina of an oblique and confus’d pulse of light, whose weakest part precedes, and whose strongest follows._ And, that _Red is an impression on the Retina of an oblique and confus’d pulse of light, whose strongest part precedes, and whose weakest follows._

Which proprieties, as they have been already manifested, in the Prisme and falling drops of Rain, to be the causes of the colours there generated, may be easily found to be the efficients also of the colours appearing in thin _laminated_ transparent bodies; for the explication of which, all this has been premised.

And that this is so, a little closer examination of the _Phænomena_ and the _Figure_ of the body, by this _Hypothesis_ will make evident.

For first (as we have already observed) the _laminated_ body must be of a determinate thickness, that is, it must not be thinner then such a determinate quantity; for I have always observ’d, that neer the edges of those which are exceeding thin, the colours disappear, and the part grows white; nor must it be thicker then another determinate quantity; for I have likewise observ’d, that beyond such a thickness, no colours appear’d, but the Plate looked white, between which two determinate thicknesses were all the colour’d Rings; of which in some substances I have found ten or twelve, in others not half so many, which I suppose depends much upon the transparency of the _laminated_ body. Thus though the consecutions are the same in the scum or the skin on the top of metals; yet in those consecutions in the same colour is not so often repeated as in the consecutions in thin Glass, or in Sope-water, or any other more transparent and glutinous liquor; for in these I have observ’d, _Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, &c._ to succeed each other, ten or twelve times, but in the other more _opacous_ bodies the consecutions will not be half so many.

And therefore secondly, the _laminated_ body must be transparent, and this I argue from this, that I have not been able to produce any colour at all with an _opacous_ body, though never so thin. And this I have often try’d, by pressing small _Globule_ of _Mercury_ between two smooth Plates of Glass, whereby I have reduc’d that body to a much greater thinness then was requisite to exhibit the colours with a transparent body.

Thirdly, there must be a considerable reflecting body adjacent to the under or further side of the _lamina_ or _plate_: for this I always found, that the greater that reflection was, the more vivid were the appearing colours.

From which Observations, is most evident, that the reflection from the under or further side of the body is the principal cause of the production of these colours; which, that it is so, and how it conduces to that effect, I shall further explain in the following Figure, which is here described of a very great thickness, as if it had been view’d through the _Microscope_; and ’tis indeed much thicker than any _Microscope_ (I have yet us’d) has been able to shew me those colour’d plates of Glass, or _Muscovie-glass_, which I have not without much trouble view’d with it, for though I have endeavoured to magnifie them as much as the Glasses were capable of, yet are they so exceeding thin, that I have not hitherto been able positively to determine their thickness. This Figure therefore I here represent, is wholy _Hypothetical_.

Let ABCDHFE in the sixth Figure be a _frustum_ of _Muscovy-glass_, thinner toward the end AE, and thicker towards DF. Let us first suppose the Ray aghb coming from the Sun, of some remote luminous object to fall _obliquely_ on the thinner plate BAE, part therefore is reflected back by cghd, the first _Superficies_; whereby the perpendicular pulse ab is after reflexion propagated by cd, cd, equally remote from each other with ab, ab, so that ag + gc, or bh + hd are either of them equal to aa, as is also cc, but the body BAE being transparent, a part of the light of this Ray is refracted in the surface AB, and propagated by gikh to the surface EF, whence it is reflected and refracted again by the surface AB. So that after two refractions and one reflection, there is propagated a kind of fainter Ray emnf, whose pulse is not only weaker by reason of the two refractions in the surface AB, but by reason of the time spent in passing and repassing between the two surfaces AB and EF, ef which is this fainter or weaker pulse comes behind the pulse cd; so that hereby (the surfaces AB, and EF being so neer together, that the eye cannot _discriminate_ them from one) this confus’d or _duplicated_ pulse, whose strongest part precedes, and whose weakest follows, does produce on the _Retina_, (or the _optick nerve_ that covers the bottom of the eye) the sensation of a _Yellow_.

And secondly, this _Yellow_ will appear so much the deeper, by how much the further back towards the middle between cd and cd the spurious pulse ef is remov’d, as in 2 where the surface BC being further remov’d from EF, the weaker pulse ef will be nearer to the middle, and will make an impression on the eye of a _Red_.

But thirdly, if the two reflecting surfaces be yet further remov’d asunder (as in 3 CD and EF are) then will the weaker pulse be so farr behind, that it will be more then half the distance between cd and cd. And in this case it will rather seem to precede the following stronger pulse, then to follow the preceding one, and consequently a _Blue_ will be generated. And when the weaker pulse is just in the middle between two strong ones, then is a deep and lovely _Purple_ generated; but when the weaker pulse ef is very neer to cd, then is there generated a _Green_, which will be _bluer_, or _yellower_, according as the _approximate_ weak pulse does precede or follow the stronger.

Now fourthly, if the thicker Plate chance to be cleft into two thinner Plates, as CDFE is divided into two Plates by the surface GH then from the composition arising from the three reflections in the surfaces CD, GH, and EF, there will be generated several compounded or mixt colours, which will be very differing, according as the proportion between the thicknesses of those two divided Plates CDHG, and GHFE are varied.

And _fifthly_, if these surfaces CD and FE are further remov’d asunder, the weaker pulse will yet lagg behind much further, and not onely be _coincident_ with the second, cd, but lagg behind that also, and that so much the more, by how much the thicker the Plate be; so that by degrees it will be _coincident_ with the third cd backward also, and by degrees, as the Plate grows thicker with a fourth, and so onward to a fifth, sixth, seventh, or eighth; so that if there be a thin transparent body, that from the greatest thinness requisite to produce colours, does, in the manner of a Wedge, by degrees grow to the greatest thickness that a Plate can be of, to exhibit a colour by the reflection of Light from such a body, there shall be generated several consecutions of colours, whose order from the thin end towards the thick, shall be _Yellow, Red, Purple, Blue, Green; Yellow, Red, Purple, Blue, Green; Yellow, Red, Purple, Blue, Green; Yellow_, &c. and these so often repeated, as the weaker pulse does lose paces with its _Primary_, or first pulse, and is _coincident_ with a second, third, fourth, fifth, sixth, &c. pulse behind the first. And this, as it is _coincident_, or follows from the first _Hypothesis_ I took of colours, so upon experiment have I found it in multitudes of instances that seem to prove it. One thing which seems of the greatest concern in this _Hypothesis_, is to determine the greatest or least thickness requisite for these effects, which, though I have not been wanting in attempting, yet so exceeding thin are these coloured Plates, and so imperfect our _Microscope_, that I have not been hitherto successfull, though if my endeavours shall answer my expectations, I shall hope to gratifie the curious Reader with some things more remov’d beyond our reach hitherto.

Thus have I, with as much brevity as I was able, endeavoured to explicate (_Hypothetically_ at least) the causes of the _Phænomena_ I formerly recited, on the consideration of which I have been the more particular.

First, because I think these I have newly given are capable of explicating all the _Phænomena_ of colours, not onely of those appearing in the _Prisme_, Water-drop, or Rainbow, and in _laminated_ or plated bodies, but of all that are in the world, whether they be fluid or solid bodies, whether in thick or thin, whether transparent, or seemingly opacous, as I shall in the next Observation further endeavour to shew. And secondly, because this being one of the two ornaments of all bodies discoverable by the sight, whether looked on with, or without a _Microscope_, it seem’d to deserve (somewhere in this Tract, which contains a description of the Figure and Colour of some minute bodies) to be somewhat the more intimately enquir’d into.

* * * * *

Observ. X. _Of _Metalline_, and other real Colours._

Having in the former Discourse, from the Fundamental cause of Colour, made it probable, that there are but two Colours, and shewn, that the _Phantasm_ of Colour is caus’d by the sensation of the _oblique_ or uneven pulse of Light which is capable of no more varieties than two that arise from the two sides of the _oblique_ pulse, though each of those be capable of infinite gradations or degrees (each of them beginning from _White_, and ending the one in the deepest _Scarlet_ or _Yellow_, the other in the deepest _Blue_) I shall in this _Section_ set down some Observations which I have made of other colours, such as _Metalline_ powders tinging or colour’d bodies and several kinds of tinctures or ting’d liquors, all which, together with those I treated of in the former Observation will, I suppose, comprise the several subjects in which colour is observ’d to be inherent, and the several manners by which it _inheres_, or is apparent in them. And here I shall endeavour to shew by what composition all kind of compound colours are made, and how there is no colour in the world but may be made from the various degrees of these two colours, together with the intermixtures of _Black_ and _White_.

