has been converted into water in consequence of this hot substance parting with its heat, we have the means of finding the specific heat of the substance in A. A stop-cock connected with B serves to carry off the water formed through melting of the ice, and hence, by weighing this water, we know approximately the quantity of ice melted in B, and if we know the quantity of heat necessary to convert ice into water we can find the heat given out by the substance in A. The chief objection to this instrument is that we cannot measure accurately the amount of water produced since a certain amount remains adhering to the ice.

291. III. Method by cooling. The third means of estimating specific heat is by the method of cooling. If two similar substances be exposed to the same cooling influence it is manifest that the one which has the smallest specific heat will cool fastest. Thus, suppose that we have two thermometers with blackened bulbs of precisely the same size, the one being filled with mercury and the other with water; further, let these instruments both cool from a common temperature under precisely the same circumstances. It will be found that the mercurial thermometer will cool more than twice as fast as the water one. For although the weight of the mercury is more than 13 times that of the water, yet the specific heat of mercury is only one-thirtieth of that of water, and hence while the same amount of heat leaves both instruments in one minute, yet this heat will produce on the water thermometer only 13ths of that diminution of temperature which it produces on the mercurial one. The idea of measuring the specific heat of bodies first originated with Black, who was also the discoverer of latent heat; and many numerous and important experiments have since been made on this subject by a number of observers.

SPECIFIC HEAT OF SOLIDS.

292. Some of the latest experiments in this branch of the subject have been made by Regnault, who used the method of cooling. In these experiments the substance was reduced to a fine powder and enclosed along with a delicate thermometer in a vessel which was exposed to the cooling influence, Although every precaution was used the result of this process was not satisfactory; one objection was that the heat was not conducted sufficiently fast from the powder to the sides of the vessel which contained it. Another is that the specific heat of the same substance in the solid state depends to some extent on the mechanical treatment which it has received. Regnault has also investigated the specific heat of various solid substances by the method of mixtures.

293. Rise of specific heat of solids with temperature. It was first shewn by the experiments of Dulong and Petit that the specific heat of a solid is greater at a high temperature than at a low one. The following table embodies the results of these experiments.

It will be noticed that for all the substances in the above table the specific heat is greater at high temperatures, with the exception of platinum, for which the specific heat remains the same between the limits of the experiment. Pro

bably the reason of this is that the highest temperature of experiment was very much below the melting-point of this metal, and it has been found by Regnault that the variation of specific heat with temperature is much more rapid when the substance approaches its melting-point. M. Pouillet, by means of the method of mixtures, has obtained the specific heat of platinum at still higher temperatures. His results are as follows

The constancy of the specific heat of platinum renders this metal serviceable as a pyrometer, and a piece of platinum may be used for estimating the temperature of a furnace. When it has attained the temperature of the furnace it is taken out and plunged into a known quantity of ice-cold water. By means of the rise of temperature produced it is easy to calculate approximately the temperature of the platinum, and hence of the furnace.

294. Circumstances which influence the specific heat of solids. The specific heat of a solid has been found to depend on the mode of aggregation of its molecules and on the nature of the mechanical action to which it has been subjected. In general whatever augments the density diminishes the specific heat, and whatever diminishes the density augments the specific heat; and it is perhaps owing to expansion that the specific heat of a body increases with its temperature. The more carbon is divided the greater is its specific heat. The following table exhibits the specific heat, in their different states of aggregation, of carbonate of lime, sulphur, and carbon.

295. Regnault has determined the specific heat of a number of liquids by the following method. The liquid under experiment is contained in a reservoir R (Fig. 63) which is

T

P

S

Fig. 63.

immersed in the middle of a bath, and by agitating the water of this bath a definite temperature is communicated to the

liquid in R. By opening the stop-cock at r and bringing to bear at the same time an atmospheric pressure upon the liquid, it is driven through the tubes at r into a vessel contained in the calorimeter C. Having entered the calorimeter and having disposed of its surplus heat, the temperature of the water of the calorimeter is observed by means of the thermometer T, and this affords the means of estimating the specific heat of the liquid. The calorimeter is defended by means of a screen P from the heat of R.

Generally speaking a substance when liquid has a greater specific heat than when solid, a fact which was discovered by Irvine. Thus the specific heat of ice is only one-half that of

water.

296. Variation with temperature of the specific heat of liquids. The specific heat of liquids increases in general with the temperature and at a rate exceeding that of solids. Thus bromine has between -6° and 10°C the mean specific heat 0.10513, while between 13° and 58° it has the mean specific heat 0.11294.

The specific heat of water at various temperatures has been especially studied by Regnault, who has obtained the following result.

SPECIFIC HEAT OF GASES.

297. In this branch of our subject there are two sets of determinations to be made. We must find, in the first place, the specific heat of gas under constant pressure; and in the second place, the specific heat of gas under constant volume.