190 



PHYSIOLOGY 



response ought to throw important light on the intimate nature of 

 excitation generally. It may be therefore advisable to consider more 

 closely the conditions which determine differences of potential in a 

 complex system of electrolytes. 



As a simple case we may take an ordinary concentration cell. 

 Two vessels (Fig. 29), A and B, are united by a glass tube C. A contains 

 a 10 per cent, solution of zinc sulphate and B a 1 per cent, solution of 

 the same salt. A rod of pure zinc is immersed in each limb. On 

 connecting the zinc by a zinc wire to a galvanometer a current is 

 observed to flow from A to B through the galvanometer, and therefore 



FIG. 29. 



FIG. 30. 



from B to A through the cell. A solution of zinc sulphate contains 



+ 

 partly undissociated ZnS0 4 and partly dissociated Zn and S0 4 ions. 



If a rod of zinc be immersed in a watery fluid the zinc tends to dis- 

 solve. The Zn passing into the fluid is, however, directly ionised, and 

 therefore carries a positive charge into the fluid, leaving the zinc 

 negatively charged (Fig. 30). This process of solution will rapidly 

 come to an end, since the positively charged ions in the fluid will 

 repel back into the zinc any ions which may be escaping from 

 the zinc. The amount of zinc actually dissolved in the fluid is 

 infinitesimal, the process of solution ceasing when the pressure (osmotic 

 pressure) of the Zn ions in the fluid equals what may be called the 

 ' electrolytic solution pressure ' of the zinc. The continued solution 

 of the zinc is therefore only possible when means are supplied for the 

 Zn ions in the fluid to get rid of their positive charges. 



In an ordinary Daniell cell the Zn ions which leave the zinc are 

 discharged by combining with the S0 4 ions passing to the zinc from 



