36 REPORT—1904. 
Tasty I1—Z.M.F. of Cadmium Cells minus EMF, of W 17. 
Differences in hundredths of a millivolt. 
Cell No. 52 Cell No. 53 Cell No. 54 
Date of ; 
Observation b. C. a b. | Ce a. b. CG: 
April 4, 1904 | +38 | —21 | —19| +38 | —14| -13] +38] +14] —18 
April 18, ,, +36:/-—80 | —20 | +39)|>—43e: —144l ce aiel eae ene 
May 3, ,, +30 |-—43 | —20.) 482.) — 14] 4 | aa ee ee 
June 13)... ,, +27 | —50 | —21 | +32 | —14 / SA Ba Son) nd 
uly, 995.5 +93 | —54 | —21" | +30'| —18 "96 eae 
The pastes 52a, 53a, 54a were prepared with the same sample of 
Hg,SO, ; it was purchased and prepared in a similar manner to the 
sulphates dealt with in Table I. 546 was also a purchased sulphate. The 
remaining specimens were prepared by Divers’ method. 
It will be observed that all the pastes change so as to reduce the 
E.M.F. of the cell ; but whereas the E.M.F. of the cells prepared with 
purchased sulphates is greater than that of W 17, those made up with 
the Nordhausen sulphates have in each case lower E.M.F.s. Cell No. 526 
is exceptional in the fall of its voltage. The difference in the prepared 
pastes, though small, condemns part of the method of preparation, and 
further investigation became necessary. 
The method of preparation adopted by Dr. Carpenter was at first 
repeated. Close observation showed that on formation the sulphate cakes 
considerably, and is accompanied at the surface of contact with the 
mercury by a compound of a light brick-red colour. If without freeing 
from the acid or SO, the product is added to distilled water, reduction of 
part of the sulphate apparently occurs, mercury is precipitated as a black 
powder, and the red compound entirely disappears. (The mercury thus 
precipitated is a valuable addition to the paste, the conversion of mercuric 
sulphate to the mercurous condition being rendered possible by its pre- 
sence.) The salt produced by freeing the first product from SO, also 
loses the brick-red tint, and is finally obtained as a pure white paste. On 
prolonged washing with water, however, hydrolysis results and the colour 
changes to pale yellow. Two samples of hydrolysed mercurous sulphate 
were thus prepared, the one being washed for one hour with water and 
the other for twenty-four hours. An experimental cell indicated that the 
more hydrolysed product if employed to set up a cadmium cell would 
cause the E.M.F. of that cell to be greater by 0-00064 volt than if pre- 
pared with the first sample. The presence of this hydrolysed product is 
therefore to be avoided, and washing by water prohibited. 
About this time, through the kindness of Professor Ayrton, the 
results of some experiments by Professor H. 8. Carhart and Mr. G. A. 
Hulett, of the University of Michigan, were communicated to the 
Laboratory. Professor Carhart has also sought a standard method of 
preparing the depolariser, and suggests that any prepared sulphate be 
washed with cadmium sulphate (or zinc sulphate for Clark cells) in order 
to prevent hydrolysis. Prior to this, Mr. Swinburne, in a letter to 
Dr. Glazebrook, suggested the precipitation of the sulphate intended for 
Clark cells from saturated solutions of mercurous nitrate and zinc 
sulphate, the washing to be effected with alcohol or saturated zine sul- 
phate solution. 
