Micrographia Part 4

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 gla.s.s-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 a.s.sign 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 Stopc.o.c.k of Bra.s.s, 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 c.o.c.k 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_, _b.u.t.ter_, 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 _Gla.s.s of Antimony_, which does very neer approach the nature of Gla.s.s,

But because these are all Examples taken from other materials then Gla.s.s, and argue only, that possibly there may be the like property also in Gla.s.s, 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 Gla.s.s: 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 Gla.s.ses with a hot Iron. For in that Experiment the top of the Iron heats, and thereby rarifies the parts of the Gla.s.s 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 Gla.s.s, which is done two wayes, either First, by suddenly heating a piece of Gla.s.s in one place more then in another. And by this means _chymists_ usually cut off the necks of Gla.s.s-bodies, by two kinds of Instruments, either by a glowing hot round Iron-Ring, which just incompa.s.ses 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 Gla.s.s 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 Gla.s.s, which is also made more probable by this circ.u.mstance which I have observed, that a piece of common window-gla.s.s 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 Gla.s.s-pipe, and fit into a solid stick of Gla.s.s, 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 Gla.s.s is more manifest in this Experiment.

Take a stick of Gla.s.s 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 Gla.s.s 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 Gla.s.s, 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 s.p.a.ce 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 Gla.s.s 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 Gla.s.s, upon the subsiding of the parts of the Gla.s.s, 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 Gla.s.s 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 Gla.s.ses) contract the bulk of the drop into a much less compa.s.s. 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 Gla.s.s 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 Gla.s.s 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 Gla.s.s-b.a.l.l.s (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 b.a.l.l.s were small, but exceeding thick bubbles of Gla.s.s, 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 Gla.s.s, which cools much quicker then green Gla.s.s, 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 Gla.s.s 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 mult.i.tude of small s.h.i.+vers 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 Gla.s.s, 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 _a.n.a.logous_ 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 Gla.s.s 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 Gla.s.s 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 Bra.s.s, &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 s.h.i.+ning 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 s.h.i.+ning 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 sh.e.l.l 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 fas.h.i.+oned Bowl without a foot. This night, making many tryals and observations of this Experiment, I met, among a mult.i.tude 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 b.a.l.l.s 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_ incompa.s.sing _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 s.h.i.+ning 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-gla.s.s_.

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 pa.s.sage 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 Carca.s.s, 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 s.h.i.+ning 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 ill.u.s.trate my present Explication, and is in it self exceeding pretty, I must not pa.s.s by: And that is a way of making small _Globules_ or _b.a.l.l.s_ 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 b.a.l.l.s be: _Stratifie_ these filings with the fine and well dryed powder of quick Lime in a _Crucible_ proportioned to the quant.i.ty 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 was.h.i.+ng 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-gla.s.ses 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 Gla.s.s, and other thin Bodies_.

Moscovy-gla.s.s, 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-gla.s.s_, 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 incompa.s.sed 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 incompa.s.sing 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 incompa.s.sing 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 Gra.s.s-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 Peac.o.c.ks, 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 circ.u.mstance. And this will be made manifest to him that considers, first, that this laminated body is more simple and regular then the parts of Peac.o.c.ks feathers, this consisting only of an indefinite number of plain and smooth Plates, heaped up, or _inc.u.mbent_ on each other.

Next, that the parts of this body are much more manageable, to be divided or joyned, then the parts of a Peac.o.c.ks 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 circ.u.mstances, 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 Circ.u.mstances will make manifest.

As _first_, if you take any small piece of the _Muscovy-gla.s.s_, 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 s.p.a.ce, 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-gla.s.s-plate, each about the bigness of a s.h.i.+lling, 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-gla.s.s_; 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-gla.s.s_ hint; but it may be confirm'd by mult.i.tudes of other Instances.

And first, we shall find, that even Gla.s.s 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 Gla.s.ses 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 Gla.s.s, 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.