An essay of the great effects of even languid and unheeded motion whereunto is annexed An experimental discourse of some little observed causes of the insalubrity and salubrity of the air and its effects / by the Honourable Robert Boyle ...

CHAP. VIII. (Book 8)

Observat. VII. Men look upon divers Bodies as having their parts in a state of absolute Rest, when indeed they are in a Forced state, as of Ten∣sion, Compression, &c.

THis Observation will proba∣bly seem paradoxicall. For, when an intire Body, especially if it be of a solid Consistence, and seem to be of an homogeneous or uniform matter, appears to be movelesse, we are wont to take it for granted, that the parts, which that body is made up of, are perfectly at Rest also. But yet this will scarce be thought a reasona∣ble supposition, if we do but rightly consider some obvious Phaenomena, which may teach us, that, whilst a whole Body, or the superficies that includes it, retains its figure, di∣mensions and distance from other stable Bodies that are near it, the Corpuscles that compose it may have

various and brisk motions and endea∣vours among themselves. As, when a bar of iron or silver, having been well hammered, is newly taken off of the Anvill; though the Eye can discern no motion in it, yet the touch will readily perceive it to be very hot, and, if you spit upon it, the brisk agitation of the insensible parts will become visible in that which they will produce in the liquour. Be∣sides, when the Lath of a Cross-bow stands bent, though a man do nei∣ther by the Eye nor the Touch per∣ceive any motion in the springy parts, yet if the string be cut or broken, the sudden and vehement motion of the Lath, tending to restore it to the figure it had before it was bent, dis∣covers a springiness; whence we conclude it was before in a state of violent Compression. And, though the string of a bent Bow do like∣wise appear to be in a state of Rest; yet, if you cut it asunder, the newly made extreams will fly from one an∣other suddenly and forcibly enough

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to manifest, that they were before in a violent state of Tension. And on this occasion I could add divers In∣stances taken not onely from the works of Art, but those of Nature too, if they did not belong to ano∣ther paper: But, one sort of Obser∣vations 'twill be proper to set down in this place; because in those al∣ready mentioned, the bow and string were brought into a violent state by the meer and immediate force of man. I shall therefore add, that there are divers bodies, in which, though no such kind of force appears to have antecedently acted on them, we may yet take notice of a state of violent Compression or Extension, and a strong endeavour or tendency of the parts, that to the Eye or the Touch seem at rest, to shrink or to fly out; and this endeavour may in some Cases be more lasting and more forcible than one would easily suspect or believe. But examples of this kind you must not expect that I should give you out of Classick Au∣thours,

since in them 'tis like you have not met with either an Instance or a Conjecture to this purpose; but some few things that I tried my self, and some others that I learnt by Inquiry from some Tradesmen, whom I judged likeliest to inform me, I shall briefly acquaint you with.

I have sometimes observed my self, and have had the Observation confirmed to me by the ingeniouser Traders in Glass; That a Glass, that seemed to have been well baked, or nealed, (as they call it) would some∣times, many days or weeks, or per∣haps months, after it is taken from the fire, crack of its own accord; which seems for the most part to happen upon the score of the strong, but un∣equall, shrinking of the parts of the Glasse. And the Glass-men will tell you, that, if they take their Glasses too hastily from the fire, not allowing them leisure to cool by degrees, they will be very apt to crack. But I remember, that, to satisfy some In∣genious

men, I devised a way of ex∣hibiting a much more quick and re∣markable Phaenomenon of that kind. Having made then, by a way I else∣where teach, a flat Lump of metal∣line Glass, two or three or four times as thick as an ordinary Drinking∣glass, I observed, as I expected, that, though it had been melted in a very gentle fire, its very fusible nature needing no other, and though it were removed but very little from the fire, it was so disposed to shrink upon a small degree of Refrigeration, or rather abatement of Heat, that, be∣fore it was sensibly cold, it would crack with a noise in so vehement a manner, that, notwithstanding the ponderousness of the matter; which had been purposely laid upon a Le∣vell, parts of a considerable bulk, weighing perhaps some Drams, would fly, to a not inconsiderable distance from one another. And this Experiment I took pleasure to make more than once. And if you will be content with an Instance

