10 CHEMICAL COMPOSITION. 



This pressure will be superior to, less than, or equal to, the 

 initial pressure according as 1 + at is greater, less than, or equal 

 to&. 



Q 



We should note that t = ; Q being the quantity of heat 







developed in the reaction and c the mean specific heat of the 

 products between zero and t. 



4. Further, the pressure increases if the condensation is nil, 

 that is if k = 1 (chlorine and hydrogen ; combustion of cyanogen 

 by oxygen). It increases especially if expansion occurs, that is 

 when k < 1 (combustion of acetylene by oxygen) assuming that 

 Q is positive in every direct and rapid reaction between gaseous 

 bodies. 



Now let Tc > 1 this condensation is always comprised between 

 certain limits for definite gaseous compounds, limits such that 

 K = 4, 3, 2, 1 J. Hence the fundamental condition, 



1 + a < or Q 



c 



a condition which is necessary for a diminution of pressure, 

 cannot be realised except in quite exceptional cases, in which 

 the heat disengaged by an internal reaction is very slight, and 

 beyond the scope of any observed reactions. We can assure 

 ourselves of this by making the calculation by means of the 

 specific heats at constant volume deduced from specific heat at 

 constant pressure which Eegnault has determined for many 

 bodies. 



5. The calculation may also be made in a more general 

 manner by admitting with M. Clausius that the specific heats 

 at constant volume have an identical value for the atomic 

 weights of the various simple bodies ; that this value is equal to 

 2 '4: a number which is found for H = 1, in fact, that it does 

 not change by the fact of combination. 



Now, W being the quantity of heat disengaged in a reaction 

 between gaseous bodies in relation to the atomic weights, and 

 M the number of atomic weights which are engaged in the 

 reaction, the pressure will only diminish if we have 



W < 655M(& - 1). 



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

 best known gaseous combinations. In making the calculation, 

 whether by the aid of this formula, or the foregoing, no example 

 has been discovered of diminution of pressure among the 

 numerous reactions which have been examined. 



It should be noted that it is sufficient to make the calculation 

 for the supposed total reaction, the result being the same for the 

 supposed partial reaction, that is to say, in the case of dis- 

 sociation. This can be easily proved, for the uncombined 



