DIFFUSION. 



kind. Wfg forced away from them temporarily by the violence of the molecular 

 If these diMOciated molecules form a small proportion of the whole, 



the velocitv of their paoage through the medium must be much greater than 

 the mean Telocity of the whole, which is the quantity calculated by Kohlrausch. 



0* Proce*** by trA/cA the M'urt'nv and Sejxtration of Fluids can be 

 effected in a Reversible Manner. 



A physical process is said to be reversible when the material system can 

 be made to return from the final state to the original state under conditions 

 which at every stage of the reverse process differ only infinitesimally from the 

 conditions at the corresponding stage of the direct process. 

 All other processes are called irreversible. 



Thus the passage of heat from one body to another is a reversible process 

 if the temperature of the first body exceeds that of the second only by an 

 infinitesimal quantity, because by changing the temperature of either of the 

 bodies by an infinitesimal quantity, the heat may be made to flow back again 

 from the second body to the first 



But if the temperature of the first body is higher than that of the second 

 by a finite quantity, the passage of heat from the first body to the second 

 is not a reversible process, for the temperature of one or both of the bodies 

 must be altered by a finite quantity before the heat can be made to flow 

 back again. 



In like manner the interdiffusion of two gases is in general an irreversible 

 process, for in order to separate the two gases the conditions must be very 

 considerably changed. For instance, if carbonic acid is one of the gases, we 

 can separate it from the other by means of quicklime ; but the absorption of 

 carbonic acid by quicklime at ordinary temperatures and pressures is an irre- 

 versible process, for in order to separate the carbonic acid from the lime it 

 must be raised to a high temperature. 



In all reversible processes the substances which are in contact must be 

 in complete equilibrium throughout the process ; and Professor Gibbs has shewn 

 the condition of equilibrium to be that not only the temperature and the 

 pressure of the two substances must be the same, but also that the potential 

 of each of the component substances must be the same in both compounds, 

 and that there is an additional condition which we need not here specify. 



