ELECTRICAL CHANGES IN LIVING TISSUES 



171 



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 dissolves 

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

 sulphate on A becomes progressively weaker, while that in B becomes 

 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 refrigerat- 

 ing 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- U V 



stances in solution, especially if the 

 two solutions be separated by a mem- 

 brane. Very large differences may be Fio. 31. 

 produced if this membrane be prac- 

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

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

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

 differences of potential of a concentration cell up to and beyond the extent 



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B 



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