And this being so, as I shall anon shew, it seems an evident argument to me, that all colours whatsoever, whether in fluid or solid, whether in very transparent or seemingly _opacous_, have the same efficient cause, to wit, some kind of _refraction_ whereby the Rays that proceed from such bodies, have their pulse _obliquated_ or confus’d in the manner I explicated in the former _Section_; that is, a _Red_ is caus’d by a duplicated or confus’d pulse, whose strongest pulse precedes, and a weaker follows: and a _Blue_ is caus’d by a confus’d pulse, where the weaker pulse precedes, and the stronger follows. And according as these are, more or less, or variously mixt and compounded, so are the _sensations_, and consequently the _phantasms_ of colours _diversified_.

To proceed therefore; I suppose, that all transparent colour’d bodies, whether fluid or solid, do consist at least of two parts, or two kinds of substances, the one of a substance of a somewhat differing _refraction_ from the other. That one of these substances which may be call’d the _tinging_ substance, does consist of distinct parts, or particles of a determinate bigness which are _disseminated_, or dispers’d all over the other: That these particles, if the body be equally and uniformly colour’d, are evenly rang’d and dispers’d over the other contiguous body; That where the body is deepest ting’d, there these particles are rang’d thickest, and where ’tis but faintly ting’d, they are rang’d much thinner, but uniformly. That by the mixture of another body that unites with either of these, which has a differing refraction from either of the other, quite differing effects will be produc’d, that is, the _consecutions_ of the confus’d pulses will be much of another kind, and consequently produce other _sensations_ and _phantasms_ of colours, and from a _Red_ may turn to a _Blue_, or from a _Blue_ to a _Red_, &c.

Now, that this may be the better understood, I shall endeavour to explain my meaning a little more sensible by a _Scheme_: Suppose we therefore in the seventh _Figure_ of the sixth _Scheme_, that ABCD represents a Vessel holding a ting’d liquor, let IIIII, &c. be the clear liquor, and let the tinging body that is mixt with it be EE, &c. FF, &c. GG, &c. HH, &c. whose particles (whether round, or some other determinate Figure is little to our purpose) are first of a determinate and equal bulk. Next, they are rang’d into the form of _Quincunx_, or _Equilaterotriangular_ order, which that probably they are so, and why they are so, I shall elsewhere endeavour to shew. Thirdly, they are of such a nature, as does either more easily or more difficultly transmit the Rays of light then the liquor; if more easily, a _Blue_ is generated, and if more difficultly, a _Red_ or _Scarlet_.

And first, let us suppose the tinging particles to be of a substance that does more _impede_ the Rays of light, we shall find that the pulse or wave of light mov’d from AD to BC, will proceed on, through the containing _medium_ by the pulses or waves KK, LL, MM, NN, OO; but because several of these Rays that go to the constitution of these pulses will be slugged or stopped by the tinging particles E, F, G, H; therefore there shall be _secundary_ and weak pulse that shall follow the Ray, namely PP which will be the weaker: first, because it has suffer’d many refractions in the impeding body; next, for that the Rays will be a little dispers’d or confus’d by reason of the refraction in each of the particles, whether _round_ or _angular_; and this will be more evident, if we a little more closely examine any one particular tinging _Globule_.

Suppose we therefore AB in the eighth _Figure_ of the sixth _Scheme_, to represent a tinging _Globule_ or particle which has a greater refraction than the liquor in which it is contain’d: Let CD be a part of the pulse of light which is _propagated_ through the containing _medium_; this pulse will be a little stopt or impeded by the _Globule_, and so by that time the pulse is past to EF that part of it which has been impeded by passing through the _Globule_, will get but to LM, and so that pulse which has been _propagated_ through the _Globule_, to wit, LM, NO, PQ, will always come behind the pulses EF, GH, IK, &c.

Next, by reason of the greater impediment in AB, and its _Globular_ Figure, the Rays that pass through it will be dispers’d, and very much scatter’d. Whence CA and DB which before went _direct_ and _parallel_, will after the refraction in AB, _diverge_ and spread by AP, and BQ; so that as the Rays do meet with more and more of these tinging particles in their way, by so much the more will the pulse of light further lagg behind the clearer pulse, or that which has fewer refractions, and thence the deeper will the colour be, and the fainter the light that is trajected through it; for not onely many Rays are reflected from the surfaces of AB, but those Rays that get through it are very much disordered.

By this _Hypothesis_ there is no one experiment of colour that I have yet met with, but may be, I conceive, very rationably solv’d, and perhaps, had I time to examine several particulars requisite to the demonstration of it, I might prove it more than probable, for all the experiments about the changes and mixings of colours related in the Treatise of Colours, published by the _Incomparable_ Mr. _Boyle_, and multitudes of others which I have observ’d, do so easily and naturally flow from those principles, that I am very apt to think it probable, that they own their production to no other _secundary_ cause: As to instance in two or three experiments. In the twentieth Experiment, this _Noble Authour_ has shewn that the deep _bluish purple-colour_ of _Violets_, may be turn’d into a _Green_, by _Alcalizate Salts_, and to a _Red_ by acid; that is, a _Purple_ consists of two colours, a deep _Red_, and a deep _Blue_; when the _Blue_ is diluted, or altered, or destroy’d by _acid Salts_, the _Red_ becomes predominant, but when the _Red_ is diluted by _Alcalizate_, and the _Blue_ heightned, there is generated a _Green_; for of a _Red_ diluted, is made a _Yellow_, and _Yellow_ and _Blue_ make a _Green_.

Now, because the _spurious_ pulses which cause a _Red_ and a _Blue_, do the one follow the clear pulse, and the other precede it, it usually follows, that those _Saline_ refracting bodies which do _dilute_ the colour of the one, do deepen that of the other. And this will be made manifest by almost all kinds of _Purples_, and many sorts of _Greens_, both these colours consisting of mixt colours; for if we suppose A and A in the ninth Figure, to represent two pulses of clear light, which follow each other at a convenient distance, AA, each of which has a _spurious_ pulse preceding it, as BB, which makes a _Blue_, and another following it, as CC, which makes a _Red_, the one caus’d by tinging particles that have a greater refraction, the other by others that have a less refracting quality then the liquor or _Menstruum_ in which these are dissolv’d, whatsoever liquor does so alter the refraction of the one, without altering that of the other part of the ting’d liquor, must needs very much alter the colour of the liquor; for if the refraction of the _dissolvent_ be increas’d, and the refraction of the tinging particles not altered, then will the preceding _spurious_ pulse be shortned or stopt, and not out-run the clear pulse so much; so that BB will become EE, and the _Blue_ be _diluted_, whereas the other _spurious_ pulse which follows will be made to lagg much more, and be further behind AA than before, and CC will become _ff_, and so the _Yellow_ or _Red_ will be heightned.

A _Saline_ liquor therefore, mixt with another ting’d liquor, may alter the colour of it several ways, either by altering the refraction of the liquor in which the colour swims: or secondly by varying the refraction of the coloured particles, by uniting more intimately either with some particular _corpuscles_ of the tinging body, or with all of them, according as it has a _congruity_ to some more especially, or to all alike: or thirdly, by uniting and interweaving it self with some other body that is already joyn’d with the tinging particles, with which substance it may have a _congruity_, though it have very little with the particles themselves: or fourthly, it may alter the colour of a ting’d liquor by dis-joyning certain particles which were before united with the tinging particles, which though they were somewhat _congruous_ to these particles, have yet a greater _congruity_ with the newly _infus’d Saline menstruum_. It may likewise alter the colour by further dissolving the tinging substance into smaller and smaller _particles_, and so _diluting_ the colour; or by uniting several _particles_ together as in precipitations, and so deepning it, and some such other ways, which many experiments and comparisons of differing trials together, might easily inform one of.

