M. Berthelot on Chemical Combination. 



153 



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

 initial system at 0° and m -/60. 



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, /60. 

 1 



The relation — : 



K 



expresses the condensation produced by the 



reaction. 



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 gene- 

 rally 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. 



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

 become 



Hx I(I-+«0i 



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



l-\-at<K, or equal if l+a£=K. Let us observe that t— — , 



c 



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 in- 

 stance chlorine and hydrogen, K = l, — and especially when there is 

 dilatation (combustion of acetylene by oxygen), t being always posi- 

 tive in a direct and rapid reaction between gaseous bodies. 



4. On the contra^, 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 developed by the reaction. This case is more rare 

 than one would think at first sight, because very few compounds 

 subsist wholly at the high temperature that would be developed by 

 the integral union of their gaseous components. 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 pres- 

 sure corresponding to effects so complex. 



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

 founded 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 of a subsequent cooling ; for instance, the for- 

 mation of water from its elements, or of chlorhydrate of ammonia 

 from hydrochloric acid and ammonia, produces 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 



