788 
LORD RAYLEIGH ON THE CLARK CELL AS 
the appearance of the metallic surface. When the mercury is quite pure, the surface 
is as bright as in a thermometer bulb. I have instituted special experiments in 
order to ascertain the effects of impure mercury. On two occasions three cells 
have been charged alike in all respects, except that in the first vacuum-distilled 
mercury was employed, in the second ordinary mercury, and in the third mercury 
purposely contaminated with tin-foil (probably containing also lead). The results 
were not very distinct; but they indicate that impurity in the mercury is apt to 
depress the E.M.F. (e.g. by per cent.), and especially to make it irregular. I am 
disposed to attach great importance to purity of mercury, and believe it to be more 
essential than purity of zinc,* although I should not recommend the use of common 
zinc when the purer metal can be obtained so easily. Other cells have been prepared 
with mercury to which a little silver and copper (filings from a silver coin) were 
added. After the first week or two the E.M.F. of these cells was normal (to within 
TFoodh part). It is probable that metals more oxidisable than mercury are removed 
from it by the paste, as certainly happens in the case of zinc. 
§ 47. It has been abundantly proved by v. Helmholtz, Alder Wright and other 
workers, that the E.M.F. of Clark’s, Daniell’s, and similar cells, rises as the zinc 
sulphate solution is diluted. In some such cells to be discussed presently (39, 40), the 
E.M.F. is about 1^ per cent, higher than for a normal Clark. Dr. Hopkinson, some 
of whose cells have been compared with mine, writes that he is pretty clear that the 
worst irregularities of Clark’s cells are due to the zinc sulphate not being saturated. 
A cell 2 per cent, in excess could be made right by simply introducing crystals of 
ZnS0 4 . It is evident that sufficient care has not been taken in this respect in the 
preparation of cells sold to the public. 
§ 48. In this matter of saturation there is another danger to be encountered, to 
which my attention was first drawn in connexion with some cells prepared with great 
care by Mr. Mortimer Evans, and left in my charge by Sir W. Thomson. Of these, 
two were in practical agreement with mine; but the other eight, though in close 
agreement with one another, were too low (according to my standard) by rather more 
than four parts per thousand. And this state of things persisted without the smallest 
change for two or three months, during which tests were applied at intervals. 
Being anxious to examine a phenomenon to which my experience had afforded no 
parallel, I opened carefully one of the abnormal cells (To) to the extent of withdrawing 
the zinc. My idea was that possibly the zinc solution was supersaturated, in which 
case the E.M.F. might be expected to be too low. Attached to the zinc, however, 
were found what appeared to be crystals of sulphate ; but in order to be on the safe 
side, a few particles from the stock bottle were added to the cell, and the zinc was 
replaced. After the lapse of a few hours the E.M.F. was tested, and was found to be 
normal, as it has remained ever since. 
* Of ten cells, prepared at the same time as a set to be used in series, five were made Avith pure zinc 
and five with common sheet zinc. No difference in the performance can be detected. 