From these Principles applied, may be made out all the varieties of colours observable, either in liquors, or any other ting’d bodies, with great ease, and I hope intelligible enough, there being nothing in the _notion_ of colour, or in the suppos’d production, but is very conceivable, and may be possible.

The greatest difficulty that I find against this _Hypothesis_, is, that there seem to be more distinct colours then two, that is, then Yellow and Blue. This Objection is grounded on this reason, that there are several Reds, which _diluted_, make not a Saffron or pale Yellow, and therefore Red, or Scarlet seems to be a third colour distinct from a deep degree of Yellow.

To which I answer, that Saffron affords us a deep Scarlet tincture, which may be _diluted_ into as pale a Yellow as any, either by making a weak solution of the Saffron, by infusing a small parcel of it into a great quantity of liquor, as in spirit of Wine, or else by looking through a very thin quantity of the tincture, and which may be heightn’d into the loveliest Scarlet, by looking through a very thick body of this tincture, or through a thinner parcel of it, which is highly _impregnated_ with the tinging body, by having had a greater quantity of the Saffron dissolv’d in a smaller parcel of the liquor.

Now, though there may be some particles of other tinging bodies that give a lovely Scarlet also, which though _diluted_ never so much with liquor, or looked on through never so thin a parcel of ting’d liquor, will not yet afford a pale Yellow, but onely a kind of faint Red; yet this is no argument but that those ting’d particles may have in them the faintest degree of Yellow, though we may be unable to make them exhibit it; For that power of being _diluted_ depending upon the divisibility of the ting’d body, if I am unable to make the tinging particles so thin as to exhibit that colour, it does not therefore follow, that the thing is impossible to be done; now, the tinging particles of some bodies are of such a nature, that unless there be found some way of comminuting them into less bulks then the liquor does dissolve them into, all the Rays that pass through them must necessarily receive a tincture so deep, as their appropriate refractions and bulks compar’d with the proprieties of the dissolving liquor must necessarily dispose them to empress, which may perhaps be a pretty deep Yellow, or pale Red.

And that this is not _gratis dictum_, I shall add one instance of this kind, wherein the thing is most manifest.

If you take Blue _Smalt_, you shall find, that to afford the deepest Blue, which _cæteris paribus_ has the greatest particles or sands; and if you further divide, or grind those particles on a Grindstone, or _porphyry_ stone, you may by _comminuting_ the sands of it, _dilute_ the Blue into as pale a one as you please, which you cannot do by laying the colour thin; for wheresoever any single particle is, it exhibits as deep a Blue as the whole mass. Now, there are other Blues, which though never so much ground, will not be _diluted_ by grinding, because consisting of very small particles, very deeply ting’d, they cannot by grinding be actually separated into smaller particles then the operation of the fire, or some other dissolving _menstruum_, reduc’d them to already.

Thus all kind of _Metalline_ colours, whether _precipitated_, _sublim’d_, _calcin’d_, or otherwise prepar’d, are hardly chang’d by grinding, as _ultra marine_ is not more _diluted_; nor is _Vermilion_ or _Red-lead_ made of a more faint colour by grinding; for the smallest particles of these which I have view’d with my greatest Magnifying-Glass, if they be well enlightned, appear very deeply ting’d with their peculiar colours; nor, though I have magnified and enlightned the particles exceedingly, could I in many of them, perceive them to be transparent, or to be whole particles, but the smallest specks that I could find among well ground _Vermilion_ and _Red-lead_, seem’d to be a Red mass, compounded of a multitude of less and less motes, which sticking together, compos’d a bulk, not one thousand thousandth part of the smallest visible sand or mote.

And this I find generally in most _Metalline_ colours, that though they consist of parts so exceedingly small, yet are they very deeply ting’d, they being so ponderous, and having such a multitude of terrestrial particles throng’d into a little room; so that ’tis difficult to find any particle transparent or resembling a pretious stone, though not impossible; for I have observ’d divers such shining and resplendent colours intermixt with the particles of _Cinnaber_, both natural and artificial, before it hath been ground and broken or flaw’d into _Vermilion_: As I have also in _Orpiment_, _Red-lead_, and _Bise_, which makes me suppose, that those _metalline_ colours are by grinding, not onely broken and separated actually into smaller pieces, but that they are also flaw’d and brused, whence they, for the most part, become _opacous_, like flaw’d Crystal or Glass, &c. But for _Smalts_ and _verditures_, I have been able with a _Microscope_ to perceive their particles very many of them transparent.

Now, that the others also may be transparent, though they do not appear so to the _Microscope_, may be made probable by this Experiment: that if you take _ammel_ that is almost _opacous_, and grind it very well on a _Porphyry_, or _Serpentine_, the small particles will by reason of their flaws, appear perfectly _opacous_; and that ’tis the flaws that produce this _opacousness_, may be argued from this, that particles of the same _Ammel_ much thicker if unflaw’d will appear somewhat transparent even to the eye; and from this also, that the most transparent and clear Crystal, if heated in the fire, and then suddenly quenched, so that it be all over flaw’d, will appear _opacous_ and white.

And that the particles of _Metalline_ colours are transparent, may be argued yet further from this, that the Crystals, or _Vitriols_ of all Metals, are transparent, which since they consist of _metalline_ as well as _saline_ particles, those _metalline_ ones must be transparent, which is yet further confirm’d from this, that they have for the most part, _appropriate_ colours; so the _vitriol_ of Gold is Yellow; of Copper, Blue, and sometimes Green; of Iron, green; of Tinn and Lead, a pale White; of Silver, a pale Blue, _&._

And next, the _Solution_ of all Metals into _menstruums_ are much the same with the _Vitriols_, or Crystals. It seems therefore very probable, that those colours which are made by the _precipitation_ of those particles out of the _menstruums_ by transparent _precipitating_ liquors should be transparent also. Thus Gold _precipitates_ with _oyl of Tartar_, or _spirit of Urine_ into a brown Yellow, Copper with spirit of _Urine_ into a Mucous blue, which retains its transparency. A solution of sublimate (as the same Illustrious Authour I lately mention’d shews in his 40. Experiment) _precipitates_ with oyl of _Tartar_ _per deliquium_, into an Orange colour’d _precipitate_; nor is it less probable, that the _calcination_ of those _Vitriols_ by the fire, should have their particles transparent: Thus _Saccarum Saturni_, or the _Vitriol of Lead_ by _calcination_ becomes a deep Orange-colour’d _minium_, which is a kind of _precipitation_ by some Salt which proceeds from the fire; common _Vitriol_ _calcin’d_, yields a deep Brown Red, &c.

A third Argument, that the particles of Metals are transparent, is, that being _calcin’d_, and melted with Glass, they tinge the Glass with transparent colours. Thus the _Calx_ of Silver tinges the Glass on which it is anneal’d with a lovely Yellow, or Gold colour, &c.

And that the parts of Metals are transparent, may be farther argued from the transparency of Leaf-gold, which held against the light, both to the naked eye, and the _Microscope_, exhibits a deep Green. And though I have never seen the other Metals _laminated_ so thin, that I was able to perceive them transparent, yet, for Copper and Brass, if we had the same conveniency for _laminating_ them, as we have for Gold, we might, perhaps, through such plates or leaves, find very differing degrees of Blue, or Green; for it seems very probable, that those Rays that rebound from them ting’d, with a deep Yellow, or pale Red, as from Copper, or with a pale Yellow, as from Brass, have past through them; for I cannot conceive how by reflection alone those Rays can receive a tincture, taking any _Hypothesis_ extant.

So that we see there may a sufficient reason be drawn from these instances, why those colours which we are unable to _dilute_ to the palest Yellow, or Blue, or Green, are not therefore to be concluded not to be a deeper degree of them; for supposing we had a great company of small _Globular_ essence Bottles, or round Glass bubbles, about the bigness of a Walnut, fill’d each of them with a very deep mixture of Saffron, and that every one of them did appear of a deep Scarlet colour, and all of them together did _exhibit_ at a distance, a deep dy’d Scarlet body. It does not follow, because after we have come nearer to this _congeries_, or mass, and divided it into its parts, and examining each of its parts severally or apart, we find them to have much the same colour with the whole mats; it does not, I say, therefore follow, that if we could break those _Globules_ smaller, or any other ways come to see a smaller or thinner parcel of the ting’d liquor that fill’d those bubbles, that that ting’d liquor must always appear Red, or of a Scarlet hue, since if Experiment be made, the quite contrary will ensue; for it is capable of being _diluted_ into the palest Yellow.

