What we have now stated may be regarded as the foun dation of the celebrated theory df latent heat ; which has been universally received, as affording a satisfactory ex planation of some of the most interesting phenomena that occur in the system of nature. A number of very impor tant consequences were immediately deduced from it, of which we may enumerate the two following, as those the most deserving our attention : 1. When solids become liquid, and liquids become elas tic fluids, a quantity of heat, previously latent, becomes sensible, and may be now measured by the thermometer, although before it was not capable of affecting this instru ment. On the contrary, when vapours are converted into liquids, or liquids into solids, a portion of heat, which was previously in the uncombined state, is now absorbed or rendered latent. In the former process, therefore, warmth is produced ; in the latter, cold is generated.
2. Substances which are at the same temperature actu ally contain different quantities of heat. Ice, just before liquefaction, and the water which is formed, are both at the temperature of 32°, and steam is no hotter than the boil ing water from which it is produced ; so that the water and the steam both contain a large quantity of heat, in the com bined or latent state, which they have absorbed in order to cause the fusion of the one, and the vaporization of the other. This property in bodies, of indicating the same temperature when they contain different quantities of heat. is denominated their capacity for heat ; while the quantity of heat which they require to bring them to the same tem perature is called theirispecific heat, in opposition to t he real quantity, which is styled their absolute heat.
It would carry us far beyond our prescribed limits, were we to give an account of all the experiments which were perforthed by Black, for the purpose of establishing his theory, and of repelling the objections against it. They consisted, in the first place, in comparing the effect pro duced by the same quantity of heat upon bodies which pos sessed different capacities, as ice and water. When the same weight of ice and water, both at the temperature of 32°, were suspended in an atmosphere of 47°, the water rose to 39° in 30 minutes, while the ice required ten hours to become liquid, and to acquire the same temperature. The ice, therefore, was 21 times as long as the water in acquiring the same degree of warmth, and we may con clude that it would absorb 21 times as much caloric. While the number of degrees gained by the water was no more than seven, the number that entered the ice would he 147° ; but of these only seven were employed in warming the substance, the rest were expended in melting it. Hence
we should say, that when ice is converted into water, 140° degrees of caloric are rendered latent. The same gene ral conclusion was obtained by a different process. When two equal portions of water, at unequal temperatures, are mixed together, the mixture indicates the mean tempera ture of the two portions ; thus a pound of water at 32°, and a pound at 172°, produce a mixture which indicates 102° ; but if we mix a pound of ice at 32°, with a pound of water at 172°, the ice is melted, but the temperature of the fluid is not raised. • Herice the 140° of heat are all em ployed in thawing the ice, and thus become latent.
The absorption of heat that takes place when solids are melted, or liquids evaporated, and the discharge of heat that occurs in the reverse operations, are often employed in different processes, for the purpose of cooling or warm ing bodies. The effect of evaporation in generating cold is well known, both as it affects the sensations, and as it actu ally reduces the temperature of substances. In hot climates, fluids and various articles of diet are preserved at a tem perature considerably below that of the atmosphere, by plac ing them in vessels, from the surface of which evapora tion is continually going forwards ; and by the proper ma nagement ofothis process, even ice is formed in considera ble quantity, for the purposes of domestic economy, at a temperature some degrees above the freezing point. By means of the evaporation of ether, water is easily frozen at the medium temperature of our climate, and indeed a degree of cold may be generated far below that of the freez ing point. A still greater diminution of temperature is pro duced by a solution of certain salts ;f a circumstance which depends partly upon the heat that is rendered latent by the conversion of a solid into a liquid, and partly by the mixture possessing a greater capacity for heat than the in gredients in their separate state. To these two circum stances, the change of state which bodies experience, and the change in their capacity, as depending either upon this alteration in their state, or upon the new combinations into which they enter, we ascribe the increase of temperature which occurs in a variety of chemical operations, where caloric is not introduced from any external source. The heat, which is excited by combustion, is to be attributed to the latter cause, the change of capacity in the sub stances; the products of combustion, particularly the car bonic acid, being supposed to possess a less capacity than the carbon and oxygen before their combination.