DIFFUSION. 643 



Now, we may obtain complete equilibrium between quicklime and the 

 mixture containing carbonic acid if we raise the whole to a temperature at 

 which the pressure of dissociation of the carbonic acid in carbonate of lime is 

 equal to the pressure of the carbonic acid in the mixed gases. By altering 

 the temperature or the pressure very slowly we may cause carbonic acid to 

 pass from the mixture to the lime, or from the lime to the mixture, in such 

 a manner that the conditions of the system differ only by infinitesimal quan- 

 tities at the corresponding stages of the direct and the inverse processes. The 

 same thing may be done at lower temperatures by means of potash or soda. 



If one of the gases can be condensed into a liquid, and if during the 

 condensation the pressure is increased or the temperature diminished so slowly 

 that the liquid and the mixed gases are always very nearly in equilibrium, 

 the separation and mixture of the gases can be effected in a reversible 

 manner. 



The same thing can be done by means of a liquid which absorbs the 

 gases in different proportions, provided that we can maintain such conditions as 

 to temperature and pressure as shall keep the system in equilibrium during the 

 whole process. 



If the densities of the two gases are different, we can effect their partial 

 separation by a reversible process which does not involve any of the actions 

 commonly called chemical. We place the mixed gases in a very long horizontal 

 tube, and we raise one end of the tube till the tube is vertical. If this is 

 done so slowly that at every stage of the process the distribution of the two 

 gases is sensibly the same as it would be at the same stage of the reverse 

 process, the process will be reversible, and if the tube is long enough the 

 separation of the gases may be carried to any extent. 



In the Philosophical Magazine for 1876, Lord Rayleigh has investigated 

 the thermodynamics of diffusion, and has shewn that if two portions of different 

 gases are given at the same pressure and temperature, it is possible, by 

 mixing them by a reversible process, to obtain a certain quantity of work. 

 At the end of the process the two gases are uniformly mixed, and occupy a 

 volume equal to the sum of the volumes they occupied when separate, but 

 the temperature and pressure of the mixture are lower than before. 



The work which can be gained during the mixture is equal to that which 

 would be gained by allowing first one gas and then the other to expand 

 from its original volume to the sum of the volumes ; and the fall of temperature 



812 