Now, that I might avoid all the Objections of this kind, by exhibiting an Experiment that might by ocular proof convince those whom other reasons would not prevail with, I provided me a _Prismatical Glass_, made hollow, just in the form of a Wedge, such as is represented in the tenth _Figure_ of the sixth _Scheme_. The two _parallelogram_ sides ABCD, ABEF, which met at a point, were made of the clearest Looking-glass plates well ground and polish’d that I could get; these were joyn’d with hard cement to the _triangular_ sides, BCE, ADF, which were of Wood; the _Parallelogram_ base BCEF, likewise was of Wood joyn’d on to the rest with hard cement, and the whole _Prismatical_ Box was exactly stopt every where, but onely a little hole near the base was left, whereby the Vessel could be fill’d with any liquor, or emptied again at pleasure.

One of these Boxes (for I had two of them) I fill’d with a pretty deep tincture of _Aloes_, drawn onely with fair Water, and then stopt the hole with a piece of Wax, then, by holding this Wedge against the Light, and looking through it, it was obvious enough to see the tincture of the liquor near the edge of the Wedge where it was but very thin, to be a pale but well colour’d Yellow, and further and further from the edge, as the liquor grew thicker and thicker, this tincture appear’d deeper and deeper, so that near the blunt end, which was seven Inches from the edge and three Inches and an half thick; it was of a deep and well colour’d Red. Now, the clearer and purer this tincture be, the more lovely will the deep Scarlet be, and the fouler the tincture be, the more dirty will the Red appear; so that some dirty tinctures have afforded their deepest Red much of the colour of burnt Oker or _Spanish_ brown; others as lovely a colour as _Vermilion_, and some much brighter; but several others, according as the tinctures were worse or more foul, exhibited various kinds of Reds, of very differing degrees.

The other of these Wedges, I fill’d with a most lovely tincture of Copper, drawn from the filings of it, with spirit of _Urine_, and this Wedge held as the former against the Light, afforded all manner of Blues, from the faintest to the deepest, so that I was in good hope by these two, to have produc’d all the varieties of colours imaginable; for I thought by this means to have been able by placing the two _Parallelogram_ sides together, and the edges contrary ways, to have so mov’d them to and fro one by another, as by looking through them in several places, and through several thicknesses, I should have compounded, and consequently have seen all those colours, which by other like compositions of colours would have ensued.

But insteed of meeting with what I look’d for, I met with somewhat more admirable; and that was, that I found my self utterly unable to see through them when placed both together, though they were transparent enough when asunder; and though I could see through twice the thickness, when both of them were fill’d with the same colour’d liquors, whether both with the Yellow, or both with the Blue, yet when one was fill’d with the Yellow, the other with the Blue, and both looked through, they both appear’d dark, onely when the parts near the tops were look’d through, they exhibited Greens, and those of very great variety, as I expected, but the Purples and other colours, I could not by any means make, whether I endeavour’d to look through them both against the Sun, or whether I plac’d them against the hole of a darkned room.

But notwithstanding this mis-ghessing, I proceeded on with my trial in a dark room, and having two holes near one another, I was able, by placing my Wedges against them, to mix the ting’d Rays that past through them, and fell on a sheet of white Paper held at a convenient distance from them as I pleas’d; so that I could make the Paper appear of what colour I would, by varying the thicknesses of the Wedges, and consequently the tincture of the Rays that past through the two holes, and sometimes also by varying the Paper, that is, insteed of a white Paper, holding a gray, or a black piece of Paper.

Whence I experimentally found what I had before imagin’d, that all the varieties of colours imaginable are produc’d from several degrees of these two colours, namely, Yellow and Blue, or the mixture of them with light and darkness, that is, white and black. And all those almost infinite varieties which Limners and Painters are able to make by compounding those several colours they lay on their Shels or _Palads_, are nothing else, but some _compositum_, made up of some one or more, or all of these four.

Now, whereas it may here again be objected, that neither can the Reds be made out of the Yellows, added together, or laid on in greater or less quantity, nor can the Yellows be made out of the Reds though laid never so thin; and as for the addition of White or Black, they do nothing but either whiten or darken the colours to which they are added, and not at all make them of any other kind of colour: as for instance, _Vermilion_, by being temper’d with White Lead, does not at all grow more Yellow, but onely there is made a whiter kind of Red. Nor does Yellow _Oker_, though laid never so thick, produce the colour of _Vermilion_, nor though it be temper’d with Black, does it at all make a Red; nay, though it be temper’d with White, it will not afford a fainter kind of Yellow, such as _masticut_, but onely a whiten’d Yellow; nor will the Blues be _diluted_ or deepned after the manner I speak of, as _Indico_ will never afford so fine a Blue as _Ultramarine_ or _Bise_; nor will it, temper’d with _Vermilion_, ever afford a Green, though each of them be never so much temper’d with white.

To which I answer, that there is a great difference between _diluting_ a colour and whitening of it; for _diluting_ a colour, is to make the colour’d parts more thin, so that the ting’d light, which is made by trajecting those ting’d bodies, does not receive so deep a tincture; but whitening a colour is onely an intermixing of many clear reflections of light among the same ting’d parts; deepning also, and darkning or blacking a colour, are very different; for deepning a colour, is to make the light pass through a greater quantity of the same tinging body; and darkning or blacking a colour, is onely interposing a multitude of dark or black spots among the same ting’d parts, or placing the colour in a more faint light.

First therefore, as to the former of these operations, that is, diluting and deepning, most of the colours us’d by the Limners and Painters are incapable of, to wit, _Vermilion_ and _Red-lead_, and _Oker_, because the ting’d parts are so exceeding small, that the most curious Grindstones we have, are not able to separate them into parts actually divided so small as the ting’d particles are; for looking on the most curiously ground _Vermilion_, and _Oker_, and _Red-lead_, I could perceive that even those small _corpuscles_ of the bodies they left were compounded of many pieces, that is, they seem’d to be small pieces compounded of a multitude of lesser ting’d parts: each piece seeming almost like a piece of Red Glass, or ting’d Crystal all flaw’d; so that unless the Grindstone could actually divide them into smaller pieces then those flaw’d particles were, which compounded that ting’d mote I could see with my _Microscope_, it would be impossible to _dilute_ the colour by grinding, which, because the finest we have will not reach to do in _Vermilion_ or _Oker_, therefore they cannot at all, or very hardly be _diluted_.

Other colours indeed, whose ting’d particles are such as may be made smaller, by grinding their colour, may be _diluted_. Thus several of the Blues may be _diluted_, as _Smalt_ and _Bise_; and _Masticut_, which is Yellow, may be made more faint: And even _Vermilion_ it self may, by too much grinding, be brought to the colour of _Red-lead_, which is but an Orange colour, which is confest by all to be very much upon the Yellow. Now, though perhaps somewhat of this _diluting_ of _Vermilion_ by overmuch grinding may be attributed to the Grindstone, or muller, for that some of their parts may be worn off and mixt with the colour, yet there seems not very much, for I have done it on a Serpentine-stone with a muller made of a Pebble, and yet observ’d the same effect follow.

And secondly, as to the other of these operations on colours, that is, the deepning of them, Limners and Painters colours are for the most part also uncapable. For they being for the most part _opacous_; and that _opacousness_, as I said before, proceeding from the particles, being very much flaw’d, unless we were able to joyn and reunite those flaw’d particles again into one piece, we shall not be able to deepen the colour, which since we are unable to do with most of the colours which are by Painters accounted _opacous_, we are therefore unable to deepen them by adding more of the same kind.

