 Section 11 of Micrographia, some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Jim Cooper. Micrographia, some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon by Robert Hook. Observation 7 of some phenomena of glass drops. These glass drops are small parcels of coarse green glass taken out of the pots that contain the metal, as they call it, infusion upon the end of an iron pipe and being exceedingly hot and thereby of a kind of sluggish fluid confidence, are suffered to drop from vents into a bucket of cold water and in it to lie till they be grown sensibly cold. Some of these I broke in the open air by snapping off a little of the small stem with my fingers, others by crushing it with a small pair of pliers, which I had no sooner done than the whole bulk of the drop flew violently with a very brisk noise into multitudes of small pieces, some of which were as small as dust, though in some there were remaining pieces pretty large without any flaw at all and others very much flawed, which by rubbing between one's fingers was easily reduced to dust. These dispersed every way so violently that some of them pierced my skin. I could not find either with my naked eye or a microscope that any of the broken pieces were of a regular figure nor any one like another, but for the most part those that flawed off in large pieces were priddly branched. The ends of others of these drops I nipped off whilst all the bodies and ends of them lay buried under the water, which, like the former, flew all to pieces with as brisk a noise and as strong a motion. Others of these I tried to break by grinding away the blunt end and though I took a seemingly good one and had ground away near two thirds of the ball, yet would it not fly to pieces, but now and then some small rings of it would snap and fly off, not without a brisk noise and quick motion, leaving the surface of the drop whence it flew very priddly branched or creased, which was easily discoverable by the microscope. This drop, after I had thus grounded without at all impairing the remnant that was not ground away, I caused to fly immediately all into sand upon the nipping off the very tip of its slender end. Another of these drops I began to grind away at the smaller end, but had not worn away on the stone above a quarter of an inch before the whole drop flew with a brisk crack into sand or small dust, nor would it have held so long had there not been a little flaw in the piece that I ground away as I afterwards found. While others of these drops I covered over with a thin but very tough skin of ichthyocola, which being very tough and very transparent, was the most convenient substance for these trials that I could imagine, having dipped, I say, several of these drops in this transparent glue whilst hot, and suffering them to hang by a string tied about the end of them till they were cold and the skin pretty tough. Then wrapping all the body of the drop, leaving out only the very tip, in fine supple kid's leather very closely, I nipped off the small top and found, as I expected, that notwithstanding this skin of glue and the close wrapping up in leather, upon the breaking of the top, the drop gave a crack like the rest, and gave my hand a pretty brisk impulse. But yet the skin and leather was so strong as to keep the parts from flying out of their former posture and the skin being transparent, I found that the drop retained exactly its former figure in polish, but was grown perfectly opatious and all over flawed, all those flaws lying in the manner of rings, from the bottom or blunt end, to the very top or small point. And by several examinations with the microscope of several thus broken, I found the flaws both within the body of the drop and on the outward surface to lie much in this order. Let A be in the figure X of the fourth scheme, represent the drop cased over with ichthyocola or isenglass, by being ordered as is before prescribed, crazed or flawed into pieces, but by the skin or case kept in its former figure, and each of its flawed parts preserved exactly in its due posture. The outward appearance of it, somewhat plainly to the naked eye, but much more conspicuous if viewed with a small lens, appeared much after the shape. That is, the blunt end B for a pretty breath, namely, as far as the ring CCC seemed irregularly flawed with diverse clefs, which all seemed to tend towards the center of it, being, as I afterwards found, and shall anon show in the description of the figure Y, the basis, as it were, of a cone, which was terminated a little above the middle of the drop, all the rest of the surface, from CCC to A, was flawed with an infinite number of small and parallel rings, which as they were for the most part very round, so were they very thick and close together, but were not so exactly flawed as to make a perfect ring, but each circular part was by irregular cracks flawed likewise into multitudes of irregular flakes or tiles, and this order was observed likewise the whole length of the neck. Now, though I could not exactly cut this conical body through the axis, as is represented by the figure Y, yet by anatomizing, as it were, of several, and taking notice of diverse particular circumstances, I was informed that could I have artificially divided a flawed drop through the axis or center, I should, with a microscope, have found it to appear much of this form, where A signifies the apex and B, the blunt end, CCC, the cone of the basis, which is terminated at T, the top or end of it, which seems to be the very middle of the blunt end in which not only the conical body of the basis CCC is terminated, but as many of the parts of the drop as reach as high as DD. And it seemed to be the head or beginning of a pith, as it were, or a part of the body which seemed more spongy than the rest and much more irregularly flawed, from which T ascended by EE, though less visible, into the small neck towards A. The grain, as it were, of all the flaws that proceeds from all the outward surface ADCCDA, was much the same as is represented by the black strokes that meet in the middle, DT, DT, DE, DE, etc. Nor is this kind of grain, as I may call it, peculiar to glass drops thus quenched for, not to mention copper stones and diverse other marcosites and minerals, which I have often taken notice of to be in the very same manner flaked or grained with a kind of pith in the middle. I have observed the same in all manner of cast iron, especially the coarser sort, such as stoves and furnaces and backs and pots are made of. For upon the breaking of any of those substances it is obvious to observe how from the outsides towards the middle there is a kind of radiation or grain much resembling this of the glass drop, but this grain is most conspicuous in iron bullets if they be broken. The same phenomena may be produced by casting Regulus of Antimony into the