1892.] The Clark Cell as a Standard of Electromotive Force. 65 



It may be well to explain the purpose of some of the precautions 

 advised in the circular. The mercurous sulphate, as ordinarily pur- 

 chased, contains some mercuric sulphate. When this is moistened 

 with water it is resolved into a yellow basic mercuric sulphate 

 (turpeth mineral) and a soluble acid mercuric sulphate. The first, 

 at any rate in moderate quantities, does not affect the E.M.F. ; the 

 latter greatly hinders it from attaining the proper value. Repeated 

 washing, however, removes most of this soluble salt. The paste, 

 when made, is shaken with mercury to remove any traces of the 

 acid sulphate which may be left, for the acid mercuric sulphate 

 attacks the mercury and forms mercurous sulphate. 



Careful precautions are necessary to ensure that the solutions should 

 be saturated with both zinc and mercurous sulphates, but the solu- 

 tions should not be raised in temperature above 30° C, for the zinc 

 sulphate may then crystallise out in the wrong form. The proper 

 crystals have the composition ZnS0 4 .7H 2 0, and are rhombic. 



But while we have had no serious difficulty with any of the cells 

 prepared in accordance with the last form of the memorandum, 

 some of the other cells we have set up have led to some interesting 

 results. 



Two sets of cells were put up with great care by Mr. Wilberforce 

 in March and April. One of us (S. S.) set up some cells in the same 

 way about the same time. The solutions were prepared from very 

 pure materials, following Lord Rayleigh's instructions. The zinc 

 sulphate was remarkably free from acid, and it appeared as if the 

 results ought to be good. 



In the first set, ISTos, 36 — 41, the E.M.F. was too low. At the end 

 of a month it was much too low, about 0'005 volt, and Mr. Wilber- 

 force noticed that a dull-grey deposit covered the zincs ; he therefore 

 removed them and scraped off this deposit, when, on replacing the 

 zincs, the cells were found to have approximately the normal E.M.F. ; 

 they have continued nearly normal since. The next set, Nos. 42 — 47, 

 were very good when first set up, but the E.M.F. soon fell rapidly, 

 until at the end of a month they were nearly O01 volt too low. 



The grey deposit again was formed over the zinc. Some of these 

 cells were left untouched till August, by which time the E.M.F. had 

 recovered somewhat, being then about 0'005 too low. Others had 

 been treated by removing the zincs and replacing them by amalga- 

 mated zincs. In August some experiments were made on the unal- 

 tered cells, which showed conclusively that it is necessary that the 

 surface of the zinc should remain bright if consistent results are to 

 be obtained. 



This bright surface may be secured by amalgamating the zinc, but 

 we are not yet sure that this alone is effective, for it seems possible 

 from various observations that some action which results in the 



VOL. LI. F 



