296 Dr. C. R. A. Wright on the Determination of 



plate. The actual value of the depreciation produced with 

 the stronger currents in either case, moreover, clearly indi- 

 cates that the diminution cannot psssibly be solely due to the 

 formation of stronger zinc-sulphate solution round the zinc 

 plate, and weaker copper-sulphate round the copper plate, in 

 the case of the smaller areas, than are produced with the 

 larger ones; and hence the conclusion is arrived at that 

 when, by reason of the production of a current, the E.M.F. of 

 a Daniell cell is diminished, and the energy developed becomes 

 proportionately non-adjuvant, the non-adjuvancy is ascribable, 

 not merely to actions taking place at the surface of the zinc 

 plate, but also, and more particularly, to those taking place 

 at the surface of the copper plate. In the former case, the 

 energy gained during the conversion of the metallic zinc of 

 the plate into solution of zinc sulphate makes its appearance 

 under such conditions partly as heat from the very com- 

 mencement of the action, and is never wholly manifested as 

 electric action expressible in volt-coulombs. In the latter 

 case, the effect of the chemical action of the cell is to set 

 free copper, which, in its transformation from the condi- 

 tion in which it is first set free (nascent copper) to the con- 

 dition ultimately assumed (more or less compact electro-de- 

 posited soft coherent metal), causes a gain of energy which, 

 like that due to the solution of the zinc, is partly manifested 

 as heat ah origine, and is never, under such conditions, 

 obtained wholly as electric action. The actual proportion of 

 the energy due to the solution of the zinc or to the agglome- 

 ration (or allotropic modification) of the copper which is thus 

 non-adjuvant, is variable within certain limits with the con- 

 ditions of the experiment, the nature of the plate- surfaces and 

 of the liquids in the cell, kc, but, cceteris paribus, is greater 

 the stronger the current : with very feeble currents (of 

 density not exceeding some 8 microamperes per square centi- 

 metre), the proportion of non-adjuvant energy is too small to 

 be measurable. 



Summary of Results. 



117. The above-described experiments, and the conclusions 

 to be drawn from them, may be thus summarized : — 



1. When a Daniell cell is constructed Avith equal-sized 

 plates of pure zinc and pure copper (either compact bright 

 metals, amalgamated plates, or plates covered with electro- 

 deposited metal) immersed respectively in solutions of pure 

 zinc and copper sulphates of the same specific gravity, 

 and is made to generate a current not exceeding in density- 

 some 8 microamperes per square centimetre, an E.M.F. is set 