bullet mold as also with Glass of Antimony and with almost any such kind of vitrified substance, either cast into a cold mold or poured into water. Others of these drops are heat red hot in the fire and then suffer them to cool by degrees, and these I found to have quite lost all their fulminating or flying quality as also their hard brittle and springy texture and to emerge of a much softer temper and much easier to be broken or snapped with one's finger, but its strong and brittle quality was quite destroyed and it seemed much of the same consistency with other green glass well kneeled in the oven. The figure and bigness of these for the most part was the same with that of the figure Z, that is, all the surface of them was very smooth and polished and for the most part round, but very rugged or knobbed about D, and all the length of the stem was here and there pitted or flatted. About D, which is at the upper part of the drop under the side of the stem which is concave, there usually was made some one or more little hillocks or prominences. The drop itself before it be broken appears very transparent and towards the middle of it to be very full of small bubbles of some kind of aerial substance, which by the refraction of the outward surface appear much bigger than really they are, and this may be in good part removed by putting the drop under the surface of clear water. Or by that means most part of the refraction of the convex surface of the drop is destroyed and the bubbles will appear much smaller. And this, by the by, mines me of the appearing magnitude of the aperture of the iris or pupil of the eye, which though it appear and be therefore judged very large, is yet not above a quarter of the bigness it appears of by the lenticular refraction of the cornea. The cause of all which phenomena I imagine to be no other than this, that the parts of the glass being by the excessive heat of the fire kept off and separated one from another and thereby put into a kind of sluggish fluid consistence, are suffered to drop off with that heat or agitation remaining in them into cold water, by which means the outsides of the drop are presently cooled and crusted and are thereby made of a loose texture. Because the parts of it have not time to settle themselves leisurely together and so to lie very close together, and the innermost parts of the drop retaining still much of their former heat and agitations remain of a loose texture also, and according as the cold strikes inwards from the bottom and sides are quenched, as it were, and made rigid in that very posture wherein the cold finds them. For the parts of the crust being already hardened will not suffer the parts to shrink any more from the outward surface inward, and though it shrink a little by reason of the small parcels of some aerial substances dispersed through the matter of the glass, yet that is not near so much as it appears as I just now hinted. Nor if it were would it be sufficient for to consolidate and condense the body of glass into a tough and close texture after it had been so excessively rarified by the heat of the glass furnace. But that there may be such an expansion of the aerial substance contained in those little blebs 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 vial, 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 boil, and the air in the bubble being good part rarified and driven out. Then by sucking at the smalling pipe more of the air or vapors in the bubble may be sucked 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 vapors in the glass will expand themselves so as to buoy up the 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 deceiting 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, etc., 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 Te, or rather from that outward, so that they cannot extricate or unbend themselves, till some part of Te, be broken and loosened, for all the parts about that are placed in the manner of an arch, and so till their hold at Te, 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 concur to the stability of the whole fabric, and no one stone can be taken away but the whole arch falls, and where so ever any of those radiating wedges, DtD, etc., are removed, which are the component parts of this arch, the whole fabric 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 itself 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 kneeling losing their springiness constitute a drop of more soft but less brittle texture, and the parts being not at all under a flexure, though any part of the middle or pith Te, be broken yet will not the drop at all fly to pieces as before. This conjecture of mine I shall endeavor 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 rarefied or take up more room than 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 rarefied texture, fourthly that there is a flexion or force remaining upon the parts of the glass thus quenched from which they endeavor to extricate themselves, fifthly that the fabric of the drop that is able to hinder the parts from extricating themselves is analogous 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 easier 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 touched 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 surpass 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 itself 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 trials at last brought to a great certainty in 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 near the very top in the heat of summer and pretty near 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 the body I have made on purpose at the glass house of the same metal where of 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 join 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 tinged with the lovely color of conchineal 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 trial found the most tender of any spiritual liquor and those are much more sensibly affected with the variations of heat and cold than other more phlegmatic and ponderous liquors and is 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 aforementioned flame of a lamp seal and join 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 shoots 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 zero or not I place according to the degrees of expansion or contraction of the liquor in proportion to the bulk it had when it endured the newly mentioned 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 a b c d of the figure z the diameter a b of whose cavity let be about two inches and the depth b c the same let each end be covered with a flat and smooth plate of the same substance closely soldered on and in the midst of the upper cover make a pretty large hole e f about the bigness of a fifth part of the diameter of the other into this fastened very