But because all those _opacous_ colours have two kinds of beams or Rays reflected from them, that is, Rays unting’d, which are onely reflected from the outward surface, without at all penetrating of the body, and ting’d Rays which are reflected from the inward surfaces or flaws after they have suffer’d a two-fold refraction; and because that transparent liquors mixt with such _corpuscles_, do, for the most part, take off the former kind of reflection; therefore these colours mixt with Water or Oyl, appear much deeper than when dry, for most part of that white reflection from the outward surface is remov’d. Nay, some of these colours are very much deepned by the mixture with some transparent liquor, and that because they may perhaps get between those two flaws, and so consequently joyn two or more of those flaw’d pieces together; but this happens but in a very few.

Now, to shew that all this is not _gratis dictum_, I shall set down some Experiments which do manifest these things to be probable and likely, which I have here deliver’d.

For, first, if you take any ting’d liquor whatsoever, especially if it be pretty deeply ting’d, and by any means work it into a froth, the _congeries_ of that froth shall seem an _opacous_ body, and appear of the same colour, but much whiter than that of the liquor out of which it is made. For the abundance of reflections of the Rays against those surfaces of the bubbles of which the froth consists, does so often rebound the Rays backwards, that little or no light can pass through, and consequently the froth appears _opacous_.

Again, if to any of these ting’d liquors that will endure the boiling there be added a small quantity of fine flower (the parts of which through the _Microscope_ are plainly enough to be perceiv’d to consist of transparent _corpuscles_) and suffer’d to boyl till it thicken the liquor, the mass of the liquor will appear _opacous_, and ting’d with the same colour, but very much whiten’d.

Thus, if you take a piece of transparent Glass that is well colour’d, and by heating it, and then quenching it in Water, you flaw it all over, it will become _opacous_, and will exhibit the same colour with which the piece is ting’d, but fainter and whiter.

Or, if you take a Pipe of this transparent Glass, and in the flame of a Lamp melt it, and then blow it into very thin bubbles, then break those bubbles, and collect a good parcel of those _laminæ_ together in a Paper, you shall find that a small thickness of those Plates will constitute an _opacous_ body, and that you may see through the mass of Glass before it be thus _laminated_, above four times the thickness: And besides, they will now afford a colour by reflection as other _opacous_ (as they are call’d) colours will, but much fainter and whiter than that of the Lump or Pipe out of which they were made.

Thus also, if you take _Putty_, and melt it with any transparent colour’d Glass, it will make it become an _opacous_ colour’d lump, and to yield a paler and whiter colour than the lump by reflection.

The same thing may be done by a preparation of _Antimony_, as has been shewn by the Learned _Physician_, Dr. _C.M._ in his Excellent Observations and Notes on _Nery’s Art of Glass_; and by this means all transparent colours become _opacous_, or _ammels_. And though by being ground they lose very much of their colour, growing much whiter by reason of the multitude of single reflections from their outward surface, as I shew’d afore, yet the fire that in the nealing or melting re-unites them, and so renews those _spurious_ reflections, removes also those whitenings of the colour that proceed from them.

As for the other colours which Painters use, which are transparent, and us’d to varnish over all other paintings, ’tis well enough known that the laying on of them thinner or thicker, does very much _dilute_ or deepen their colour.

Painters Colours therefore consisting most of them of solid particles, so small that they cannot be either re-united into thicker particles by any Art yet known, and consequently cannot be deepned; or divided into particles so small as the flaw’d particles that exhibit that colour, much less into smaller, and consequently cannot be _diluted_; It is necessary that they which are to imitate all kinds of colours, should have as many degrees of each colour as can be procur’d.

And to this purpose, both Limners and Painters have a very great variety both of Yellows and Blues, besides several other colour’d bodies that exhibit very compounded colours, such as Greens and Purples; and others that are compounded of several degrees of Yellow, or several degrees of Blue, sometimes unmixt, and sometimes compounded with several other colour’d bodies.

The Yellows, from the palest to the deepest Red or Scarlet, which has no intermixture of Blue, are _pale and deep Masticut, Orpament, English Oker, brown Oker, Red-Lead, and Vermilion, burnt English Oker, and burnt brown Oker_, which last have a mixture of dark or dirty parts with them, &c.

Their Blues are several kinds of _Smalts_, and _Verditures_, and _Bise_, and _Ultramarine_, and _Indico_, which last has many dirty or dark parts intermixt with it.

Their compounded colour’d bodies, as _Pink_, and _Verdigrese_, which are Greens, the one a _Popingay_, the other a _Sea-green_; then _Lac_, which is a very lovely _Purple_.

To which may be added their Black and White, which they also usually call Colours, of each of which they have several kinds, such as _Bone Black_, made of _Ivory_ burnt in a close Vessel, and _Blue Black_, made of the small coal of _Willow_, or some other Wood; and _Cullens earth_, which is a kind of brown Black, &c. Their usual Whites are either artificial or natural _White Lead_, the last of which is the best they yet have, and with the mixing and tempering these colours together, are they able to make an imitation of any colour whatsoever: Their Reds or deep Yellows, they can _dilute_ by mixing pale Yellows with them, and deepen their pale by mixing deeper with them; for it is not with _Opacous_ colours as it is with transparent, where by adding more Yellow to yellow, it is deepned, but in _opacous_ _diluted_. They can whiten any colour by mixing White with it, and darken any colour by mixing Black, or some dark and dirty colour. And in a word, most of the colours, or colour’d bodies they use in Limning and Painting, are such, as though mixt with any other of their colours, they preserve their own hue, and by being in such very smal parts dispers’d through the other colour’d bodies, they both, or altogether represent to the eye a _compositum_ of all; the eye being unable, by reason of their smalness, to distinguish the peculiarly colour’d particles, but receives them as one intire _compositum_: whereas in many of these, the _Microscope_ very easily distinguishes each of the compounding colours distinct, and exhibiting its own colour.

Thus have I by gently mixing _Vermilion_ and _Bise_ dry, produc’d a very fine Purple, or mixt colour, but looking on it with the _Microscope_, I could easily distinguish both the Red and the Blue particles, which did not at all produce the _Phantasm_ of Purple.

To summ up all therefore in a word, I have not yet found any solid colour’d body, that I have yet examin’d, perfectly _opacous_; but those that are least transparent are _Metalline_ and _Mineral_ bodies, whose particles generally, seeming either to be very small, or very much flaw’d, appear for the most part _opacous_, though there are very few of them that I have look’d on with a _Microscope_, that have not very plainly or circumstantially manifested themselves transparent.

And indeed, there seem to be so few bodies in the world that are _in minimis_ opacous, that I think one may make it a rational _Query_, Whether there be any body absolutely thus _opacous_? For I doubt not at all (and I have taken notice of very many circumstances that make me of this mind) that could we very much improve the _Microscope_, we might be able to see all those bodies very plainly transparent, which we now are fain onely to ghess at by circumstances. Nay, the Object Glasses we yet make use of are such, that they make many transparent bodies to the eye, seem _opacous_ through them, which if we widen the Aperture a little, and cast more light on the objects, and not charge the Glasses so deep, will again disclose their transparency.

Now, as for all kinds of colours that are dissolvable in Water, or other liquors, there is nothing so manifest, as that all those ting’d liquors are transparent; and many of them are capable of being _diluted_ and compounded or mixt with other colours, and divers of them are capable of being very much chang’d and heightned, and fixt with several kinds of _Saline menstruums_. Others of them upon compounding, destroy or vitiate each others colours, and _precipitate_, or otherwise very much alter each others tincture. In the true ordering and _diluting_, and deepning, and mixing, and fixing of each of which, consists one of the greatest mysteries of the Dyers; of which particulars, because our _Microscope_ affords us very little information, I shall add nothing more at present; but onely that with a very few tinctures order’d and mixt after certain ways, too long to be here set down, I have been able to make an appearance of all the various colours imaginable, without at all using the help of _Salts_, or _Saline menstruums_ to vary them.

As for the mutation of Colours by _Saline menstruums_, they have already been so fully and excellently handled by the lately mention’d Incomparable _Authour_, that I can add nothing, but that of a multitude of trials that I made, I have found them exactly to agree with his Rules and Theories; and though there may be infinite instances, yet may they be reduc’d under a few Heads, and compris’d within a very few Rules. And generally I find, that _Saline menstruums_ are most operative upon those colours that are Purple, or have some degree of Purple in them, and upon the other colours much less. The _spurious_ pulses that compose which, being (as I formerly noted) so very neer the middle between the true ones, that a small variation throws them both to one side, or both to the other, and so consequently must make a vast mutation in the formerly appearing Colour.

