The condensible vapours, such as the steam of water, are more expansible than the permanent gases.; and it has therefore been conceived, that they will acquire a greater degree of beat when they are permitted to en large themselves, by removing from them the pressure of the atmosphere. Mr Watt proved that this was the case, in some experiments which he performed on the distilla tion of fluids in vacuo; a process which has been recom mended as an economical project, in consequence of the smaller quantity of caloric which is necessary, under these circumstances, to convert the liquid into a vapour. But this he found was counterbalanced by the greater capacity for heat of the vapour, when in its most rarefied state. By taking off a considerable part of the atmospherical pressure, water could be distilled at a temperature of 100°; but its latent heat then appeared to be 1048°, instead of the latent heat of steam generated in the atmos phere. With the most perfect vacuum that could be formed, water may be distilled at 78° ; but then the la tent heat is still farther increased, so as to be about 1300°. IVIr Watt estimated, that when water is converted into a vapour, under the ordinary pressure of the atmosphere, it is expanded into about 1800 times its former bulk. Phil. Trans. 1784, 335 ; and Black's Lectures, vol. i. p. 190.
Dr Crawford, from his experiments on the specific heat of bodies, was led to form the conclusion, that while their state remains unchanged, their specific heat continues to bear the same relation to their absolute heat. Taking water as the standard of comparison, if we can discover what proportion the specific heat of this fluid bears to mercury at the usual temperature of the atmosphere, we may suppose that the same proportion will exist at all tem peratures. Upon this principle was founded the proposal of Irvine, for ascertaining the real zero, or that point of the thermometric scale at which, if it be supposed to be sufficiently extended, bodies would be deprived of the whole of their heat. Thus it had been found by experi ment, that the capacity of ice is to that of water as 9 to 10, and the actual quantity of latent heat in water was esti mated at 140° ; therefore ten times 140°, or 1400° below the freezing point, was supposed to be the temperature at which water is absolutely deprived of all heat. (Nichol son's Chemistry, p. 16 ; Crawford On .4n. Heat, p. 435.) The proposal of Dr Irvine is certainly ingenious ; but we conceive the problem to be one, which we are still very far from being able to solve with any degree of accuracy. Indeed, the fundamental positions, upon which the whole hypothesis rests, are themselves very dubious, and the experiments which have been performed on the subject by different philosophers, have been attended with such very discordant results, as to show, that either the theory or practice most he extremely imperfect. Besides Irvine's, the principal experiments which were performed, are those of Crawford, Gadolin, Lavoisier in conjuction with La place, and Seguin. Of these, Gadolin's seem to have been executed with the greatest care and accuracy, and they approach the most nearly to those of Irvine. His method was to ascertain the capacity of common salt, and also of its solution in water, and then he observed the de gree of cold which is produced during the solution. Ile afterwards examined the quantity of heat which was ab sorbed by the mutual action of common salt and snow, and the heat extricated by the mixture of sulphuric acid and water ; and, by comparing these with the capacity of the bodies before their union, he endeavours, as before, to de termine the proportion which their absolute bore to their specific heat. (Crawford on 4n. Heat, p. 457, et seg.) Lavoi
sier and Laplace employed the calorimeter, to measure the quantity of heat disengaged during the mixture of certain substances, comparing this with the capacity of the bodies separately ; but it would appear, that their experi ments were more liable to inaccuracy than those of Gado lin, and their results are completely at variance with each other : -dead. Scien. 1780, p. 384, et seq.) Mr Dal ton has attempted to discover the real zero, by a calcula tion depending upon his idea of the constitution of an aeriform fluid, and the relation which subsists between its basis, and the caloric which enters into its composition. `Without attempting a detail of his hypothesis, we shall merely state his conclusion, that the absolute quantity of caloric in elastic fluids at different temperatures, is in the direct ratio of the cube roots of their bulk at the same temperature : (Manch. film. vol. v. p. 601.) Ilow far this hypothesis may have any foundation in fact, we do not feel at present competent to decide ; but it has been asserted, that experiments made upon this principle at different temperatures, and upon various substances, do not cor respond to each other.
From the remarks that have been made, it will appear, that Dr Black's discovery of the existence of what he called latent heat, or of a difference in the capacity of bo dies for heat, is a fact completely established by experi ment, and one of the most important in the whole range of chemical science. Very different opinions, however, have been entertained respecting the manner of accounting for the fact, and especially the question has been warmly agi tated, whether the absorption of heat when a solid is con verted into a liquid, is to be regarded as the cause or the effect of this change of state. Dr Black conceived that it was the cause of the change ; that the body in questioo, as, for example, a portion of ice, being exposed to the influ ence of caloric, this principle necessarily enters into it, be comes combined with its particles, and is not capable of producing any effect until it be again extricated by the li quid changing its form, or entering into some new combina tion. Dr Irvine adopted the contrary opinion, and regard ed the absorption of heat as the consequence of a change of capacity which the ice undergoes when it is converted into water, which change of capacity necessarily causes it to absorb, and render latent a quantity of heat : (Black's Lett. vol. i. p. 194.) Dr Irvine's opinion seems to be supported by the fact, that there actually is this difference of capacity in the substance ; hut then it does not appear to assign cause for the change of state which the body undergoes. If we adopt it, we are obliged to assume as a principle, that when the solid is so situated as to receive above a certain quantity of heat, its particles are 'suddenly forced to a con siderable distance from each other, in consequence of some thing peculiar in its nature and constitution ; and that when this separation has been effected, the capacity becomes in creased, and the caloric of course absorbed. Dr Black conceived, that the caloric was united to the body by a pow er similar to chemical attraction; and if we adopt this opi nion, it would follow, that at certain temperatures substan ces acquire different degrees of affinity for heat, and form with these quantities an intimate union, which renders it no longer cognizable by the ordinary means.