well with cement a straight and even cylindrical pipe of glass e f g h the diameter of whose cavity let be exactly one tenth of the diameter of the greater cylinder let this pipe be marked at g h with a diamond 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 e g exactly into ten parts so the capacity of the hollow of each of these divisions will be one one thousand 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 the cylindrical vessel with the same liquor where with the thermometers are filled then place both it and the thermometer you are to graduate in water that is already to be frozen and bring the surface of the liquor in the thermometer to the first mark or zero then so proportion the liquor in the cylindrical vessel that the surface of it may be just at the lower end of the small glass cylinder then very gently and gradually warm the water in which both the thermometer and the cylindrical vessel stand and as you perceive the tinged liquor to rise in both stems with the point of a diamond gives 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 easy 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 around then heat this cock in the fire and you'll 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 easy to be turned as before this quality is also very observable in lead tin silver and timony pitch rosin beeswax 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 infusion the like I have also observed in the cooling of glass of antimony which does very near approach the nature of glass but because these are all examples taken from other materials than glass and argue only that possibly there may be the like property also in glass not that really there is we shall buy three or four experiments endeavor 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 around if the drop had cooled leisurely is by being quenched hastily very irregularly flatted and pitted which I suppose proceeds partly from the water's 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 VM and C 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 than 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 when each of those agitated parts endeavoring to expand itself 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 ways either first by suddenly heating a piece of glass in one place more than in another and by this means chemists usually cut off the necks of glass bodies by two kinds of instruments either by a glowing hot round iron ring which just encompasses the place that is to be cut or else by a sulfured three which is often around 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 phenomena 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 easy 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 laugh that it may but just easily turn and that the very ends of it may be just touched and sustained 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 easy to be turned around as 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 than when they were at rest and lay quietly upon each other and this is further confirmed by a pot of boiling alabaster which will manifestly rise a sixth or eighth part higher in the pot whilst it is boiling then it will remain at both before and after it be boiled the reason of which odd phenomenon to hint it here only by the way is this that there is in the curious powder of alabaster and other calcinating stones a certain watery substance which is so fixed 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 degree they break out every way in vapors and thereby so shake and loosen the small core puzzles 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 vapors burst and break out in bubbles just as in boiling 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 boiling 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 near 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 near together the parts next adjoining to the water must needs lose much of their motion and impart it to the ambient water which the evolution 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 agile substance contained in them has liberty of expanding itself 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 itself 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 one's 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 rarefied texture for if the outward parts of the drop by reason of its hard crust will endure very little contraction and the agile 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 form their sluggishness and glutinousness i suppose to be much after the manner of the sticks in the thorn bush or a lock of wool it will follow i say that the parts will hold each other very strongly together and endeavor to draw each other near together and consequently their texture must be very hard and stiff but very much rarefied 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 endeavor 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 in the outward parts cool leisurely as in baking of glasses contract the bulk of the drop into a much less compass for sense as i proved before the internal parts of the drop when fluid were of a very rarefied texture end as it were tossed open like a lock of wool and if they were suffered leisurely to cool would be again pressed as it were close together and sense that the heat which kept them bended and open is removed and yet the parts not suffered to get as near together as they naturally would it follows that the particles remain under a kind of tension and flexure and consequently have an endeavor 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 kneeled 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 anywhere 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 fabric is shattered and falls to pieces and each of the springs is left at liberty suddenly to extricate itself for since i've 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 condensed into a less room in the same manner as the stones of an arch conduced to 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 and all kind of knowledge especially in that of natural things and his generous disposition and communicating encouraged 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 ampersand would upon rubbing or scratching the inward surface fly all in thunder with a pretty brisk noise whereas neither before nor after the inner surface had been the scratched 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 ampersand these balls were small but exceeding thick bubbles of glass which being cracked off from the puntillion whilst very hot and so suffered to cool without kneeling them in the oven over the furnace due thereby being made of white glass which cools much quicker than green glass and is thereby made much brittle or 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 pretty 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 