210 Prof. E. F. Herroun on the Divergence of 



but the effect of solution of the oxides of nitrogen or their 

 evolution as gas, together with other effects of dilution of the 

 acid, complicate the subject from the theoretical standpoint, 

 and their ron-reversibility renders them unsuited for ex- 

 periment. Cells of the Daniell type are therefore greatly 

 to be preferred, both for theoretical consideration and direct 

 experiment. 



Thanks to the careful and accurate data furnished by the 

 published researches of Dr. C. R. A. Wright and Mr. C. 

 Thompson, and their clearly tabulated results*, one can see 

 at a glance whether the electromotive force furnished by 

 two metals immersed in solutions of their corresponding salts 

 accords with, exceeds, or falls below the value calculated 

 from thermochemical data, and the exact amount of the 

 discrepancy. 



The explanations which might be given of the cause of the 

 departure of observed E.M.F.s from the calculated values in 

 different cells may be classified under the following heads : — 



1. Certain cells in which the anticipated chemical change 

 does not occur, but some other, different, reaction which 

 evolves heat, to an extent which would account for the 

 observed E.M.F. This is brought about by the special 

 behaviour of a metal surface, and probably depends upon — 



2. The coating of a metal surface by films of oxide or 

 subsalts either more or less readily reduced than the normal 

 salt of that metal. 



3. The effect of dissolved gases, notably oxygen, in the 

 solutions of the metallic salts. 



4. The hydration or solution of the salts formed in some 

 cases, but not in all, supplementing or diminishing the E.M.F. 

 due to the reaction of the solid salts. 



5. The evolution or absorption of sensible heat in the 

 cell, and the loss or gain in electric energy, corresponding 

 thereto. 



In illustration of case 1 may be mentioned a cell consisting 

 of aluminium in aluminic sulphate opposed to zinc in zinc 

 sul])hate, in which the thermochemical data would indicate 

 that aluminium would be the metal attacked, giving, when 

 opposed to zinc, an E.M.F. of about 1 volt ; whereas^ in 

 practice, zinc is the metal which is attacked, and the cell 

 furnishes an E.M.F. of about '53 volt. But we know that 

 the chemical reaction Al2(S04)3 + Zn3 = 3(ZnS04) + Al2 does 



* " The Determination of Chemical Affinity in terms of Electromotive 

 Force," pt. ix., Proc. Phys. Soc. vol. vi. pt. iv., and Phil. Mag. ser. 5, 

 vol. xix. No. 118. 



