as a Standard of Electromotive Force. 133 



E.M.F. of cell. 

 Freshly electro-deposited surface of copper 1*105 volt. 

 Slightly oxidized, faint brown tinge ... 1*106 „ 



More oxidized , . . 1*107 „ 



Still more oxidized 1*109 „ 



Re- electroplated surface 1*105 „ 



Hence these and many similar experiments all teach that 

 oxidization, even the slightest trace, of the copper raises the 

 E.M.F. ; and that, in order to get the real value proper to the 

 combination, the copper must be electrotyped over with a thin 

 pure film of copper, and exhibit no trace of brown spots of 

 oxide, and be used at once. 



2. The Effect of Variation in the Zinc Surface and Condition. 



Numerous experiments have been made to investigate 

 whether there is any certain difference between zinc amal- 

 gamated or unamalgamated or cast or rolled. There is very 

 little, if any, certain difference between perfectly pure cast 

 zinc unamalgamated and the same amalgamated with pure 

 mercury. There are greater differences if the zinc is not 

 pure, and variations are introduced if impure mercury is used 

 for amalgamation, all of them uncertain in amount. 



The effect of oxidation of the zinc is to lower the electro- 

 motive force. If the bright surface of the zinc becomes 

 tarnished, it always shows a slightly lower E.M.F. The 

 smallest deposit of copper upon the zinc, due to diffusion of 

 the copper salt into the zinc, is indicated by a marked depres- 

 sion, amounting to 2 or 3 per cent. On the whole, the only 

 consistent values are obtained from chemically pure zinc with 

 a bright fresh untarnished surface, whether amalgamated or 

 not ; and the best results are given with pure zinc amalgamated 

 with pure mercury. In Dr. Alder Wright's memoir the 

 above conclusions are enforced by tabulated results of most 

 extensive experiments. 



On the question of amalgamation of the zinc we may quote 

 a note on the subject by M. G. Lippmann in the Journal de 

 Physique. There is an opinion expressed by some authors 

 that amalgamated zinc has a higher E.M.F. than unamal- 

 gamated, even if pure. If a plate of each of the two substances 

 be immersed in sulphate-of-zinc solution, a couple is formed in 

 which amalgamated zinc forms the negative pole, inasmuch as 

 it is the more readily acted upon by oxygen. Such, at least, 

 is the result obtained if ordinary commercial zinc is used and 

 ordinary sulphate-of-zinc solution. In a recent work, M. W. 

 Robb shows that if care be taken to use electro-deposited zinc, 



