446 M. Bcrthelot on the Change of Pressure and Volume [Apr. 27, 



2. The pressure depends upon the temperature evolved, and upon the 

 state of condensation of the products. Let us determine this quantity. 



Let t be the temperature produced by the real reaction, this being 

 effected at a constant volume, admitting that the whole of the disengaged 

 heat was employed in warming the products. 



Let V be the sum'of the volumes of the gaseous bodies in the initial 

 system at 0° and ro v60. 



At the temperature t, the final system contains in general certain gaseous 

 bodies. 



Let V be the volume of these bodies, supposed to be brought, without 

 changing their state, to 0° and m, 760. 



The relation -^=i expresses the condensation produced by the reaction. 

 V K. 



When certain bodies, contained in the initial system at 0°, or in the final 

 system at t°, are in the solid or the liquid state, you can generally neglect 

 their volume in comparison with that of the gas, when the pressures are not 

 too considerable. Let us calculate the pressure during the reaction which 

 takes place at a constant volume and at the temperature t, the initial 

 temperature and pressure being 0° and H. 



In admitting Marriotte's and Gay-Lussac's laws, the pressure will 

 become 



Hx i(i+«0; 



it will be greater than the initial pressure if 1 -fa£>K, less if l+a£<K, 

 or equal if l+at=K. Let us observe that t— — , Q being the quantity of 



heat produced in the reaction, and c being the mean specific heat of the 

 products between 0° and t°. 

 Let us develope this solution. 



3. The pressure augments when the condensation is null, for instance 

 chlorine and hydrogen, K=l ; and especially when there is dilatation 

 (combustion of acetylene by oxygen), t being always positive in a direct and 

 rapid reaction between gaseous bodies. 



4. On the contrary, the pressure diminishes if K is very great — that is, 

 in the case of a system containing gaseous bodies transformed entirely into 

 products which are in the solid or liquid state at the temperature deve- 

 loped hy the reaction. This case is more rare than one would think at 

 first sight, because very few compounds subsist wholly at the high tempe- 

 rature that would be developed by the integral union of their gaseous com- 

 ponents. Generally a portion of these remain free at the moment of the 

 reaction ; but in the present state of our knowledge it is impossible to 

 estimate the pressure corresponding to effects so complex. 



It is necessary to consider that the present case must not be confounded 

 with the case in which the products formed in the gaseous state and at the 

 temperature of the reaction are liquefied or solidified under the influence 



