172 



PHYSIOLOGY 



concentration cell already contains Zn ions. Since their pressure in the 10 per 

 cent, solution is greater than in the 1 per cent, solution, fewer Zn ions will 

 leave the zinc in A than in B. The negative charge on the Zn in A will 

 therefore be less than that on the rod in B, and positive electricity will there- 

 fore flow from A to B. This will disturb the equilibrium at the surface both 

 of B and A, so that Zn ions will be deposited from the fluid on the surface of 

 the zinc in A and will continue to pass from zinc into solution in B. At the 

 same time there is a movement of S0 4 ions, set free at the surface of A 

 towards B. The ultimate result, therefore, is that the zinc in B dis- 

 solves and the same amount of zinc is deposited on A. The solution of 

 zinc sulphate on A becomes progressively weaker, while that in B be- 

 comes stronger, until finally the concentrations in the two limbs are 

 identical and the current ceases. In this process no chemical energy is 

 involved, the energy set free by the conversion of zinc into zinc sulphate 

 in B being exactly balanced by the energy lost by the deposition of zinc 

 from zinc sulphate in A. Yet the current which is produced has a certain 

 amount of energy which can be utilised for heating a wire through 

 which it is made to pass. Since this energy must be taken from the 

 cell, the cell is cooled during the passage of the current. We have 

 here a close analogy with the case of compressed gases. If the 10 

 per cent, and 1 per cent, solutions were mixed together in a calorimeter, 

 no change of temperature would be produced, since no work is done 

 in the process. In the same way no cooling effect is observed if compressed 

 gas be allowed to expand into a vacuum. If, however, the compressed gas 

 be allowed to expand from a narrow orifice against the pressure of the external 

 air, so that it does work in the process, it is cooled, and this cooling effect is 

 made use of in the working of refrigerating machines or for the liquefaction 

 of gases. We may therefore regard the concentration battery as a machine 

 for making the substances in solution do work as they expand from a strong 

 into a dilute solution. 



The differences of potential obtained from an ordinary concentration 



cell are very small and would not 

 suffice to account for such a high 

 electromotive force as is set up, e.g. 

 in the contraction of a muscle. We 

 have seen earlier, however, that even 

 in isosmotic solutions differences of 

 pressure may be brought about by 

 differences in diffusibility of the sub- 

 stances in solution, especially if the 

 two solutions be separated by a mem- 

 brane. Very large differences may be 

 * produced if this membrane be practic- 



ally impermeable to one or other of the dissolved substances. In the same 

 way a semipermeable membrane, i.e. a membrane with different permea- 

 bilities for the different ions of the two solutions, may suffice to bring the 



FIG. 31. 



