﻿152 M. C. Marignac on the Specific Heats, Densities, 



proximately the measure, of the work produced by the chemical 

 reaction determined by the change in the temperature of the 

 liquid. 



We ought hence to conclude that,, for example, when a mix- 

 ture of water and alcohol is heated, there is produced between 

 these bodies a reaction by which heat is absorbed, while solu- 

 tions of saline compounds undergo, when their temperature is 

 raised, a chemical transformation which disengages heat ; finally, 

 certain solutions (as the aqueous ones of ammonia and sugar, 

 and the solutions of simple bodies in sulphide of carbon) should 

 present no change in their chemical constitution at different 

 temperatures. 



It would be venturing too far into the domain of hypothesis 

 to attempt to state precisely the nature of the chemical reactions 

 that may be determined by change of temperature in a solution. 

 It may be that they affect only the state of combination, more 

 or less intimate, of the solvent and the body dissolved ; but it 

 is possible that they may be of a nature more profound, and in 

 certain cases affect even the composition of the body dissolved — 

 for example, that a chloride may be transformed into a hydro- 

 chlorate. It is certain, however, that if two classes be made, 

 the first including those bodies the dissolving of which induces a 

 considerable change in their specific heats, the second those 

 which are not so affected, it will be remarked that the latter are 

 of such a nature that it is impossible to suppose any alteration 

 produced in their chemical constitution, and that nothing seems 

 to indicate that they can form definite combinations with the 

 solvent* ; while the same cannot be said of the saline compounds, 

 acid or basic, all of which belong to the first class. 



Besides, I do not pretend to be the first to advance this hypo- 

 thesis. I cannot say where it is first to be found; but the 

 same idea is expressed by M. Pfaundler in a note on the specific 

 heats of the first three hydrates of sulphuric acidf. Having 

 ascertained that the addition of one molecule of water to the 

 monohydrated acid augments the specific heat by a quantity 

 equal to that of the water added, while the addition of a se- 

 cond molecule only increases it by a little more than half 

 that quantity, he thence concludes that probably the action 

 of the heat determines a chemical modification in the solution 



* Ammonia (which, from M. Thomsen's experiments, belongs to this 

 second class) seems, at first view, to form an exception, its chemical cha- 

 racter giving an air of probability to the notion that it would combine 

 with water ; nevertheless the properties of the solution, and its total de- 

 composition by spontaneous evaporation, have always caused the rejection 

 of the idea that it could be regarded as a combination. 



t Journ.fur prakt. Chem. vol. ci. p. 507. 



