310 SCIENCE PROGRESS. 



Electrochemistry.- -The most important researches in 

 electrochemistry have been concerned with the development 

 of Nernst's theory, which is one of the most important 

 advances of the new theory of solutions, an advance which 

 enables the electromotive force of a cell to be correlated 

 with the concentration of electrolytic solutions. The basis 

 of the theory is the assumption that when a metal is 

 immersed in a solution it exerts a definite solution pressure 

 P, whereby ions tend to pass into the solution. This 

 passage is opposed by the electrostatic attraction which it 

 sets up, and also by the osmotic pressure p of ions of the 

 . same kind present in the solution. The larger P, other 

 things being the same, the more strongly will the solution 

 be positively charged and the electrode negatively charged. 

 The potential difference between an electrode and a solution 

 is held, therefore, to depend upon two constants, one of 

 which, P, is due to the metal, and the other,/, to the cathion 

 concentration of the solution. If a cell be formed in which 

 the electrodes are the same, P may be eliminated from the 

 equations, and on neglecting the potential differences 

 between the solutions, the E.M.F. may be expressed in 

 terms of the osmotic pressures or the concentrations of the 

 solutions around the electrodes. Goodwin (173) has made 

 experiments on the chain — 



Tl I T1C1 and KN0 3 | T1C1 and KC1 | Tl. 



Here as in the simple case the cathion of the salt is the 

 same as that of the electrode. He also makes observations 

 on two chains consisting of 



Zn I ZnCL ! Hg 2 CL | Hg, 

 in which the concentrations vary, and which are opposed to 

 one another. Here the solution around the electrode con- 

 tains a sparingly soluble salt of the electrode in presence of 

 another salt with the same anion. In both cases theory 

 and practice agree, and a number of alterations in the 

 solutions, such as the substitution of salts of other metals 

 for those of potassium in case 1, or of bromides for chlorides 

 in case 2, are found in accordance with theory to have no 

 effect on the E.M.F. 



The E.M.F., or the conductivity, is a convenient mode 



