M. H. Sainte-Claire Deville on Affinity and Heat. 373 



lieat of contraction, which is very great, and then some sensible 

 heat taken from itself and from the adjacent bodies. MM. Bussy 

 and Buignet have recently observed the same fact and others of 

 the same kind, especially in the mixture of hydrocyanic acid and 

 water. 



It is very probable that the preceding considerations would 

 apply to solid bodies, if we knew their latent heat of fusion, to 

 calculate formulae which give the heat of contraction. 



When a solid dissolves in water, it first absorbs the quantity 

 of heat necessary to melt it, then a certain quantity of heat 

 which increases with the proportion of the solvent, and which 

 corresponds to the extension of the body dissolved in its men- 

 struum. These facts, which are of great importance, were deter- 

 mined in the most rigorous manner by M. Person, and deduced 

 from observations made more especially on nitrate of potash. 

 To these thermal units, expended in the work of solution, must 

 be added the heat of contraction absorbed and determined by the 

 methods I have described. 



Thus the more a soluble substance is diffused in its solvent, 

 the more heat it has absorbed, which it has borrowed either at 

 the contraction (which appeared indefinite) in the act of solution, 

 or from the adjacent bodies when there is cooling during solution 

 and extension. Thus, though I use an expression paradoxical in 

 form, it may be affirmed that every body which cools while it 

 undergoes molecular modification, is really heated, by borrowing 

 heat from the adjacent bodies, and from itself first of all. The 

 latent heat which it possesses is increased to the extent of all the 

 heat which has disappeared either by the fact of spontaneous 

 cooling or by diminution of the heat of contraction. It is, in 

 fact, so much heat lost, and therefore changed into motion of 

 the molecules, or, if we like, into affinity, if we thus designate 

 the force which produces this motion. 



All this heat, lost apparently, will reappear completely when 

 the solution is made to undergo the phenomenon the inverse of 

 extension, that is to say, concentration. Hence, in the cycle 

 comprehending the solution of a salt at the ordinary tempera- 

 ture and its crystallization by spontaneous evaporation (suppo- 

 sing the salt to be anhydrous like nitrate of potash), all the quan- 

 tities of heat borrowed from without, and from the contraction 

 during the period of solution and of extension, will reappear and 

 be given out during concentration and crystallization. Hence it 

 might be. said that these solutions contain (and that will only 

 be true potentially) all the heat which water and the salt have 

 absorbed during the development of the phenomenon. 



Thus a body which cools spontaneously does really become 

 heated by all the latent heat Vhich its molecules fix ; and it may 



