116 GENERAL PRINCIPLES OF THERMO-CHEMISTRY. 



as far back as 1780. It enables us to measure the chemical 

 work of electricity, of light, of heat, etc. 



Theorem II. The quantity of heat liberated in a series of 

 chemical and physical transformations accomplished successively or 

 simultaneously, in one and the same operation, is the sum of the 

 quantities of heat liberated in each isolated transformation, (all the 

 bodies being brought to absolutely identical physical conditions.) 



It is in this way that the heat liberated by reactions referred 

 to the solid state is calculated. 



Theorem III. If two series of transformations be carried out, 

 starting from two distinct initial states, and arriving at the same 

 final state, the difference between the quantities of heat liberated 

 in the two cases will be precisely the quantity liberated or 

 absorbed when the transformation is from one of the initial 

 states to the other. 



In this way is calculated the heat liberated by the union of 

 water with acids, bases, anhydrous salts, by the synthesis of 

 alcohols, etc. 



The same theorem is employed to calculate the heat liberated 

 by the transformation of an explosive substance, whenever this 

 transformation does not occasion a total combustion, but the 

 products are defined by analysis. In a word, it is sufficient to 

 know, first, the heat produced by the total combustion of this 

 substance, a heat which may be experimentally measured by 

 detonating the substance in pure oxygen; second, the heat 

 liberated by the total combustion of the products of explosion, 

 which may be calculated when these products are known and 

 well-defined. The difference between these two quantities 

 represents the value sought. 



Theorem IY. The same conclusion is arrived at when the 

 two initial states are identical, the two final states being different. 



This relation serves as base to a number of calorimetric 

 methods introduced into thermo-chemistry during the last few 

 years, because it renders it unnecessary to define the inter- 

 mediate states in complex reactions. 



It is specially applicable to explosive substances when com- 

 bustion is incomplete and gives rise to imperfectly known 

 products. In short, it is sufficient to detonate the substance, 

 first, in pure oxygen, which gives rise to total combustion ; then 

 in nitrogen, which yields incompletely burnt products. The 

 heat liberated in each of the explosions is measured, and the 

 difference between the two figures expresses the heat of com- 

 bustion of the products of the second explosion ; that is, the 

 energy capable of being utilised in total combustion. 



Theorem V. Substitutions. If one body be substituted for 

 another in a combination, the heat liberated "by the substitution is 

 the difference between the heat liberated by the direct formation of 

 the new combination, and by that of the original combination. 