* * * * *

Observ. XI. _Of _Figures_ observ’d in small Sand._

Sand generally seems to be nothing else but exceeding small Pebbles, or at least some very small parcels of a bigger stone; the whiter kind seems through the _Microscope_ to consist of small transparent pieces of some _pellucid_ body, each of them looking much like a piece of _Alum_, or _Salt Gem_; and this kind of Sand is angled for the most part irregularly, without any certain shape, and the _granules_ of it are for the most part flaw’d, through amongst many of them it is not difficult to find some that are perfectly _pellucid_, like a piece of clear Crystal, and divers likewise most curiously shap’d, much after the manner of the bigger _Stiriæ_ of Crystal, or like the small Diamants I observ’d in certain Flints, of which I shall by and by relate; which last particular seems to argue, that this kind of Sand is not made by the comminution of greater transparent Crystaline bodies, but by the _concretion_ or _coagulation_ of Water, or some other fluid body.

There are other kinds of courser Sands, which are browner, and have their particles much bigger; these, view’d with a _Microscope_, seem much courser and more _opacous_ substances, and most of them are of some irregularly rounded Figures; and though they seem not so _opacous_ as to the naked eye, yet they seem very foul and cloudy, but neither do these want curiously transparent, no more than they do regularly figur’d and well colour’d particles, as I have often found.

There are multitudes of other kinds of Sands, which in many particulars, plainly enough discoverable by the _Microscope_, differ both from these last mention’d kinds of Sands, and from one another: there seeming to be as great variety of Sands, as there is of Stones. And as amongst Stones some are call’d precious from their excellency, so also are there Sands which deserve the same Epithite for their beauty; for viewing a small parcel of _East-India_ Sand (which was given me by my highly honoured friend, Mr. _Daniel Colwall_) and, since that, another parcel, much of the same kind, I found several of them, both very transparent like precious Stones, and regularly figur’d like Crystal, _Cornish_ Diamants, some Rubies, &c. and also ting’d with very lively and deep colours, like _Rubys_, _Saphyrs_, _Emeralds_, &c. These kinds of granules I have often found also in _English_ Sand. And ’tis easie to make such a counterfeit Sand with deeply ting’d Glass, Enamels and Painters colours.

It were endless to describe the multitudes of Figures I have met with in these kind of minute bodies, such as _Spherical_, _Oval_, _Pyramidal_, _Conical_, _Prismatical_, of each of which kinds I have taken notice.

But amongst many others, I met with none more observable than this pretty Shell (described in the _Figure_ X. of the fifth _Scheme_) which, though as it was light on by chance, deserv’d to have been omitted (I being unable to direct any one to find the like) yet for its rarity was it not inconsiderable, especially upon the account of the information it may afford us. For by it we have a very good instance of the curiosity of Nature in another kind of Animals which are remov’d, by reason of their minuteness, beyond the reach of our eyes, so that as there are several sorts of Insects, as Mites, and others, so small as not yet to have had any names; (some of which I shall afterwards describe) and small Fishes, as Leeches in Vinegar; and smal vegetables, as Moss, and Rose-Leave-plants; and small Mushroms, as mould: so are there, it seems, small Shel-fish likewise, Nature shewing her curiosity in every Tribe of _Animals_, _Vegetables_, and _Minerals_.

I was trying several small and single Magnifying Glasses, and casually viewing a parcel of white Sand, when I perceiv’d one of the grains exactly shap’d and wreath’d like a Shell, but endeavouring to distinguish it with my naked eye, it was so very small, that I was fain again to make use of the Glass to find it; then, whilest I thus look’d on it, with a Pin I separated all the rest of the granules of Sand, and found it afterwards to appear to the naked eye an exceeding small white spot, no bigger than the point of a Pin. Afterwards I view’d it every way with a better _Microscope_ and found it on both sides, and edge-ways, to resemble the Shell of a small Water-Snail with a flat spiral Shell: it had twelve wreathings, a, b, c, d, e, &c. all very proportionably growing one less than another toward the middle or center of the Shell, where there was a very small round white spot. I could not certainly discover whether the Shell were hollow or not, but it seem’d fill’d with somewhat, and ’tis probable that it might be _petrify’d_ as other larger Shels often are, such as are mention’d in the seventeenth _Observation_.

* * * * *

Observ. XII. _Of _Gravel_ in Urine._

I Have often observ’d the Sand or Gravel of Urine, which seems to be a _tartareous_ substance, generated out of a _saline_ and a _terrestrial_ substance _crystalliz’d_ together, in the form of _Tartar_, sometimes sticking to the sides of the _Urinal_, but for the most part sinking to the bottom, and there lying in the form of coorse common Sand; these, through the _Microscope_, appear to be a company of small bodies, partly transparent and partly _opacous_, some White, some Yellow, some Red, others of more brown and duskie colours.

The Figure of them is for the most part flat, in the manner of Slats or such like plated Stones, that is, each of them seem to be made up of several other thinner Plates, much like _Muscovie Glass_, or _English Sparr_ to the last of which, the white plated Gravel seems most likely; for they seem not onely plated like that, but their sides shap’d also into _Rhombs_, _Rhomboeids_, and sometimes into _Rectangles_ and _Squares_. Their bigness and Figure may be seen in the second _Figure_ of the seventh _Plate_, which represents about a dozen of them lying upon a plate ABCD, some of which, as a, b, c, d seem’d more regular than the rest, and e, which was a small one, sticking on the top of another, was a perfect _Rhomboeid_ on the top, and had four _Rectangular_ sides.

The line E which was the measure of the _Microscope_, is ¹⁄₃₂ part of an _English_ Inch, so that the greatest bredth of any of them, exceeded not ¹⁄₁₂₈ part of an Inch.

Putting these into several liquors, I found _oyl of Vitriol_, _Spirit of Urine_, and several other _Saline menstruums_ to dissolve them; and the first of these in less than a minute without _Ebullition_, Water, and several other liquors, had no sudden operation upon them. This I mention, because those liquors that dissolve them, first make them very white, not _vitiating_, but rather rectifying their Figure, and thereby make them afford a very pretty object for the _Microscope_.

How great an advantage it would be to such as are troubled with the Stone, to find some _menstruum_ might dissolve them without hurting the Bladder, is easily imagin’d, since some _injections_ made of such bodies might likewise dissolve the stone, which seems much of the same nature.

It may therefore, perhaps, be worthy some Physicians enquiry, whether there may not be something mixt with the Urine in which the Gravel or Stone lies, which may again make it dissolve it, the first of which seems by it’s regular Figures to have been sometimes _Crystalliz’d_ out of it. For whether this _Crystallization_ be made in the manner as _Alum_, _Peter_, &c. are _crystallized_ out of a cooling liquor, in which, by boyling they have been dissolv’d; or whether it be made in the manner of _Tartarum Vitriolatum_, that is, by the _Coalition_ of an _acid_ and a _Sulphureous_ substance, it seems not impossible, but that the liquor it lies in, may be again made a _dissolvent_ of it. But leaving these inquiries to Physicians or Chymists, to whom it does more properly belong, I shall proceed.

* * * * *

Observ. XIII. _Of the small _Diamants_, or _Sparks_ in _Flints_._

Chancing to break a Flint stone in pieces, I found within it a certain cavity all crusted over with a very pretty candied substance, some of the parts of which, upon changing the posture of the Stone, in respect of the _Incident_ light, exhibited a number of small, but very vivid reflections; and having made use of my _Microscope_, I could perceive the whole surface of that cavity to be all beset with a multitude of little _Crystaline_ or _Adamantine_ bodies, so curiously shap’d, that it afforded a not unpleasing object.

Having considered those vivid _repercussions_ of light, I found them to be made partly from the plain external surface of these regularly figured bodies (which afforded the vivid reflections) and partly to be made from within the somewhat _pellucid_ body, that is, from some surface of the body, opposite to that superficies of it which was next the eye.

