312 Affinity and Fleat. 
the product at first renders latent all the heat of contrac- 
tion, 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 formule which give the heat of con- 
traction. 
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 menstruum. These facts, which are 
of great importance, were determined in the most vigorous 
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 indefi- 
nite) 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 con- 
traction. 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 temperature and its crystallisation by 
spontaneous evaporation (supposing the salt to be an- 
hydrous like nitrate of potash), all the quantities of heat 
borrowed from without, and from the contraction during 