of thunder on the table now though the pieces are not so small as those of a fulminating drop yet they as plainly show that the outward parts of the glass have a great conatus to fly a thunder 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 devulsion and the broken pieces will if the concave service of them be further scratched with a diamond fly again into smaller pieces from which proceeding considerations it will follow sixly that the sudden flying a thunder of the parts as soon as this arch is anywhere disordered or broken proceeds from the springing of the parts which endeavoring 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 lie 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 somebody that is either very rarefied 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 exhibit now though i doubt not but that the air in them is very much rarefied 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 than 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 pretty 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 snapped anywhere 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 waste some part of the glass itself but ranges all the parts into a better order and gives each particle an opportunity of relaxing itself 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 hardened steel and hammered metals by kneeling them in the fire seemed to proceed from much the same cause for both by quenching suddenly such metals as have vitrified parts interspersed as steel has and by hammering of other kinds that do not so much about with them as silver brass etc 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 end of section 11 recorded by Jim Cooper Jim Cooper voice artist com section 12 of micrographia this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org micrographia by Robert Hook observation 8 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 phenomenon by the most ingenious Descartes 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 seem to vanish I found certain very small black but glittering spots of immovable substance each of which examining with my microscope I found to be a small round globule some of which as they looked pretty small so did they from their surface yield a very bright and strong reflection on that side which was next the light and each looked almost like a pretty bright iron ball whose surface was pretty regular such as is represented by the figure a in this I could perceive the image of the window pretty 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 pretty 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 far with all 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 trials 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 compositor seemed to be nothing else but a long thin chip of iron one of whose ends was melted into a small round globule the other end remaining unmelted and irregular and perfectly iron the second instance was not less remarkable than 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 phenomena of this experiment will I doubt not find cause to believe that the reason I have here to for 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 severed and heat red hot and that sometimes to such a degree is 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 vitrify it but many times the heat is so gentle as to be able to make the sliver only red hot which not withstanding falling upon the tinder that is only a very curious small coal made of the small threads of linen burnt to coals and charred it easily sets it on fire nor will any part of this hypothesis seems 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 one's 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 pools 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 sulfurous 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 transitors of the small chips of iron they are heat red hot and that combustible sulfurous body is presently preyed upon and devoured by the aerial encompassing menstruum whose office in this particular I have shown in the explication of charcoal 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 formally found the spark 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 untouched and rough filings or chips of iron so that it seems iron does contain a very combustible sulfurous body which is in all likelihood one of the causes of this phenomenon and which may be perhaps very much concerned in the business of its hardening and tempering of which somewhat is said in the description of muskovy glass so that these things considered we need not trouble ourselves to find out what kind of pores they are both in the flint and steel that contain the atoms of fire nor how these atoms come to be hindered from running all out when a door or passage in their pores is made by the concussion nor need we trouble ourselves to examine by what prometheus the element of fire comes to be fetched 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 putrefying 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 full 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 than that a body as silver for instance put into a weak menstruum as unrectified aquafortus 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 Descartes 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 stale 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 phenomenon whereas by not examining so far as he might he has set down an explication which experiment does 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 phenomenon as i have elsewhere more largely shun 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 near as is possible a spherical or globular form which propriety and several of the phenomena 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 itself 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 physician 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 stratify these filings with the fine and well dried powder of quick lime in a crucible proportion 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 near 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 mixed with the quick lime to melt and no more for if the fire be too hot many of these filings will join and run together whereas if the heat be proportioned upon washing the lime dust in fair water although 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 our glasses of now though quick lime be the powder that this direction makes choice of yet i don't not but that there may be much more convenient ones found out one of which i have made trial 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 end of section 12 section 13 of micrographia this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org micrographia by Robert Hook section 13 observation 9 part 1 of the colors observable in muscovy glass and other thin