which, though otherwise much in∣feriour, may not be unwelcome, for its being easily and readily made; I shall offer you one that I have often repeated. Take a piece of Copper, (if the Plate be thick, 'tis so much the better,) and, having throughly brought it to a red or white Heat among kindled Coals, take it from the fire, and when it begins to cool a little, hold it over a sheet or two of white Paper, and you will per∣ceive good store of flakes to fly off, not without some little noise, one af∣ter the other, and sometimes perhaps as far as the farthermost edges of the paper; which flakes or scales seem by their brittleness and colour, to be but parts of the surface of the me∣tal vitrified by the vehement action of the fire, and afterwards by a too hasty refrigeration shrinking so vi∣olently, as to crack and leap from one another, like the contiguous parts of the string of a Viol or other Musical Instrument, that breaks by the moisture of the Air. And on

this occasion I shall add, that, having afterwards inquired of an expert Ar∣tificer, that made metalline Concaves, about the shrinking of his mixtures of metalls, he confessed to me, that he usually observed them to shrink upon Refrigeration. And the like I my self have observed in Iron of a great thickness, and purposely fitted to a hollow body of metall, which it would not enter when it was ignited, though it would when 'twas cold. But to shew you by a notable Instance or two, both that Metals may shrink, and that they may doe so with a very con∣siderable force, I shall add, that I found by inquiry, that the lately mentioned Artificer, after he had made some large Concaves of an unfit mixture of metals, and having removed them from the fire, had been very carefull to keep the cold Air from them, lest they should cool too hastily, obser∣ved yet to his great loss, that, when they came to be further refrigerated, they would (perhaps after three hours) crack with a great noise,

though this metalline mixture were perchance harder than Iron, and three or four times as thick as com∣mon Looking-glasses. But the mis∣fortune of another Tradesman af∣forded me a yet more considerable Phaenomenon. For this excellent Ar∣tificer, whom I often employ, and with whom I was a while since dis∣coursing of these matters, complain'd to me, that, having lately cast a kind of Bell-metall upon a very strong so∣lid Instrument of Iron of a conside∣rable superficial Area, though the metal were suffer'd in a warm room to cool, from about eight a clock on Saturday night till about ten or twelve on Monday morning, and were then (which is to be noted) considerably hot to the touch; yet it cool'd so far, that, shrinking from the Iron that would not shrink with it, the Bell-metall cracked in divers places with noises loud as the Report of a Pistoll, though the metall, he affirm'd to me, was an inch and half, or two inches thick. And the same

person shewed me a large Cylinder of Iron, about which, for a certain purpose, a Coat of Bell-metall had been cast some days before, on which (Bell-metall) there was a crack near one end made by the coldness of the Iron, though the thickness of the Bell-metall, as near as I could mea∣sure it, exceeded an inch, and (as the Workman affirmed) an inch and a quarter.

Nor is it onely in such mixtures as Bell-metall, which, though very hard, may be very brittle, but even in a metal that is malleable when cold, that the like Phaenomenon may be met with, as I have been assured by an∣other ingenious Artificer, of whom I inquired, whether he had taken notice of the shrinking of metalls; who affirm'd to me, that, having had occasion to cast about a Cylinder of Iron a ring or hoop of Brass, he found to his trouble, that, when the metall began to cool, the parts shrunk from one another so as to leave a gaping crack, which he was fain to

fill up with soulder quite crosse the breadth of the ring, though this were above an inch thick.

I should not, Pyrophilus, have in this Chapter entertained you with more Experiments of others than of my own, if I had the conveniency of living near Founders of metalls, as the Tradesmen had whose Observa∣tions I have rectied, and whose sin∣cerity in them I had no cause to que∣stion. And both their Experiments and mine seem to teach, that a body may be brought into a state of Ten∣sion, as well by being expanded and stretch'd by the action of the fire up∣on the minute parts, as by the action of an external Agent upon the intire body. And, to speak more generally, the state of violent Contraction and Compression may not unfitly be illu∣strated by a Bow that is bent. For, as the Bow it self is brought to a state of Compression by the force of the Archer, that bent it; so by the Elastical force of the bent Bow, the string is brought into a violent state

