ed.

The Gaffendifts and ancient corpufcularians required only three conditions as neceffary thereto, viz. a fmallness and fmoothness of the particles of the body, vacuities interfperfed between them, and a spherical figure.

The Cartefians, and, after them, Dr. Hooke, Mr. Boyle, &c. befides the circumftances abovementioned, require a various, perpetual, inteftine motion of the particles of the body, as that which principally contributes to fluidity.

According to these philofophers, therefore, fluidity confifts in this: That the parts of the body, being very fine and fmall, are fo difpofed, by motion or figure, as that they can eafily flide over one another's furfaces all manner of ways; and that they be in conftant, various, feparate agitation to and fro; and that they only touch one another in fome parts of their furfaces.

Mr. Boyle, in his Hiftory of Fluidity, mentions these three, as the principal conditions required to fluidity:

1. The minutenefs of parts: As, in effect, we find that fire, by dividing metals into fine, fmall parts, renders them fluid; and that acid menftruums diffolve and render them fluid after the like manner; and that fire turns the hard body of common falt almost wholly into a liquor, by diftillation Not but that the figure of the particles may have a confiderable fhare in fluidity,

Thus mercury, whofe parts are, doubtlefs, much groffer than thofe of oil and water, is yet more fluid than either of them: And thus oil, by the action of fire, may be converted into a confiftent fubftance, like butter.

2. Store of vacuities interfperfed

mong themselves.

3. A motion and agitation of the corpufcles, either from fome principle of mobility within themselves, or from fome extraneous agent, penetrating and entering the pores, moving varioufly among them, and communicating to them part of its motion.

That this laft is the qualification chiefly required in fluidity, he argues from divers obfervations and experiments:

Thus, a little dry powder of alabafter, or plaifter of Paris finely fifted, being put in a veffel over the fire, it foon begins to boil like water, exhibiting all the motions and phænomena of a boiling liquor. It will tumble varioufly in great waves like that; will bear ftirring with a stick or ladle, like that, without refifting; nay, if strongly stirred near the fide of the veffel, its waves will apparently dafh against them; yet is it, all the while, a dry, parched powder.

The like is obferved in fand: A difh of which being fet on a drumhead brifkly beaten by the sticks, of on the upper ftone of a mill, it in all refpects emulates the properties of a fluid body: A heavy body, e. gr. will immediately fink in it to the bottom, and a light one emerge to the top. Each grain of fand has a constant vibratory and dancing motion; and, if a hole be made in the fide of the dish, the fand will spin out like water.

That the parts of fluids are in continual motion, the Cartefians bring divers confiderations to prove: As, 1, The tranfmutation of folids into fluids, e. gr. ice into water, and vice verâ; the chief difference between the body, in those two ftates, confisting in this, that the parts, being fixed and at reft in the one, refift the touch; whereas, in the other, being

B

already

ed in a clofe and an open veffel, and that the veffel, closely confined, receives, without boiling, fo much greater degree of heat than the other; it muft of confequence follow, that, the more the water is at liberty to evaporate, the lefs degrees of heat it can contain without boiling. Now all water, even in an open veffel, is neceffarily preffed with the weight of the atmofphere, which is an obstacle to its evaporation; and, therefore, the bubbles of the vapour must not only o vercome the refiftance of the water above them, but also the preffure of the air on its furface; and, confequently, there requires a greater degree of heat to make water boil in the air, than in an exhaufted receiver; and this we find to be true from experience; the water when freed from the weight of the air, boils in a degree of heat only fufficient to keep it lukewarm, when expofed to its action: Betwixt these two extremes there is an infinity of cafes, and, the greater the preffure of the atmosphere is, the greater will be the difficulty of making the water boil: This has been likewife confirmed by the experiments of Meffieurs de Thury and le Monnier.

It also follows, from what we have already faid, that, the more easily a fluid may be reduced into vapour, the lefs heat will be requifite to make it boil ; and, on the contrary, those fluids which cannot be reduced to vapour, cannot likewise be forced to boil, whatever degree of heat is communicated to them: This is confirmed by experience; for fpirit of wine boils with a lefs degree of heat than water, or even mercury; and melted metals, though less heavy than the latter, will fooner confume in the fire than boil. A kind of ebullition may, however, be procured, by introducing to the bottom a matter capable of fmoaking, or being reduced into vapour. Lead, when melted and very hot, may easily be brought to boil, by plunging a flick into it, the end of which, whilft burning, produces a

fmoke, or (which answers to the fame purpofe) a vapour; but great care fhould be taken not to introduce water, for a cubic inch only of that fluid, rarefied by a degree of heat fufficient to melt copper, would take up about 14000 inches; and, confequently, caufe fuch a quantity of the melted metal to fly over the furnace, as might prove of very bad confe. quence. For which reafen, the founders are very careful in keeping off all moisture from the moulds into which they run their metal; experience having fufficiently taught them how dangerous a thing it would be to neglect this precaution. By a confequence equally neceffary, if we boil a liquor in a matrafs, the neck of which being long enough, that the liquor which is at its extremity may have a degree of heat far different from that which toucheth the bottom of the matrafs where the fire is applied, we shall see bubbles arifing from the bottom of this matrafs, which will not rife to the top, the vapours condenfing in their paffage through the more cold part of the fluid. This phænomenon cannot, without fome difficulty, be observed in the ebullition of water, because it will be neceffary to make use of veffels of an enormous length; but M. Nollet has often obferved it, in changing barometers; for bubbles generally arife from the bottom, expofed to the fire, but lofe themselves in the length of the tube, without breaking on the furface.

