FORMATION OF SALTS. 117 



This theorem is applicable to reciprocal replacements among 

 the metals, the metalloids, bases, acids, etc. 



Theorem VI. Indirect reactions. If a compound yield one of 

 its elements to another 'body, the heat liberated by this reaction is 

 the difference between the heat liberated by the formation of the first 

 compound, by means of the free element, and the heat liberated by the 

 formation of the new compound, by means of the same free element. 



The theorem is applicable to indirect oxidations, hydrogena- 

 tions, and chlorinations, to metallurgical reactions, to the study 

 of explosive substances, etc. 



In the latter study it gives the difference between the heat of 

 combustion by free oxygen, and the heat of combustion by 

 combined oxygen. 



The oxidiser (nitrate, chlorate, bichromate, metallic oxide, etc.) 

 is not a simple magazine of oxygen, as was formerly said ; for 

 generally this oxygen has lost a portion of its energy, equivalent 

 to the heat of the first combination. In certain cases, on the 

 contrary, such as where potassium chlorate is employed, the 

 combined oxygen liberates more heat than the free would do. 



Theorem VII. Slow reactions. The heat liberated in a slow 

 reaction is the difference between the quantities of heat liberated 

 when the system of the components and that of the products of the 

 slow reaction are brought by the aid of the same reagent to the same 

 final state. 



This finds numerous applications in organic chemistry, in the 

 study of ethers, amides, etc. 



2. Theorems on the Formation of Salts. 



Theorem I. The heat of formation of a solid salt is obtained 

 by adding the heats liberated by the successive actions of the 

 acid on water (Dt at the temperature t\ of the base on water 

 (D7), and of the dissolved acid on the dissolved base (Q), then 

 by subtracting from the sum the heat of solution of the salt 

 (A), all being measured at the same temperature. 



In general, calling S the heat liberated in the reaction of a 

 system of solid bodies, transformed into a new system of solid 

 bodies, by means of a solvent, we shall have 



S = 2Dt + Q* - SA. 



D, D', Qt, A2, are obtained by experiment. They are quantities 

 such that all of them vary considerably with the temperature t ; 

 while the quantity S is almost independent of the temperature 

 at least, within very wide limits, as will be presently shown. 



Theorem II. The heat of formation of saline, add, and 

 alkaline hydrates is the difference between the heat of solution of 

 the anhydrous body and that of the hydrated body, in the same 

 proportion of water and at the same temperature. 



Theorem III. The heat of formation of a double crystallised 

 salt is equal to the difference between the heat of solution of the 



