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. II. (Book 2)

Observat. I. Men are not usually aware of the great efficacy of Celerity, even in small Bodies, and especially if they move but through a small space.

WHat a rapid Motion may en∣able a Body to doe, may be judged by the powerfull and de∣structive Effects of Bullets shot out of Cannons, in comparison of the Bat∣tering Engines of the Ancients, which, though I know not how many times bigger then the Bullets of whole Cannon, were not able to batter down Walls and Towers like Bullets, whose bulk compared with theirs is inconsiderable. Other examples

of a like nature might be without impertinency alledged on this occa∣sion; but, because the latter part of our Proposition contains that which I chiefly aim at, I shall proceed to Instances fit to prove That.

I have sometimes caused a skilfull Turner to turn for me an oblong piece of Iron or Steel, and placing my naked hand at a convenient distance to receive the little fragments, per∣haps for the most part lesser then small pins heads, as they flew off from the rod, they were, as I expected, so intensely heated by the quick action of the Tool upon them, that they seemed almost like so many sparks of fire; so that I could not endure to continue my hand there. And I re∣member, that once asking an expert workman, whether he (as I had some∣times done) did not find a trouble∣some heat in the little fragments of Brass that were thrown off when that metall was turning? He told me, that heat was sometimes very offen∣sive to his eyes and eye-lids. And

when I asked, whether it was not ra∣ther as Dust cast into them, than from their Heat; he replied, that besides the stroke, he could sensibly feel a trouble∣some heat, which would make even his Eye-lids sore: And that sometimes, when he employed a rough Tool, that took off somewhat greater Chips, he had found the heat so vehement, that not onely 'twould scorch his tender Eye-lids, but the thick and hard skin of his hands: for proof whereof he shewed me in one of his hands a little blister, that had been so raised, and was not yet quite gone off.

And inquiring about these matters of a famous Artist, imployed about the finishing up of cast Ordnance, he confess'd to me, That, when with a strong as well as peculiar Engine he and his associates turned great Guns very swiftly, to bring the surface to a competent smoothness, the tools would sometimes throw off bits of me∣tal of a considerable bigness, which, by reason of their bulk and their rapid motion, would be so heated as to burn

the fingers of the Country-people that came to gaze on his work, when he, for merriment sake, desired them to take up some of those pieces of metall from the ground. Which I thought the more remarkable, because by the Contact and Coldness of the ground I could not but suppose their Heat to have been much allayed. Not to mention, that I learnt from an experienced Artificer, that in turning of Brass the little fragments of that metall acquire an intenser Heat than those of Iron.

I remember also, that, to vary the Experiment mentioned just before this last, by making it with a bodie far less solid and heavy than Brass or Iron, I caused an Artificer to turn very nimbly a piece of ordinary wood, and holding my hand not far off, the powder, that flew about upon the operation, struck my hand in many places with that briskness, that I could but uneasily endure the Heat which they produced where they hit. Which Heat whether it were com∣municated

from the little, but much heated, fragments to my hands, or produced there by the brisk percus∣sion on my hand, or were the joint effect of both those Causes; it will however be a good Instance of the power of Celerity even in very small bodies, and that move but a very lit∣tle way.

'Tis considerable to our present purpose, that by an almost momen∣tany percussion, and that made with no great force, the parts, even of a vegetable, may be not onely intensely heated, but brought to an actuall Ig∣nition; as we have severall times tried, by striking a good Cane (of that sort which is fit for such Expe∣riments) with a steel, or even with the back of a knife. For, upon this Col∣lision, it would send forth sparks of fire like a flint.

