﻿and Expansions of some Liquids. 151 



hydrochloric acid containing 24*5 per cent, of acid, or one mole- 

 cule for 6*25 of water, has a specific heat about 11 per cent, less 

 than that of the water alone which it contains. 



This is a very remarkable fact. Indeed, some years ago* 

 M. H. Kopp showed, after a general recapitulation of the re- 

 searches made by himself and his predecessors on the specific 

 heats of combinations, that the molecular specific heat of any 

 definite compound, organic or inorganic, is nearly equal to the 

 sum of the specific heats of its elements, — the differences being 

 either of the same order as the errors of the determinations or 

 as the differences which may result from a simple change in the 

 physical state of one and the same body. We know, besides, 

 from the experiments of M. Regnault on alloys (that is, on the 

 combinations which, by their indefinite composition, seem to 

 come the nearest to solutions), that they are subject to the 

 same approximate law. 



It seems, then, that we are justified in regarding this as a 

 general principle. How is it that it does not apply to solutions ? 

 Two hypotheses may be made on this subject : — 



This anomaly may be due to a purely physical cause. The 

 laws resulting from the observations of MM. Regnault and 

 Kopp refer to the solid state of compound bodies ; perhaps they 

 are true for that state only. 



But if this were the fact, all solutions ought to behave in the 

 same manner and present an analogous diminution of specific 

 heat in proportion as the dilution is increased. Now we have 

 just seen that it is not so. There is sometimes an increase in 

 the specific heat ; and in other cases it remains equal to that of 

 the liquid mixed. The chemical nature of the bodies evidently 

 exercises a great influence on these phenomena; they must, 

 then, have a chemical cause. 



We are thus led to the second hypothesis. The specific heat 

 of a solution ought always to be equal to the sum of the specific 

 heats of the bodies mixed, except the small variations resulting 

 from modification of the physical properties (cohesion, dilatabi- 

 lity, &c). But most solutions have a very unstable chemical 

 constitution, and one that varies both with the degree of dilu- 

 tion and with the temperature. Every change of temperature 

 involves, then, a change in the chemical constitution, a chemical 

 work, which is itself the source of an absorption or a disengage- 

 ment of heat. This, of necessity, augments or diminishes by so 

 much that which must be transmitted to the solution to change 

 its temperature, and consequently its apparent specific heat. 

 Therefore the difference between the specific heat of a solution 

 and the sum of those of its elements will be the proof, and ap- 

 * Annalen der Chernie und Pharmacie, Suppl. vol. iii. p. 1. 



