PRINCIPLE OF INITIAL AND FINAL STATE. 115 



the real work of the chemical forces (affinities) must be referred to 

 the reaction of perfect gases taking place at constant volume ; that is 

 to say, that the components and the compounds must all be brought 

 to the state of perfect gases and react in an unvarying space. 



In the cases in which the reaction of the gases with formation 

 of gaseous products gives rise to a change of volume at constant 

 pressure the heat liberated necessarily varies with the tempera- 

 ture ; but the variation is slight enough to be neglected, as long 

 as we consider intervals of temperature which are not very far 

 apart, and even up to 100 or 200. 



Table I. (p. 125) gives the principal data known on the subject. 

 It expresses the heat liberated in reactions between gaseous 

 bodies at constant pressure with formation of gaseous products. 



3. In default of these conditions, which it is rarely possible 

 to realise, it is permissible to refer the reactions of the bodies to 

 the solid state; as has already been done in the case of the 

 specific heats, according to the law of Dulong. In this state 

 the influences of the external pressure and changes of tempera- 

 ture become only slightly sensible, and in consequence all 

 bodies are more comparable than in the other states. The 

 quantities of heat liberated hardly vary as long as the interval 

 between the temperatures at which the reactions are carried out 

 does not exceed 100 to 200. 



4. There remain the following definitions: we shall term 

 exothermal every reaction which liberates, and endothermal every 

 reaction which absorbs heat. 



SECOND PRINCIPLE THE CALORIFIC EQUIVALENCE OF CHEMICAL 

 TRANSFORMATIONS; OTHERWISE TERMED PRINCIPLE OF THE 

 INITIAL AND FINAL STATE. 



If a system of simple or compound bodies, under given con- 

 ditions, undergo physical or chemical changes capable of bringing 

 it to a new state without giving rise to any mechanical effect 

 exterior to the system, the quantity of heat liberated or absorbed by 

 the effect of these changes depends solely on the initial and final 

 state of the system. It is the same whatever the nature or the 

 sequence of the intermediate states may be. This principle is 

 demonstrated by the aid of the preceding, combined with the 

 principle of energy. From it there follow various very important 

 consequences, such as the following, which are simply stated, 

 those who wish to go more fully into this subject being referred 

 to the author's " Essai de Me'canique Chimique." 



1. General Theorems on Reactions. 



Theorem I. The heat absorbed in the decomposition of a body is 

 exactly equal to the heat at the time of the formation of the same 

 compound, since the initial and final states are identical. 



This relation has been pointed out by Laplace and Lavoisier 