And because these bodies were so small, that I could not well come to make Experiments and Examinations of them, I provided me several small _stiriæ_ of Crystals or Diamants, found in great quantities in _Cornwall_ and are therefore commonly called _Cornish Diamants_: these being very _pellucid_, and growing in a hollow cavity of a Rock (as I have been several times informed by those that have observ’d them) much after the same manner as these do in the Flint, and having besides their outward surface very regularly shap’d, retaining very near the same Figures with some of those I observ’d in the other, became a convenient help to me for the Examination of the proprieties of those kinds of bodies.

And first for the Reflections, in these I found it very observable, That the brightest reflections of light proceeded from within the _pellucid_ body; that is, that the Rays admitted through the _pellucid_ substance in their getting out on the opposite side, were by the contiguous and strong reflecting surface of the Air very vividly reflected, so that more Rays were reflected to the eye by this surface, though the Ray in entring and getting out of the Crystal had suffer’d a double refraction, than there were from the outward surface of the Glass where the Ray had suffer’d no refraction at all.

And that this was the surface of the Air that gave so vivid a _re-percussion_ I try’d by this means I sunk half of a _stiria_ in Water, so that only Water was contiguous to the under surface, and then the internal reflection was so exceedingly faint, that it was scarce discernable. Again, I try’d to alter this vivid reflection by keeping off the Air, with a body not fluid, and that was by rubbing and holding my finger very hard against the under surface, so as in many places the pulp of my finger did touch the Glass, without any _interjacent_ air between, then observing the reflection, I found, that wheresoever my finger or skin toucht the surface, from that part there was no reflection, but in the little furrows or creases of my skin, where there remain’d little small lines of air, from them was return’d a very vivid reflection as before. I try’d further, by making the surface of very pure Quicksilver to be contiguous to the under surface of this _pellucid_ body, and then the reflection from that was so exceedingly more vivid than from the air, as the reflection from air was than the reflection from the Water; from all which trials I plainly saw, that the strong reflecting air was the cause of this _Phænomenon_.

And this agrees very well with the _Hypothesis_ of light and _Pellucid_ bodies which I have mention’d in the description of _Muscovy-glass_; for we there suppose Glass to be a _medium_, which does less resist the pulse of light, and consequently, that most of the Rays incident on it enter into it, and are refracted towards the _perpendicular_; whereas the air I suppose to be a body that does more resist it, and consequently more are _re-percuss’d_ then do enter it: the same kind of trials have I made, with _Crystalline Glass_, with drops of fluid bodies, and several other ways, which do all seem to agree very exactly with this _Theory_. So that from this Principle well establish’d, we may deduce severall Corollaries not unworthy observation.

And the first is; that it plainly appears by this, that the production of the Rainbow is as much to be ascribed to the reflection of the concave surface of the air, as to the refraction of the _Globular_ drops: this will be evidently manifest by these Experiments, if you _foliate_ that part of a Glass-ball that is to reflect an _Iris_, as in the _Cartesian_ Experiment, above mention’d, the reflections will be abundantly more strong, and the colours more vivid: and if that part of the surface be touch’d with Water, scarce affords any sensible colour at all.

Next we learn, that the great reason why _pellucid_ bodies beaten small are white, is from the multitude of reflections, not from the particles of the body, but from the _contiguous_ surface of the air. And this is evidently manifested, by filling the _Interstitia_ of those powder’d bodies with Water, whereby their whiteness presently disappears. From the same reason proceeds the whiteness of many kinds of Sands, which in the _Microscope_ appear to be made up of a multitude of little _pellucid_ bodies, whose brightest reflections may by the _Microscope_ be plainly perceiv’d to come from their internal surfaces; and much of the whiteness of it may be destroy’d by the affusion of fair Water to be contiguous to those surfaces.

The whiteness also of froth, is for the most part to be ascribed to the reflection of the light from the surface of the air within the Bubbles, and very little to the reflection from the surface of the Water it self: for this last reflection does not return a quarter so many Rays, as that which is made from the surface of the air, as I have certainly found by a multitude of Observations and Experiments.

The whiteness of _Linnen_, _Paper_, _Silk_, &c. proceeds much from the same reason, as the _Microscope_ will easily discover; for the Paper is made up of an abundance of _pellucid_ bodies, which afford a very plentifull reflection from within, that is, from the concave surface of the air contiguous to its component particles; wherefore by the affusion of Water, Oyl, Tallow, Turpentine, &c. all those reflections are made more faint, and the beams of light are suffer’d to traject & run through the Paper more freely.

Hence further we may learn the reason of the whiteness of many bodies, and by what means they may be in part made _pellucid_: As white Marble for instance, for this body is composed of a _pellucid_ body exceedingly flaw’d, that is, there are abundance of thin, and very fine cracks or chinks amongst the multitude of particles of the body, that contain in them small parcels of air, which do so _re-percuss_ and drive back the penetrating beams, that they cannot enter very deep within that body; which the _Microscope_ does plainly inform us to be made up of a _Congeries_ of _pellucid_ particles. And I further found it somewhat more evidently by some attempts I made towards the making transparent Marble, for by heating the Stone a little, and soaking it in Oyl, Turpentine, Oyl of Turpentine, &c., I found that I was able to see much deeper into the body of Marble than before; and one trial, which was not with an unctuous substance, succeeded better than the rest, of which, when I have a better opportunity, I shall make further trial.

This also gives us a probable reason of the so much admired _Phænomena_, of the _Oculus Mundi_, an _Oval_ stone, which commonly looks like white Alabaster, but being laid a certain time in Water, it grows _pellucid_, and transparent, and being suffer’d to lie again dry, it by degrees loses that transparency, and becomes white as before. For the Stone being of a hollow spongie nature, has in the first and last of these appearances, all those pores fill’d with the obtunding and reflecting air; whereas in the second, all those pores are fill’d with a _medium_ that has much the same refraction with the particles of the Stone, and therefore those two being _contiguous_, make, as ’twere, one _continued medium_, of which more is said in the 15. _Observation_.

There are a multitude of other _Phænomena_, that are produc’d from this same Principle, which as it has not been taken notice of by any yet that I know, so I think, upon more diligent observation, will it not be found the least considerable. But I have here onely time to hint _Hypotheses_, and not to prosecute them so fully as I could wish; many of them having a vast extent in the production of a multitude of _Phænomena_, which have been by others, either not attempted to be explain’d, or else attributed to some other cause than what I have assign’d, and perhaps than the right; and therefore I shall leave this to the prosecution of such as have more leisure: onely before I leave it, I must not pretermit to hint, that by this Principle, multitudes of the _Phænomena_ of the air, as about _Mists_, _Clouds_, _Meteors_, _Haloes_, &c. are most plainly and (perhaps) truly explicable; multitudes also of the _Phænomena_ in colour’d bodies, as liquors, &c. are deducible from it.

And from this I shall proceed to a second considerable _Phænomenon_ which these Diamants exhibit, and that is the regularity of their _Figure_, which is a propriety not less general than the former, It comprising within its extent, all kinds of _Metals_, all kinds of _Minerals_, most _Precious stones_, all kinds of _Salts_, multitudes of _Earths_, and almost all kinds of _fluid bodies_. And this is another propiety, which, though a little superficially taken notice of by some, has not, that I know, been so much as attempted to be explicated by any.