By mixing together liquors capable of being differently evaporated, we may arrive at compofing a fluid, fufceptible of feveral fucceffive ebullitions. Every body may obferve, that, when fresh butter in a fauce-pan is fet on the fire, it begins, almoft, inftantaneously, to boil; but this boiling noise foon ceafes, and there paffeth a very perceptible interval before the butter really boils. The first boiling is only owing to a certain portion of ferum of the milk remaining in the butter; this, being reduced to vapour, raifes the butter which floats on its

furface,

furface, giving it all the appearance of a boiling fluid; but it is entirely evaporated fome time before the butter begins to boil.

The above theory is founded on the following experiments made by the Abbe Nollet for that purpose:

I took, fays he, the glass of a thermometer, the ball of which was twothirds full of mercury, the outfide being very clean and thoroughly dry; I tied it to the end of the tube, which was not fealed, a bladder, as may be feen in the figure 3. I made the mercury to boil for more than a quarter of an hour over a chafingdifh of coals; and although I faw the mafs perpetually agitated by large bubbles which refembled large quantities of air raifing to the furface of the liquor, the bladder was but very little diftended; began to fink again on removing the glafs, and recovered its firft fize, when the mercury and the glafs were entirely cold.

:

Above a chafingdifh of live coals, without flame or fmoke, I hung up,, with two pack-threads, a cylindrical glafs veffel, very thin, which contained about a pint of water, the veffel being not above two-thirds full; the ftaff of fupport, which resembled a gibbet, flid in a box made on purpose at the end of the table where the chafingdish ftood, and could be ftopped by the means of a screw at any heighth This apparatus is reprefented by figure 6. I immediately ftopped, at the heighth of 8 inches above the fire, the veffel that contained the water, and into which I had plunged a thermometer made on the principles of M. de Reaumur; this veffel having received thirty degrees of heat, I obferved the bottom to be covered with fmall, and fcarcely perceptible bubbles, fome of which de. tached themselves to rife to the fu. perficies of the liquor, and I faw others like them which formed themfelves round the ball of the thermo

meter.

Having fixed the veffel at 6 inches above the fire, in 40 degrees of hear,

I faw that the bubbles were larger, in greater number, and that there were others like them within the veffel.

I caused the heat to be augmented to 50 degrees, and having attentively examined all these bubbles with a large convex glass, I obferved that thofe were always the biggeft which detached themselves from the glafs, that they came from the bottom chiefly, that their places were not fupplied by any others in the stations they came from, and that, when they were come to the furface of the water, they burft like little bladders.

I placed the veffel at 5 inches diftance above the chafingdifh, and waited for 60 degrees of heat; then I obferved, that the bubbles were very much rounded, that most of them appeared of a line diameter, that the number of them was much lefs; and having gently ftruck the staff with the handle of a knife, more than one half of these bubbles difengaged themselves from the glafs, and rofe to the fuperficies of the water.

.

gure

gure 4, having put in the bottom of this veffel fome water to the height of 2 inches, and above that, 5 inches of olive-oil, I hung it up by two fmall wires, at fome inches above a chafingdifh of lighted coals, when I gently began to warni, with a great deal of caution, the part that contained the oil, by the means of a large candle, the flame of which I directed by the wind of a blow-pipe, being very careful not to let this flame touch the glafs, but only to communicate its heat at fome diftance: Thus I kept the upper part of the oil a little lefs hot than the boiling water, and waited till the water which was below had acquired 75 degrees of heat; then I perceived the fmall transparent globules which troubled the water, and remained imperceptible as far as its fuperficies; but when they had entered into the oil, they increased all of a fudden in fize, and their figure became confiderably altered. From whence it appears, that a liquor does not boil only because it becomes a vapour, but because its evaporated particles, even in the interior of the mals, receive a degree of heat capa

ble of rarefying them, and of extending them in a certain space.

Thus, being contented to fee my principle fquare with the phænomena of ebullition, I was going to put an end to this examen, when I bethought myfelf to make another proof, which is this:

I plunged a fmall thin glass bottle, half filled with water, into the middle of a veffel like to that of figure 6, containing water like that, and alfo fufpended over a chafingdifh of coals, towards which I brought it nearer by degrees to make it hot (fee fig. 5.) the water heated in both veffels, but with this difference, that it began to boil very faft in the biggest, and gave no manner of fign of ebullition in the other, although I had continued this experiment for more than half an hour. I was defirous of knowing if this difference was occafioned by the matter in the fmall vessel not having acquired a fufficient degree of heat, but the thermometer fhewed me that it was within half a degree as hot as that in the vessel which contained it.

[To be continued.]

Yours, &c. B. D.

in violating that piety which they owed to God, and destroying faith among men, the strongest pillar of human fociety.

As an oath is a religious affeveration, by which we either renounce the mercy, or imprecate the vengeance of Heaven, if we fpeak not the truth, we should be very careful that the truth, and that only, be fpoken. For, when a fuperior, who has a right of punifhing, is appealed to as a witnefs, his vengeance is alfo invoked, in cafe of breach of faith; and God, who

knows