The cases in which the conver§ion of a body from the liquid to the solid form actually produces an increase of tem perature, are not very numerous ; but there are not unfre quent instances in which the rate of cooling is obviously retarded by this process. Andin no operation is this more remarkable than in the natural freezing of water. When water is exposed to the influence of external cold, its tem perature sinks in proportion as the heat is removed from it, until it arrives at 32° ; the refrigeration is then suspend ed, but the water begins to become solid, and the tempe rature remains stationary, until the freezing is completed ; then the cooling recommences, and is continued until the ice arrives at the temperature of the surrounding medium. The freezing of water, however, under particular circum stances, affords an example of the actual evolution of heat. This takes place in that case which has been described above, where water has been cooled below 32° without be coming solid ; if it be then agitated, or a small spicula of ice be introduced into it, the congelation takes place with great rapidity, and the portion of water which still remains fluid instantly rises to 32°. An experiment of an analo gous kind may be performed on the crystallization of salts. By proper management, a warm saturated solution of the sulphate of soda may be cooled for several degrees, without any of it beginning to crystallize ; if the vessel be then slightly shaken, the proces. suddenly commences, and a large part of the salt immediately becomes solid, in consequence of which the temperature will be very sensi bly raised. The general law, both in this and the reverse operation, is, that the most powerful effect, either of heat ing or cooling, takes place when the process is performed with the most rapidity, when the substance that has its state or its capacity changed, has its equilibrium with the neighbouring bodies restored in a short space of time, and therefore most perceptibly affects their temperature.
The same kind of experiments which Black performed, to establish the absorption of heat, when a solid is convert ed into a liquid, he afterwards made on the conversion of a liquid into an elastic fluid. He also attempted to ascer tain the exact quantity of heat which is rendered latent in this operation, in which he was essentially aided by Mr Watt ; and the result of their inquiry was, that when water assumes the state of vapour, it absorbs 950° of caloric.* This number he obtained in two ways : 1st, By comparing the heat necessary to raise the temperature of a certain portion of water to the boiling point, with the effect pro duced by an equal addition of heat in afterwards evaporat ing the water ; and, 2dly, By finding what quantity of ca loric was extricated, when steam is reconverted into water by condensation. We have already mentioned, that when water is strongly compressed, as by being inclosed in Pa pin's digester, its temperature may be raised far above the boiling point, without its assuming the aeriform state. In this case its tendency to evaporation is mechanically pre vented, by the particles not being allowed to separate from each other ; and therefore, as it cannot alter its form, its capacity remains the same, and its caloric all continues to be in the uncombined state. But if the pressure be sud
denly removed from the water, its particles now being at liberty to expand themselves, they unite to a portion of the heat, instantly assume the elastic state, and the remainder of the fluid sinks to 212°.
We have already explained, in a general way, the meaning of the term capacity for heat, and the difference between the absolute and the specific heat of bodies ; but we must illustrate the subject with a little more minute ness. As a foundation for our reasoning, it may be as sumed, that when equal quantities of the same substance, but at different temperatures, are mixed together, the temperature of the mixture indicates that of the arithme tical mean of the two ingredients. This, however, only applies to bodies of the same kind ; for when different substances are employed, it is impossible to predict what will be the temperature of the mixture. Thus if a pound of water and a pound of mercury he mixed at different temperatures, the result will not be the mean temperature ; but it will be found that the mercury loses 28°, while the water gains only I°. Water is therefore said to ha% e 28 times the capacity for heat that mercury has, because it requires 28 tunes as much to produce the same change of temperature ; or, to use a difkrent, and perhaps a more collect phraseology, we say that the specific caloric of water is to that of mercury as 28 to I. Proceeding upon this principle, and taking water as a standard of compari son, iminerous experiments have been nia•,e on the spe cific heat of various substances, particularly by Crawford, Irvine, and NVucke.
The method employed by Crawford was to mix together, at the same temperature, the substance to be examined and water, the specific heat of which is considered as one, being that with which all the rest are compared. He then multiplied the weight of each body by the number of de grees between its original temperate,' e and the common tem perature obtained by their mixture, and the capacities will he inversely as the products: (On Animal Heat 2 I edit. p 96, et seq.) It is generally more coe‘el.ient to employ a &finite weight of the substance to be examined, than to measure it by its volume; but when we examine the specific heat of equal volumes of different bodies, we find the same want of con es pondence between the results, so as to prove that the capacity for heat is a property which bears no exact ratio to any other physical quality. In the performance of these ex periments, much dexterity is requisite, in order to ensure even a tolerable degree.of accuracy ; and after all, the results of different trials, made upon the same substance, will not always be found to correspond, yet there seems to be DO doubt of the correctness of the principle, and, in many cases, we may conceive that we arrive at a near ap proximation to the truth. See CHEMISTRY.