To the same purpose may be al∣ledged, that, by but dextrously scra∣ping good loaf-sugar with a knife, there will be made so brisk an agita∣tion of the parts, that store of sparks

will be produced. But that is more considerable, which happens upon the collision of a flint and a steel: For, though Vitrification be by Chymists esteemed the ultimate action of the fire, and though, to turn sand or stones, though very finely poudered, into glass, 'tis usually required that it be kept for divers hours in the intense fire of a glass-house; and though, last∣ly, the glass-men complain, that they cannot bring flints or sand to fusion without the help of a good proporti∣on of Borillia or some other fixed salt: yet both actuall Ignition and Vitrification are brought to pass al∣most in a moment by the bare vehe∣mence of that motion that is excited in the parts of a flint when it is struck with a steel: For those sparks that then fly out, (as an Ingenious per∣son* 1.1 has observed, and as I have often seen with a good Micro∣scope,) are usually real and permanent parcels (for the most part globulous) ofstone vitrified and ignited by the vehemence of the motion. And that

this vitrification may be of the stone itself, though steel be a metal of a far more fusible nature then a flint, I am induced to think, because I have tried, that not only flints with steel, but flints with flints, and more easi∣ly pieces of Rock Crystal between themselves, will by collision strike fire. And the like effect of collision I have found my self in some precious stones, harder than Crystall. And after∣wards inquiring of an ingenious Arti∣ficer that cuts Diamonds, Whether he had not observed the like, when Dia∣monds were grated on by the rapid motion of his mill? He replied, that he observed Diamonds to strike fire al∣most like Flints; which afterwards was confirmed to me by another ex∣perienced cutter of Gems; and yet having made divers trials on Dia∣monds with fire, he would not allow that fire itself can bring them to fusion.

Nor are fluid Bodies, though but of small Dimensions, to be altogether excluded from the power of making considerable impressions on solid

bodies, if their celerity be great.

Whether the Sun-beams consist, according to the Atomical Doctrine, of very minute Corpuscles, that, con∣tinually issuing out of the body of the Sun, swiftly thrust on one ano∣ther in Physically-straight Lines; or whether, as the Cartesians would have it, those beams be made by the brisk action of the Luminary upon the con∣tiguous fluid, and propagated every way in straight lines through some Ethereal matter harboured in the pores of the Air; it will be agreeable to either Hypothesis, that the Sun∣beams, refracted or reflected by a burning-glass to a focus, do there, by their concourse, compose a small portion of fluid matter; and yet the Celerity, wherewith the soft and yielding substance is agitated, enables so few of them as can be circumscri∣bed by a Circle, not a quarter of an inch in Diameter, to set afire green wood in lesse than a minute, and (perhaps in as little time as that) to melt not onely Tinn and Lead thinly

beaten, but, as I have tried, foliated Silver and Gold.

The operation of small portions of fluid matter on solid bodies will be farther exemplified in the IV. Chapter, by the effects of the blown flame of a Lamp on glass and me∣talls; so that I shall here need but to point in general at the wonderfull effects that Lightning has produced, as well by the Celerity of its motion, as by the matter whereof it consists. Of which Effects, Histories and the writings of Meteorologists afford good store; and I have been an admiring observer of some of them, one of the last of which was the mel∣ting of metal by the flame in its pas∣sage, which probably lasted but the twinckling of an Eye.

And even a small parcel of Air, if put into a sufficiently-brisk motion, may communicate a considerable motion to a solid body; whereof a notable Instance (which depends chiefly upon the Celerity of the sprin∣gy corpuscles of the Air) is afforded

by the violent motion communica∣ted to a bullet shot out of a good wind-gun. For, when this Instrument is well charged, the strongly-comprest Air being set at liberty, and forcibly endeavouring to expand it self to its wonted laxity, its corpuscles give a multitude of impulses to the bullet, all the while that it continues moving along the barrel, and by this means put it into so rapid a motion, that I found by trial, the bullet would in a moment be flatted, almost into the figure of a Hemisphere, by being shot off against a metalline plate.

And farther to shew, How swift that motion must have been, and with what Celerity a vehement agitation may be communicated to the parts of a Solid body, I shall add here (though the Phaenomenon might be referred to the V. Chapter,) that, though the con∣tact of the Bullet and the metalline plate lasted probably but a physical moment; yet the minute parts of the bullet were put into so various and brisk an agitation, that making hast

to take it up before it should cool, I found it too hot to be with over∣much ease held between my fingers.

Notes

• * 1.1

  Mr. Hooke