170 



PHYSIOLOGY 



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 from B to A through the cell. A 



Fio. 29. 



FIG. 30. 



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 dissolve. 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 possible only 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 dis- 

 charged by combining with the S0 4 ions passing to the zinc from 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 con- 

 tact 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 