bodies muscovy glass or lapis specularis is a body that seems to have as many curiosities in its fabric 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 joined or generated one upon another so close and 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 flipped 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 rhomboid this figure is much more conspicuous in our english talk much whereof is found in the lead mines and is commonly called spar and caulk 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 tough but is much more clear and transparent and much more curiously shaped and yet may be cleft and flaked 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 colors 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 diversely colored with all the colors of the rainbow and with the microscope i could perceive that these colors were ranged in rings that encompassed the white speck or flaw and were round or irregular according to the shape of the spot which they terminated and the position of colors in respect of one another was the very same as in the rainbow the prosecution of those colors from the middle of the spot outward being blue purple scarlet yellow green blue purple scarlet and so onwards sometimes half a score times repeated that is there appeared six seven eight nine or ten several colored rings or lines each encircling the other in the same manner as i have often seen a very vivid rainbow to have four or five several rings of colors that is accounting all the gradations between red and blue for one but the order of the colors in these rings was quite contrary to the primary or innermost rainbow and the same with those of the secondary or outermost rainbow these colored lines or irises as i may so call them were some of them much brighter than others and some of them also very much broader they being some of them 10 20 nay i believe near 100 times broader than others and those usually were broadest which were nearest the center or middle of the flaw and oftentimes i found that these colors reached 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 color which was very vivid and all the other colors encompassing it gradually ascending and growing narrower towards the edges keeping the same order as in the secondary rainbow that is if the middle were blue the next encompassing it would be a purple the third a red the fourth a yellow etc as above if the middle were a red the next without it would be a yellow the third a green the fourth a blue and so onward and this order it always kept whatsoever were the middle color there was further observable in several other parts of this body many lines or threads each of them of some one peculiar color 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 colored 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 color one eighth of a lovely grass green three fourths of an inch more of a bright scarlet and the rest of the line of a watch it blue others of them were much otherwise colored the variety being almost infinite another thing which is very observable is that if you find any place where the colors are very broad and conspicuous to the naked eye you may by pressing that place with your finger make the colors change places and go from one part to another there is one phenomenon more which may if care be used exhibit to the beholder as it has diverse 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 and one eighth or a one sixth part of an inch over each of them appearing through the microscope most curiously entirely and uniformly adorned with some one vivid color 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 colors which produce such a composite them as one would scarce imagine should be the result of such ingredients as perhaps a faint yellow and a blue may produce a very deep purple but when on on we come to the more strict examination of these phenomena and to inquire into the causes and reasons of these productions we shall I hope make it more conceivable how they are produced and show them to be no other than the natural and necessary effects arising from the peculiar union of concurrent causes these phenomena 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 colors and I the rather now do it instead of an appendix or digression to this history then upon the occasion of examining the colors in peacocks or other feathers because this subject as it does afford more variety of particular colors so does it afford much better ways of examining each circumstance and this will be made manifest to him that considers first that this laminated body is more simple and regular than 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 joined than the parts of a peacocks feather or any other substance that I know and thirdly because that in this we are able from a colorless body to produce several colored bodies affording all the variety of colors 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 colors is some lamina or plate of a transparent or polluted body of a thickness very determinant and proportioned according to the greater or less refractive power of the polluted 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 determinant thinness of them they shall all appear transparent and colorless but if you continue to split and divide them further you shall find at last that each plate after it comes to such a determinant thickness shall appear most lovely tinge or imbued with a determinant color 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 polluted medium those laminated polluted bodies that fill that space shall exhibit several rainbows or colored lines the colors 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 polished looking glass plate each about the bigness of a shilling take these two dry and with your forefingers 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 colored lines in the same manner almost as in the muscovy glass and you may very easily change any of the colors 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 colored with a red may be presently tinged with a yellow blue green purple or the like by altering the apropin equation 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 tried 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 compressed 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 tinged body should be of a fluid substance any other substance provided it be thin enough and transparent doing the same thing this the laminate of our muscovy glass hint but it may be confirmed by multitudes of other instances and first we shall find that even glass itself may by the help of a lamp be blown thin enough to produce these phenomena of colors which phenomena accidentally happening as i have been attempting to frame small glasses with a lamp did not a little surprise me at first having never heard or seen anything of it before though