1871.] 



produced by Chemical Combination. 



447 



of a subsequent cooling ; for instance, the formation of water from its ele- 

 ments, or of chlorhydrate of ammonia from hydrochloric acid and am- 

 monia, produce equally a diminution in the final pressure. 



5. In theory the most interesting case is that in which the initial and 

 final systems are wholly constituted of gaseous bodies, whose volume (cal- 

 culated at 0° and ,n, 760) is more condensed in the final than in the initial 

 system. But this condensation is always comprised within very narrow 

 limits, such as K = 4 (formation of arsenious acid by the elements), 

 K=3, 2, 1 j, &c. ; so the fundamental condition 



l+a^<K, or Q<273 (K-l) c, 

 c 



that determines a diminution of pressure, should be realized only In very 

 exceptional cases and when the heat evolved by an integral reaction is very 

 little. 



One can ascertain it in making the calculation by means of the specific 

 heats at constant volume (deduced with ordinary coefficient from the 

 specific heats at constant pressure which M. Regnault has determined for 

 many bodies). One can also make the calculation in a more general 

 manner, in admitting with Clausius that the specific heats at constant 

 volume have an identical value for the atomic weights of elements, that 

 this value is equal to 2, 4, the number found for H=l, and that it does 

 not change from the fact of combination. "W being the quantity of heat 

 produced in a reaction between gaseous bodies calculated for atomic 

 weights, and n the number of atoms in the reaction, the pressure will 

 diminish only if 



W<655n (K-l). 

 It is easy to see that this condition is not fulfilled in the combinations 

 best known. Calculating, either by means of this formula or by means of 

 the preceding, I have not succeeded in discovering any example of diminu- 

 tion of pressure among the numerous reactions I have examined in this 

 present research. 



Besides, it is sufficient to make the calculation for the reaction supposed 

 integral, the conclusion being generally the same for the reaction supposed 

 incomplete, that is to say in the case of dissociation, as it would be easy to 

 prove. 



6. Without further extending this discussion, I believe that a new 

 general proposition relative to chemical combination can be deduced from 

 it. It is known that every direct reaction which can be accomplished in 

 a very short time between gaseous bodies with formation of gaseous com- 

 pounds, produces a disengagement of heat : this is true for all reactions 

 evolved by chemical forces alone, acting without help of any work done 

 by exterior forces*. The new proposition is the following :-— 



The heat produced in a reaction of this sort, supposing it to be applied 

 * Tbis proposition is contained in a more general one, which I have given in 

 1 Annales de Chimie et Physique,' 4 me serie, t. xviii. 



