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 there.

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 twofold 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 Phaenomena 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 Phaenomenon (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 corpusles 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 vitrifed 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 Phaenomenon, 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 glittering 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 Phaenomena 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 Phaenomenon, and which may be perhaps very much concerned in the business of its hardening and tempering: of which somewhat it 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 Phaenomenon; 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 Phaenomenon, 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 Phaenomena 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 flit 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, 1/8 of a lovely Grass-green, 3/4 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 Phaenomenon 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 1/8 or a 1/6 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 Phaenomena, 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 Phaenomena, 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 Laminae, 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 Laminae 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 Phaenomena of Colours: which Phaenomena 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 Phaenomena, 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 Phaenomena 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 Trades-men 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 Phaenomena 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 Phaenomena 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 rigidnesse 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 Laminae on Steel, Lead, and other Metals, one surface of which Laminae is contiguous to the surface of the Metal, the other to that of the Air.

Nor is it necessary, that these colour'd Laminae 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 vertuity 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 Energyes 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 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 vitration 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 Chimical 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 Rain-bow, 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 Phaenomena 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 side 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 Phaenomena, 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 Phaenomena 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 alia contraria (refractione) non destruatur: Nam experientia docet si superficies NM & NP (nempe refringentes) Parallelae 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 Phaenomena 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 aetherei, 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. Sec. 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. Sec. 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 laminae 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. Sec. 5. Et praeterea observavi umbram quoque, aut limitationem luminis requiri: and Chap. 8. Sec. 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 Phaenomena 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 Phaenomena: 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 Phaenomena, 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 deadned 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.