Tlie Estimation oj Acidity. 239 



lively charged ions to the solution. The other is a tendency for 

 the dissociated zinc to abandon the ionic form, and to appear upon 

 the rod in the metallic state; but while this latter will be recog- 

 nised as the osmotic pressure of the dissolved metal, the former 

 has been termed by Nernst the " electrolytic solution pressure " 

 of the metal in question. 



It will be evident then, that where the osmotic pressure of the 

 •dissolved salt exceeds the electrolytic solution pressure of the 

 metal, the plus charges imparted in this way to the rod will cause 

 it to become positively charged with respect to the solution. 



It must be understood that owing to electrostatic attraction, 

 the plus charged ions cannot move more than an infinitesimally 

 small distance from the rod unless an external circuit be pro- 

 vided, when a current will pass until equilibrium is established. 

 Until the external circuit is closed we must assume the existence 

 of a " Helmholtz double layer." 



We may perhaps make the matter clearer and trace a closer 

 correspondence between theory and practice by studying the 

 development of an e.m.f. in such a cell as the Daniell. 



The Daniell cell consists of a glass vessel, within which is 

 •placed a porous cylinder. 



The outer vessel contains a solution of copper sulphate in 

 which is immersed a copper plate, while the inner vessel is sup- 

 plied with a solution of zinc sulphate in which rests an electrode 

 of zinc. 



If electrical connection be made between the zinc and the 

 copper plate, a current will follow, and a galvanometer inserted 

 in the circuit will indicate an e.m.f. of 1.1 volt. 



The production of this e.m.f. may be explained thus:' — 



Copper exhibits in solution a strong tendency to abandon the 

 ionic form, and its electrolytic solution pressure is therefore low, 

 being lower than its osmotic pressure. In consequence of this 

 a copper electrode is positive to solutions of its salts. 



On the other hand, as we have seen, the zinc readily gives off 

 positively charged ions to its solution, and its charge is therefore 

 negative. 



The combination in the Daniell cell, i.e., copper — copper sul- 

 phate; zinc — zinc sulphate, may be represented graphically thus: 



