ELECTRICAL CHANGES IN LIVING TISSUES 191 



the copper sulphate in the outer cell. It is a well-known fact that 

 pure zinc does not dissolve in acid until some other metal, such as 

 copper, is brought into contact with it, so as to set up an electric 

 couple, i.e. to provide means for the discharge of the Zn ions passing 

 into the solution. When the zinc is immersed in the two solutions 

 of zinc sulphate in the concentration battery, the same change will 

 occur. The ZnS0 4 solution in the two limbs of the 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 therefore 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 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 concentra- 

 tion 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 