afterwards comparing it with the phenomena i had often observed in those bubbles which children used to make with soap water i did the less wonder especially when upon experiment i found i was able to produce the same phenomena in thin bubbles made with any other transparent substance thus have i produced them with bubbles of pitch rosin calafany turpentine solutions of several gums as gum arabic in water any glutinous liquor as wort wine spirit of wine oil of turpentine glare of snails etc it would be needless to enumerate the several instances these being enough to show the generality or universality of this propriety only i must not omit that we have instances also of this kind even in metaline bodies and animal for those several colors which are observed to follow each other upon the polished surface of hardened 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 vitram 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 hardening 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 softening 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 tough that is will endure 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 etc 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 accepting gold and silver which do not so much with the bare fire unless assisted by other saline bodies do more or less vitrify by the strength of fire that is are corroded by a saline substance which i elsewhere show to be the true cause of fire and are thereby as by several other minstrums converted into scoria and this is called calcining of them by chemists 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 metaline particles nor do i know anyone calcination wherein a saline body may not with very great probability be said to be an agent or co-agitor 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 hardened as the tradesmen call it by casing the iron to be hardened 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 contain 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 vitrified substances by being suddenly cooled become very hard and brittle and then surrises the pretty phenomena of the glass drops which i have already further explained in its own place eighthly that those metals which are not so apt to vitrify do not acquire any hardness by quenching in water as silver gold etc these considerations premised will i suppose make way for the more easy reception of this following explication of the phenomena of hardened and tempered 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 mixed with the more tough and unaltered parts of the iron that when by the great heat of the fire this vitrified substance is melted and consequently rarefied 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 hardened that is stayed 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 colors appear upon the surface of the brightened 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 stretched or set a tilt 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 color increases to a deep purple and so onward by these gradations or consecutions white yellow orange minimum scarlet purple blue watch it et cetera and the parts within are more conveniently and proportionately mixed and so they gradually subside into a texture which is much better proportioned and closer joined whence that rigidness of parts ceases and the parts begin to acquire their former ductilness now that is nothing but the vitrified metal that sticks upon the surface of the colored body is evident from this that if by any means it be scraped and rubbed 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 confirmed by this observable that that iron or steel will keep longer from resting which is covered with this vitrified case thus also lead will by degrees be all turned into a litharge for that color which covers the top being scummed or shoved aside appears to be nothing else but a litharge or vitrified lead this is observable also in some sort on brass copper silver gold tin but is most conspicuous in lead all those colors that cover the surface of the metal being nothing else but a very thin vitrified part of the heated metal the other instance we have is in animal bodies as in pearls mother of pearl shells oyster shells and almost all other kinds of stony shells whatsoever this have I also sometimes with pleasure observed 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 polished metal line body you shall find the like effects produced and in general where so ever you meet with a transparent body thin enough that is terminated by reflecting bodies of differing refraction from it there will be a production of these pleasing and lovely colors 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 colored 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 color but they may resemble a lens that is have their middles thicker than their edges or else a double concave that is be thinner in the middle than at the edges in both which cases there will be various colored rings or lines with differing consecutions or orders of colors the order of the first from the middle outwards being red yellow green blue etc and the latter quite contrary but further it is altogether necessary that the plate in the places where the colors appear should be of a determinant thickness first it must not be more than such a thickness for when the plate is increased to such a thickness the colors 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 colors but in that place where they were united and constituted one double plate as i may call it they appear transparent and colorless nor secondly may the plates be thinner than such a determinant size for we always find that the very outmost rim of these flaws is terminated in a white and colorless ring further in this production of colors there is no need of a determinant light of such a bigness and no more nor of a determinant position of that light that it should be on this side and not on that side nor of a terminating shadow as in the prism and rainbow or water ball for we find that the light in the open air either in or out of the sunbeams and within a room either from one or many windows produces much the same effect only where the light is brightest there the colors 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 colored rings be towards the light for the whole ring keeps its proper colors 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 colors generated in a pollucid body where there is properly no such refraction as Descartes supposes his globules to acquire a virtuity by for in the plane and even plates it is manifest that the second refraction according to Descartes his principles in the fifth section of the eighth chapter of his meteors does regulate and restore the supposed terminated globules unto their former uniform motion this experiment therefore will prove such a one as our