of Tension, as may be made evident by the cutting off the string in the middle; for then both the Bow will fly suddenly outwards, and the parts of the string will swiftly and violent∣ly shrink from one another. And according to this Doctrine, the effect of other bodies upon such as are thus brought into, what men call, a Preter∣natural state, is not to be judg'd barely according to usual measures, but with respect to this latent Dispo∣sition of the Patient: as, for instance, though the string of a Viol not screw∣ed up, will not be hardned by the vapours that imbue the Air in moist weather; yet a neighbouring string of the same Instrument, though per∣haps much stronger, being screw'd up, and thereby stretched, will be so affected with those vapours, as to break with noise and violence. And so when one part of a piece of Glass is made as hot as can be, without appearing discolour'd to the Eye, though a drop or two of cold water have no effect upon the other part of

the same Glasse, yet if it touch the heated part, whose wonted extension (as I have elsewhere proved) is al∣ter'd by the fire that vehemently agitates the component particles, the cracking of the Glass will almost al∣ways presently ensue.

If against these Instances it be al∣ledged, that it is possible to assign an∣other cause of the seemingly sponta∣neous breaking of the bodies men∣tion'd in this Chapter, than that which I have propos'd, it will not much concern this Discourse to ex∣amine the Allegation; for, whatever the latent Cause of the Phaenomena may be, the manifest Circumstances of them suffice to shew, that bodies, which, as to sense, are in a natural state of Rest, may be in a violent one, as of Tension, and may have, either upon the score of the contex∣ture of the parts among themselves, or upon that of some interfluent sub∣tile matter, or some other Physical Agent, a strong endeavour to fly off or recede from one another; and that,

in divers bodies, the cause of this en∣deavour may act more vigorously than one would easily believe: and this suffices to serve the turn of this Discourse. For I presume that a person of your Principles will allow, that Local Motion must be produc'd by Local Motion, and consequently, that, without a very strong, though invisible and unheeded one, such hard and solid bodies as thick pieces of metall could not be made to crack.

I know not whether I may on this occasion acquaint you with an odd Relation I had from a very honest and credible, as well as experienced, Artist, whom I, for those reasons, have several times made choice to deal with about precious Stones, and other things belonging to the Jewel∣lers and Goldsmiths trades. For, considering with him one day a large lump of matter, which contained se∣veral Stones that he took for course Agats, and which were joyned to∣gether by a Cement, that in most places was harder than most ordina∣ry

Stones, I perceived that there re∣mained divers pretty large cavities in this Cement, which seemed to have contained such Stones as those that yet made parts of the lump. Upon which occasion he affirmed to me, that several of the Stones grew whilst they were lodg'd in those cavities. And when I told him, that, though I had been long of an opinion, that Stones may receive an increment after their first formation, yet I did not see how any such thing appeared by those we were looking upon: He gave me in many words an account of his Asser∣tion, which I reduced to this, that the Stones he spoke of, did, after they were first formed, really tend to expand themselves by virtue of some Principle of growth, which he could not intelligibly describe; but that these Stones being lodg'd in a Ce∣ment extreamly hard, and therefore not capable of being forced to give way, their expansive endeavour was rendered ineffectual, but not destroy∣ed: so that when afterwards these

Stones came to be taken out of the Cement wherein they were bedded, and to whose sides 'tis like they were not exquisitely congruous, the com∣prest Stones, having their sides now no longer wedged in by the harder Cement, quickly expanded them∣selves, as if 'twere by an internal and violently comprest spring, and would presently burst asunder, some into two, and some into more pieces: of which he presented many to his friends, but yet had reserved some, whereof he presented me one, that I have yet by me, together with some of the mass, whose Cement I find to bear a better polish than mar∣ble, and to be very much harder than it. And, in answer to some questions of mine, he told me, that he had taken up these Stones himself, naming the place to me, which was not very far off, and that he observed all that he told me himself, and more than once or twice, and that I needed not sus∣pect, as I seemed to doe, that 'twas the strokes employed to force the

Stones out of their Beds, that made them break. For, besides that many of them, which (it seems) were not comprest enough, did not break, se∣veral of those, that did, were taken out, without offering them any such violence, as that their bursting could with any probability be imputed to it.