Micrographia, by Robert Hooke

Observ. vii. Of some Phænomena of Glass drops.

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

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

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

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

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

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

Schem. 4.
Fig. X.
Let AB in the Figure X of the fourth Scheme represent the drop cased over with Icthyocolla or Isinglass, (by being ordered as is before prescribed) crazed or flawed into pieces, but by the skin or case kept in its former figure, and each of its flawed parts preserved exactly in its due posture; the outward appearance of it somewhat plainly to the naked eye, but much more conspicuous if viewed with a small lens appeared much after this shape. That is, the blunt end B for a pretty breadth, namely, as far as the Ring CCC seemed irregularly flawed with divers clefts, which all seemed to tend towards the Center of it, being, as I afterwards found, and shall anon shew in the description of the figure Y, the Basis, as it were, of a Cone, which was terminated a little above the middle of the drop, all the rest of the Surface from CCC to A was flawed with an infinite number of small and parallel Rings, which as they were for the most part very round, so were they very thick and close together, but were not so exactly flaw'd as to make a perfect Ring, but each circular part was by irregular cracks flawed likewise into multitudes of irregular flakes or tiles; and this order was observed likewise the whole length of the neck.

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

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

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

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

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

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

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


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

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

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

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

Secondly, That the parts of the drop do suffer a twofold contraction.

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

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

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

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

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

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

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

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

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

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

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

This Quality is also very observable in Lead, Tin, Silver, Antimony, Pitch, Rosin, Bees-wax, Butter, and the like; all which, if after they be melted you suffer gently to cool, you shall find the parts of the upper Surface to subside and fall inwards, losing that plumpness and smoothness it had whilst in fusion. The like I have also observed in the cooling of Glass of Antimony, which does very neer approach the nature of Glass,

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

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

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

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

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

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

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

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

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

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

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

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

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

http://ebooks.adelaide.edu.au/h/hooke/robert/micrographia/observ7.html

Last updated Monday, March 17, 2014 at 16:42