Chemical Affinity in terms of Electromotive Force. 283 



gravity-cells it is almost impracticable to prevent traces of 

 copper reaching the zinc after some twenty-four hours at 

 latest : the effect of the deposition of even the faintest traces of 

 copper on the zinc surface is to cause a considerable fraction 

 of the energy due to the solution of the zinc to become non- 

 adjuvant, and thus materially to diminish the E.M.F. Thus, 

 for instance, the following figures were obtained with one cell, 

 and similar ones in numerous other cases: — ■ 



E.M.F. of gravity-cell newly set up: zinc plate ")-..-. ™ 

 wholly free from copper J 



After 8 hours: faint tarnish visible on the zinc . 1*095 

 „ 24 hours: slight film of copper on the zinc . 1*070 

 „ 48 hours: thick film of copper on the zinc . 1*045 



In much the same way, the presence of even small quantities 

 of impurities in the zinc causes an appreciable diminution in 

 the E.M.F. In all the above-described observations, some of 

 the purest zinc that could be bought was employed, being- 

 fashioned into plates and rods by melting in a porcelain 

 crucible, pouring out on a fire-clay tile, and cutting into slips 

 with a chisel, &c. When commercial sheet or cast zinc was 

 used, or when pure zinc was amalgamated with imperfectly 

 purified mercury, the E.M.F. developed when such zinc was 

 opposed to a given copper plate was often very materially less 

 than the value obtained with pure zinc, or pure zinc and 

 pure mercury. Thus, whilst values varying between 1*111 

 and 1*116 were obtained with pure amalgamated zinc opposed 

 to fresh electro-copper as above described, values varying 

 from 1*080 to 1*109 were obtained with commercial zinc 

 amalgamated with pure mercury, and with pure zinc amal- 

 gamated with impure mercury, similarly opposed. 



It is abundantly evident from the above-described results 

 (not to speak of those detailed later on), that " the E.M.F. 

 of a Daniell cell " is a unit of comparison subject to decidedly 

 wide limits of fluctuation ; but that it is possible to reproduce 

 a standard cell of the kind within a maximum limit of 

 variation of about +0*25 per. cent., by using Raoult's form of 

 construction together with a recently electro-coppered or 

 amalgamated copper plate, and a pure zinc plate amalga- 

 mated with pure mercury, the zinc- and copper-sulphate 

 solutions used being both of the same specific gravity, the 

 precise value of the specific gravity being immaterial. Even 

 when made, however, such a standard cell cannot be relied 

 on for more than a few hours. It will be shown in a 

 subsequent paper that whilst Latimer Clark's mercurous- 

 sulphate cell is subject to an even wider range of fluctuation 

 in E.M.F. than the best forms of Daniell cell, its permanence 



