Mr Leslie has proposed another method of ascertain ing the specific heat of bodies ; it consists in comparing the time which they occupy in cooling with the cooling of water placed under precisely the same circumstances. The apparatus which he employed was a thin glass globe, furnished with a neck capable of receiving a delicate ther mometer. He observed the time necessary for water to cool a certain number of degrees in this globe suspended in the air, then he fills the instrument with another fluid, the specific heat of which he wished to examine, and heats it to the same degree, and suspends it in air of the same temperature, and observes the time necessary for it to cool the same number of degrees. It is found, that, under the same circumstances, water cooled in 70 minutes, while oil required 32 minutes only ; therefore, by esti mating water at unity, as the standard of comparison, the specific heat of oil will be .46, if we estimate it by bulk, or .5, if we estimate it by weight. Inquiry, p. 170.
The great difficulty there is in executing experiments like those of Crawford's, where the substances are mixed together, and the' specific heat calculated from the tem perature of the mixture, induced Lavoisief and Laplace to attempt to solve the problem by a new method. They proceeded upon the fact, that when ice is melted, it must always absorb the same quantity of heat, and therefore that the caloric which is disengaged from any body, by a change in its form, or from the union of two or more bodies, may be accurately measured, by placing the substance in such a situation as that all the heat must necessarily pass into a stratum of ice ; and from the amount of water pro duced by the melting of the ice, the quantity of heat given out may be estimated. The apparatus which they invented to accomplish this purpose, they called a calorime ter ; it consists of three vessels inclosed one within another, so as to leave a cavity between each of them. The sub stance which is to he the subject of experiment is placed in the innermost vessel ; the second is filled with pounded ice, and is provided with a tube at the bottom, through which all the water that is formed is conducted into a suitable vessel, where it may be collected and measured. The outermost space is also filled with ice, in order to pre serve the interior of the apparatus at the proper tempera ture: (Mem. Scien. 1780, p. 368.) This instrument is constructed upon scientific principles, and seems both simple and ingenious; yet we are informed that there are some circunistal ces which interfere with its practical utility, and we believe it has never been employed except by the inventors. See Wedgewood in Phil. Trans. 1784, p. 37l, et seq. ; and CeemisTav• The capacity of gaseous bodies for heat, like that of liquids and solids, varies according to nature of the individual gas; but the determination of their respective capacities requires a delicacy of expel iment, proportion ate to the difficulty of operating upon substances of so subtle a nature. It has, however, been attempted by Craw ford, and prosecuted by him with much perseverance and ingenuity, and made the foundation of some very important doctrines, both in chemistry and physiology. It must in deed be confessed, that the extreme nicety of the process necessarily makes, the conclusions liable to much uncer tainty ; yet the general results have been, for the most part, acquiesced in, although they have been considerably altered since they were originally brought forward. Of late, however, they have been controverted by MM. Dela roche and B.rard, who have published a very elaborate set of experiments, which they performed upon the spe cific heat of the gases, in which they employed an appa ratus of a new construction. It is in fact a calorimeter; but instead of measuring the heat which is extricated by the quantity of ice which it will melt, they endeavour to ascertain what quantity of heat the gas will impart to a mass of water; and for this purpose a current of the gas is sent along a serpentine tube, which traverses a cylinder of water. The instrument appears to us to be very com
plicated, and to require a great share of manual dexterity in the operator, a circumstance which we always consider as very objectionable: (?Inn. Chico. lxxxv. 72.) But al though their results differ considerably from those of Craw ford, they agree with him in the general principle, that the specific heat of the gases is different from each other, whether we attend to their volumes or their weights, and that there is no relation between the specific gravity of the gases and their specific heat. See Description of Plates at the end of the Volume.
Besides the difference in their capacities for heat, which gases possess in consequence of a difference in their na ture, this property is also much affected by the degree of compression which they experience, or is in proportion to their density. It is consequently observed, both in expe riments performed for the pin pose, and in many natural processes, that the sudden coadensatitin of air generates heat ; and, on the contrary, that sudden dilation produces cold. Whenever an expansion of air takes place, either from the removal of pressure, or from any other cause, the particles are more widely separated from each other, in consequence of their elasticity being now at liberty to exert itself; and it would appear, that a quantity of calo ric necessarily rushes in to supply the vacuity. This, although merely a mechanical view of the subject, ap pears to be rendered probable by the fact, that the emis sion and absorption of heat are respectively produced by causes, which can only be supposed to act by bringing the particles of the gas nearer together, or removing them farther asunder. Ha thermometer be placed in the receiv er of an air pump, and the air be quickly exhausted, the mercury will sink several degrees; and if, on the con trary, air be rapidly condensed by the appropriate appa ratus, the mercury is considerably elevated. (Dalton in Manch. Mem. vol. v. p. 515.) The heat which is excited by condensing the atmosphere in the barrel of an air-gun is well known; and by employing it in the most advan tageous manner, sufficient heat may be extricated to set fire to a piece of tinder. if a part of the apparatus be composed of glass, a flash of light is perceptible, when the stroke of the piston is made with great force and rapidity. The effect of the rarefaction of air in promo ting the absorption of heat is well illustrated in Profes sor Leslie's new method of forming ice. (See our article COLD, for a full account of this process.) In this pro cess, water is placed in a shallow dish over a vessel con taining sulphuric acid ; the whole being enclosed in the receiver of an air-puilip, and a vacuum produced. As the exhaustion proceeds, the quantity of heat abstracted horn the water, i11 order to supply the evaporation, quickly converts the whole into ice. The sulphuric acid, in this case, promotes the effect, by its attraction for the aque ous vapour, which it absorbs as fast as it is generated. A still more striking effect of evaporation has been ex hibited by Dr Marcet. He enclosed a quantity of mer cury in a bulb of thin glass ; this was wrapped up in cot ton, which was soaked in the very evaporable fluid, the sulphuret of carbon ; the apparatus was then placed un der the receiver of an air-pump, and the exhaustion pro duced, when the mercury was completely frozen : (Phil. Trans. 1813, 252.) The ingenious instrument invented by Dr Wollaston, which be called the cryophorus, acts upon the same principle. It consists of a glass tube of some length, each extremity of which terminates in a globe about one inch in diameter, and bent at a right angle to the tube ; one of the globes is half filled with water, while all the remainder of the apparatus is carefully exhausted of air. if the empty globe be plunged into a freezing mixture, the aqueous vapour with which it is filled is so rapidly condensed, that a portion of the water in the se cond globe is immediately frozen. Phil. Trans. 1813, 71.