CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
607 
will be remembered that a good many other cells showed a fall at first, but most of 
them recovered in time. 
The observations on July 22 show a tendency to recover here also. 
The mercury used in the cells was the purest attainable, and the cells contained 
visible crystals of ZnS0 4 .7JI 3 0 of the proper form. It was not possible, therefore, 
to attribute the fall of E.M.F. to impurity of the mercury or to supersaturation, and 
some other source had to be sought for the difficulty. 
It was noticed that the grey deposit had formed on the zincs, and we had seen (see 
Table VI., April 20) that by removing this deposit arise in the E.M.F. was produced. 
From this, and from some experiments which were being made on two cells, Nos. 80 
and 81, it appeared probable that the fault lay at the zinc pole; that, in order that 
the cell should, have its proper E.M.F., some action between the zinc and the soluble 
mercurial compounds was necessary, and that this action did not occur when the 
grey deposit was formed all over the zinc, but only when part, at least, of the zinc 
surface became bright. It is not clear whether the necessity for this action arises 
from impurities or from some other unexplained cause. 
Clark liquor-- 
Zinc filings 
Paste ' 
-Zinc rod 
Pure saturated Zinc 
Sulphate 
--Crystals of ZnSO + 7H 2 0 
N Parchment paper 
membrane 
Crystals of ZnSQ J .7H,0 
The diagram (fig. 4) shows how Nos. 80 and 81 were constructed. There is inside 
the cell surrounding the zinc rod a small glass tube, closed at its lower end by a 
membrane of parchment paper. On this is a layer of zinc filings. The tube sur¬ 
rounding the zinc is filled with saturated solution of zinc sulphate with an excess of 
crystals, and dips into the ordinary Clark liquor in the outer cell. Thus none of the 
soluble mercurial compounds in the liquor can reach the zinc pole, for in diffusing into 
the inner cell they have to pass the zinc filings which would react with them. 
The cell is not really a Clark cell for the liquid in the glass tube is not a saturated 
solution of zinc sulphate and mercurous sulphate. 
Observations on these cells showed that they were always considerably lower than 
the standard. After they had been compared the zinc of No. 80 was removed and 
amalgamated ; the cell then became correct. This is shown in Table XI. 
