154 ■ Royal Society. 



volume (calculated at 0° and ra, 760) is more condensed in the final 

 than in the initial system. But this condensation is always com- 

 prised within very narrow limits, such as K = 4 (formation of 

 arsenious acid by its elements), K=3, 2, 1|, &c. ; so the funda- 

 mental condition 



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



. . 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 by 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, by 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 reac- 

 tion between gaseous bodies calculated for atomic weights, and n 

 the number of atoms in the reaction, the pressure will diminish 

 only if 



W<655»(K-1). 



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

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

 by means of the preceding, I have not succeeded in discovering any 

 example of diminution 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 dissocia- 

 tion, 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 de- 

 duced from it. It is known that every direct reaction which can be 

 accomplished in a very short time between gaseous bodies with for- 

 mation of gaseous compounds, 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 pro- 

 position is the following : — 



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

 applied exclusively and without any loss to warm the products, is 

 such that an augmentation of pressure always takes place at a con- 

 stant volume, or, what is the same thing, an augmentation of volume 

 at a constant pressure. 



* This proposition is contained in a more general one, which I hare given 

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



