118 GENERAL PRINCIPLES OF THERMOCHEMISTRY. 



double salt and the sum of the heats of solution of the component 

 salts, increased by the heat liberated ly the mixture of the 

 solutions of the separate salts, the whole at the same temperature 

 and in presence of the same quantity of water. 



Theorem IV. The heat of formation of acid salts is calculated 

 in a similar manner. 



Theorem V. Changes of state of precipitates. The difference 

 between the quantities of heat liberated or absorbed during the re- 

 dissolving of a precipitate, under two different states, at the same 

 temperature, and in the same solvent is equal to the heat brought 

 into action when the precipitate passes from one state to another. 



Theorem VI. Influence of dilution. The heat of formation of 

 dissolved salts varies in general with the dilution and temperature. 

 The variation of this quantity of heat with the dilution at a given 

 temperature is expressed by the formula 



M' - M = A - (8 -f S')> 



M being the heat liberated by the reaction of an acid and a 

 base, taken at a certain degree of concentration at this tempera- 

 ture; M', the heat liberated by the same reaction, the two 

 bodies being taken at a different degree of concentration ; 

 A the heat liberated (or absorbed) when the solution of the salt 

 is brought from the degree of concentration corresponding to 

 the first reaction to the concentration corresponding to the 

 second. S and ' are the analogous values, which correspond to 

 the respective changes of concentration of the acid and of the 

 base, always at the given temperature. 



From a suitable degree of dilution, such as 1QOH 2 to 1 

 equiv. of an acid or of a base, the variation M' M generally 

 reduces itself to negligable quantities, that is to say, within the 

 limits of experimental error. But it should be remarked that 

 the variation M' M ceases to be negligable, even within these 

 limits, for salts formed by the union of bases with alcohols or 

 weak acids, or by the union of any acid with wedk bases, such 

 as the metallic oxides. For such salts, moreover, the variation 

 M' M tends to reduce itself to A, because S and ' become 

 inappreciable. Thus 



Theorem VII. Under these conditions the heat of dilution of 

 the salt represents the variation in the heat of combination. 



This action of water constitutes a true characteristic of weak 

 acids and bases. The preceding theorems are applicable not 

 only to salts but to every compound, or system of compounds 

 solid or in solution. 



Theorem VIII. The reciprocal action of acids on the salts which 

 they form with the same base, in presence of the same quantity, of 

 water, may be expressed at a given temperature by the relation 



K! - K = M - M 1? 

 M, M! being the heats liberated by the separate union of the 



