gained during the Mixing of Gases. 313 



compress the hydrogen from the original volume v x + v 2 to the 

 volume v x if no steam were present, together with the work 

 necessary to compress the steam from v l +v 2 to v 2 if no 

 hydrogen were present. And since every step of this process is 

 reversible, the same amount of work might be gained in making 

 the mixture, and is dissipated if the mixture is allowed to take 

 place by free diffusion. 



The same argument will apply when the condensation of one 

 of the gases is effected by chemical means. Suppose, for exam- 

 ple, that we have a mixture of carbonic anhydride and hydrogen 

 at a red heat, and that it is proposed to absorb the carbonic gas 

 with quicklime. It has been proved by Debray that at every 

 temperature above a certain point carbonic gas in contact with 

 quicklime and carbonate has a definite tension ; any excess will 

 be absorbed by the lime, and any deficiency supplied by a de- 

 composition of the carbonate. 



If the tension of the carbonic gas in the given mixture be 

 higher than that proper to the temperature, absorption will 

 take place in an irreversible manner. In order to prevent dissi- 

 pation, the mixture of gases must be first expanded until the 

 tension of the carbonic gas is no higher than that corresponding 

 to the temperature at which it is proposed to work. When the 

 contact is made, the mixture may be very slowly condensed, so 

 that after the point is passed at which chemical action commences, 

 the tension of the carbonic anhydride remains constant. This 

 process may be continued until nearly all the carbonic anhydride 

 is absorbed. The hydrogen may then be separated. The space 

 over the carbonate of lime must next be slowly increased until 

 the original quantity of carbonic gas has been again evolved, 

 when the connexion with the quicklime must be cut off. It now 

 only remains to reduce the separated gases to the same pressure 

 and to a total volume equal to that of the original mixture. 



From the preceding considerations we may, I think, infer that 

 the law above stated is general whenever the gases really press 

 independently; for it is difficult to see how its truth could depend 

 on what would seem to be the accident of the existence or non- 

 existence of a chemical capable of absorbing one or other of the 

 gases. 



It is worthy of notice that exactly the same rule applies for 

 the mechanical value of the separation of two gases, even when 

 the pressures are different; for we get the same result whether 

 we first before mixing allow the pressures to become equal and 

 add the work gained in this process to that due to the subsequent 

 mixing, or whether we calculate at once the work due to the se- 

 parate expansion of the two gases from their original volumes to 

 the total volume of the mixture. 



Phil. Mag. S. 4. Vol. 49. No. 325. April 1875. Y 