This propriety of bodies, as I think it the most worthy, and next in order to be consider’d after the contemplation of the _Globular Figure_, so have I long had a desire as wel as a determination to have prosecuted it if I had had an opportunity, having long since propos’d to my self the method of my enquiry therein, it containing all the allurements that I think any enquiry is capable of: For, first I take it to proceed from the most simple principle that any kind of form can come from, next the _Globular_, which was therefore the first I set upon, and what I have therein perform’d, I leave the Judicious Reader to determine. For as that form proceeded from a propiety of fluid bodies, which I have call’d _Congruity_, or _Incongruity_; so I think, had I time and opportunity, I could make probable, that all these regular Figures that are so conspicuously _various_ and _curious_, and do so adorn and beautifie such multitudes of bodies, as I have above hinted, arise onely from three or four several positions or postures of _Globular_ particles, and those the most plain, obvious, and necessary conjunctions of such figur’d particles that are possible, so that supposing such and such plain and obvious causes concurring the _coagulating particles_ must necessarily compose a body of such a determinate regular Figure, and no other, and this with as much necessity and obviousness as a fluid body encompast with a _Heterogeneous_ fluid must be protruded into a _Spherule_ or _Globe_. And this I have _ad oculum_ demonstrated with a company of bullets, and some few other very simple bodies; so that there was not any regular Figure, which I have hitherto met withall, of any of those bodies that I have above named, that I could not with the composition of bullets or globules, and one or two other bodies, imitate, even almost by shaking them together. And thus for instance may we find that the _Globular_ bullets will of themselves, if put on an inclining plain, so that they may run together, naturally run into a _triangular_ order, composing all the variety of figures that can be imagin’d to be made out of _æquilateral triangles_; and such will you find, upon trial, all the Surfaces of _Alum_ to be compos’d of: For three bullets lying on a plain, as close to one another as they can compose an _æquilatero-triangular_ form, as in A in the 7. _Scheme_. If a fourth be joyn’d to them on either side as closely as it can, they four compose the most regular Rhombus consisting of two _æquilateral triangles_, as B. If a fifth be joyn’d to them on either side in as close a position as it can, which is the propriety of the _Texture_, it makes a _Trapezium_, or four-sided Figure, two of whose angles are 120. and two 60. degrees, as C. If a sixth be added, as before, either it makes an _æquilateral triangle_, as D, or a Rhomboeid, as E, or an _Hexangular Figure_, as F, which is compos’d of two _primary Rhombs_. If a seventh be added, it makes either an _æquilatero-hexagonal_ Figure, as G, or some kind of six-sided _Figure_, as H, or I. And though there be never so many placed together, they may be rang’d into some of these lately mentioned Figures, all the angles of which will be either _60_. degrees, or 120. as the figure K. which is an _æquiangular hexagonal_ Figure is compounded of 12. _Globules_, or may be of 25, or 27, or 36, or 42, &c. and by these kinds of texture, or position of globular bodies, may you find out all the variety of regular shapes, into which the smooth surfaces of _Alum_ are form’d, as upon examination any one may easily find; nor does it hold only in superficies, but in solidity also, for it’s obvious that a fourth _Globule_ laid upon the third in this texture, composes a regular _Tetrahedron_, which is a very usual Figure of the _Crystals_ of _Alum_. And (to hasten) there is no one Figure into which _Alum_ is observ’d to be crystallized, but may by this texture of _Globules_ be imitated, and by no other.

I could instance also in the Figure of _Sea-salt_, and _Sal-gem_, that it is compos’d of a texture of _Globules_, placed in a _cubical_ form, as L, and that all the Figures of those Salts may be imitated by this texture of _Globules_ and by no other whatsoever. And that the forms of _Vitriol_ and of _Salt-Peter_, as also of _Crystal_, _Hore-frost_, &c. are compounded of these two textures, but modulated by certain proprieties: But I have not here time to insist upon, as I have not neither to shew by what means _Globules_ come to be thus context, and what those _Globules_ are, and many other particulars requisite to a full and intelligible explication of this propriety of bodies. Nor have I hitherto found indeed an opportunity of prosecuting the inquiry so farr as I design’d; nor do I know when I may, it requiring abundance of time, and a great deal of assistance to go through with what I design’d; the model of which was this:

First, to get as exact and full a collection as I could, of all the differing kinds of Geometrical figur’d bodies, some three or four several bodies of each kind.

Secondly, with them to get as exact a History as possibly I could learn of their places of Generation or finding, and to enquire after as many circumstances that tended to the Illustrating of this Enquiry, as possibly I could observe.

Thirdly, to make as many trials as upon experience I could find requisite, in Dissolutions and Coagulations of several crystallizing Salts; for the needfull instruction and information in this Enquiry.

Fourthly, to make several trials on divers other bodies, as Metals, Minerals, and Stones, by dissolving them in several _Menstruums_, and crystalizing them, to see what Figures would arise from those several _Compositums_.

Fifthly, to make Compositions and Coagulations of several Salts together into the same mass, to observe of what Figure the product of them would be; and in all, to note as many circumstances as I should judge conducive to my Enquiry.

Sixthly, to enquire the closeness or rarity of the texture of these bodies, by examining their gravity, and their refraction, &c.

Seventhly, to enquire particularly what operations the fire has upon several kinds of Salts, what changes it causes in their Figures, Textures, or Energies.

Eighthly, to examine their manner of dissolution, or acting upon those bodies dissoluble in them; The texture of those bodies before and after the process. And this for the History.

Next for the Solution, To have examin’d by what, and how many means, such and such Figures, actions and effects could be produc’d possibly.

And lastly, from all circumstances well weigh’d, I should have endeavoured to have shewn which of them was most likely, and (if the informations by these Enquiries would have born it) to have demonstrated which of them it must be, and was.

But to proceed, As I believe it next to the Globular the most simple; so do I, in the second place, judge it not less pleasant; for that which makes an Enquiry pleasant, are, first a noble _Inventum_ that promises to crown the successfull endeavour; and such must certainly the knowledge of the efficient and concurrent causes of all these curious Geometrical Figures be, which has made the Philosophers hitherto to conclude nature in these things to play the Geometrician, according to that saying of _Plato_, Ὁ Θεὸς γεομετρεῖ. Or next, a great variety of matter in the Enquiry; and here we meet with nothing less than the _Mathematicks_ of nature, having every day a new Figure to contemplate, or a variation of the same in another body,

Which do afford us a third thing, which will yet more sweeten the Enquiry, and that is, a multitude of information; we are not so much to grope in the dark, as in most other Enquiries, where the _Inventum_ is great; for having such a multitude of instances to compare, and such easie ways of generating, or compounding and of destroying the form, as in the _Solution_ and _Crystallization_ of Salts, we cannot but learn plentifull information to proceed by. And this will further appear from the universality of the Principle which Nature has made use of almost in all inanimate bodies. And therefore, as the contemplation of them all conduces to the knowledg of any one; so from a Scientifical knowledge of any one does follow the fame of all, and every one.

And fourthly, for the usefulness of this knowledge, when acquir’d; certainly none can doubt, that considers that it caries us a step forward into the Labyrinth of Nature, in the right way towards the end we propose our selves in all Philosophical Enquiries. So that knowing what is the form of Inanimate or Mineral bodies, we shall be the better able to proceed in our next Enquiry after the forms of Vegetative bodies; and last of all, of Animate ones, that seeming to be the highest step of natural knowledge that the mind of man is capable of.

* * * * *

Observ. XIV. _Of several kindes of frozen _Figures_._

I have very often in a Morning, when there has been a great _hoar-frost_, with an indifferently magnifying _Microscope_, observ’d the small _Stiriæ_, or Crystalline beard, which then usually covers the face of most bodies that lie open to the cold air, and found them to be generally _Hexangular prismatical_ bodies, much like the long Crystals of _Salt-peter_, save onely that the ends of them were differing: for whereas those of _Nitre_ are for the most part _pyramidal_, being terminated either in a point or edge; these of Frost were hollow, and the cavity in some seem’d pretty deep, and this cavity was the more plainly to be seen, because usually one or other of the six _parallelogram_ sides was wanting, or at least much shorter then the rest.

But this was onely the Figure of the _Bearded hoar-frost_; and as for the particles of other kinds of _hoar-frosts_, they seem’d for the most part irregular, or of no certain Figure. Nay, the parts of those curious branchings, or _vortices_, that usually in cold weather tarnish the surface of Glass, appear through the _Microscope_ very rude and unshapen, as do most other kinds of frozen _Figures_, which to the naked eye seem exceeding neat and curious, such as the Figures of _Snow_, frozen _Urine_, _Hail_, several _Figures_ frozen in common Water, &c. Some Observations of each of which I shall hereunto annex, because if well consider’d and examin’d, they may, perhaps, prove very instructive for the finding out of what I have endeavoured in the preceding Observation to shew, to be (next the _Globular Figure_ which is caus’d by _congruity_, as I hope I have made probable in the sixth _Observation_) the most simple and plain operation of Nature, of which, notwithstanding we are yet ignorant.