thrice excellent virulom calls experimentum crutches serving as a guide or landmark by which to direct our course in the search after the true cause of colors affording us this particular negative information that for the production of colors there is not necessary either a great refraction as in the prism nor secondly a determination of light and shadow such as is both in the prism 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 the 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 putrefying bodies that the bononian stone shines no longer than it is either warmed by the sunbeams or by the flame of a fire or of a candle is the general report of those that ride 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 shown to the royal society by mr clayton a worthy member thereof which does make this assertion more evident than all the rest and that is that a diamond being rubbed 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 rubbed 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 colors by the truly honorable mr boil what may be said of those ignis fatui that appear in the night i cannot so well affirm having never had the opportunity to examine them myself nor to be informed by any others that had observed 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 seawater 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 ways broken or agitated by storms or oars or other percussing bodies and that the animal energies or spiritual agile parts are very active in cat's eyes when they shine seems evident enough because their eyes never shine but when they look very intensely 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 light may be said of the shining bellies of glow worms since tiz evident they can at pleasure either increase or extinguish that radiation it would be somewhat too long a work for this place is a technically to examine and positively to prove what particular kind of emotion it is that must be the efficient of light for though it be a motion yet tiz not every motion that produces it since we find there are many bodies very violently moved which yet afford not such an effect and there are other bodies which to our other senses seem not moved 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 etc though very often struck with a hammer shine not presently though they will all of them grow exceeding hot whereas rotten wood rotten fish seawater glow worms etc 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 inquired after the properties 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 moved and that because we find those motions are able more minutely to shatter and divide the body than the most violent heats minstrooms we yet know and that fire is nothing else but such a dissolution of the burning body made by the most universal minstrum of all so furious bodies namely the air we shall in another place of this tractate endeavor to make probable and that in all extremely 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 bodies are dissolved next it must be a vibrative motion and for this the newly mentioned 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 supposed 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 moved only with a terminated motion i know not any cause they can impress that motion upon the polluted 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 principle proprieties of emotion requisite to produce the effect called light in the object the next thing we are to consider is the way or manner of the projection of this motion through the interposed polluted body to the eye and here it will be easily granted first that it must be a body susceptible and impartable 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 urged i have this to answer that i can as easily deny as they affirm for i would faint know by what means anyone can be assured any more of the affirmative than i of the negative if indeed the propagation were very slow tis possible something might be discovered by eclipses 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 eclipses that may seem very much to favor this supposition of the slower progression of light than most imagine and the light may be said of the eclipses of the sun et cetera 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 when necessarily every pulse or vibration of the luminous body will generate a sphere which will continually increase and grow bigger just after the same manner though indefinitely swifter as the waves or rings on the surface of the water do swell into bigger and bigger circles about a point of it where by the sinking of a stone the motion was begun once 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 Descartes 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 only to the projection 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 endeavor to explicate we will suppose therefore in the first figure a c f d to be a physical ray or a b c and d e f to be two mathematical rays projected from a very remote point of a luminous body through and homogeneous transparent medium l l l and d a e b f c to be small portions of the orbicular impulses which must therefore cut the rays at right angles these rays meeting with the plane surface in o of a medium that yields an easier transitus to the propagation of light and falling obliquely on it they will in the medium m m m be refracted towards the perpendicular of the surface and because this medium is more easily projected than the former by a third therefore the point c of the orbicular pulse f c will be moved to h four spaces in the same time that f the other end of it is moved to g three spaces therefore the whole refracted pulse g h shall be oblique to the refracted rays c h k and g i and the angle g h c 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 g f to t c the distance between the point c and the perpendicular from g on c k which being as four to three h c being longer than g f is longer also than t c therefore the angle g h c is less than g t c so that henceforth the parts of the pulses g h and i k are moved a skew 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 only thus much in general i shall hint that i suppose the medium m m m to have less of the transparent undulating subtle matter and that matter to be less implicated by it whereas l l l 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 easy into a more difficult medium as by the calculations of g q and c s r which are refracted from the perpendicular in both which calculations it is 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 oftener 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 straightened by a contrary refraction we will next examine the refractions of the sunbeams as they are suffered only to pass through a small passage obliquely out of a more difficult into a more easy medium end of section 13