S. L. Harding—Bichromate of Soda Cell. - 61 
Art. IX.—The Bichromate of Soda Cell; by SELWYN Lewis 
HARDING. 
In view of the recent claims of superiority of bichromate.of 
soda as a depolarizing agent over its companion salt, bichro- 
mate of potash, I have made a series of comparative tests of 
the two salts, the results of whieh are contained in the follow- 
ing paper: 
The points to be examined were the relative constancy or 
powers of endurance, the electromotive force, and the resist- 
ance of the two batteries. 
The cells, in every case, were set up in the manner known as 
the Bunsen battery, namely, dilute sulphuric acid and zine 
outside, bichromate solution and carbon inside, the porous | 
cup. The proportions used were those given by the chemical 
reactions which take place within the cells; these are: 
Na,Cr,O,+4H,SO,+6H=Na,Cr,(SO,),+7H,O (A) 
and similarly 
K,Cr,O,+4H,SO,+6H=K,Cr, (SO,),+7H,0, (B) 
giving, from (A) 
27 parts of Na,Cr,O, 
40 parts of H,SO, 
and from (B) 3 parts of K,Cr,O, 
4 parts of H,SO 
For every part of the bichromate of soda, seven parts of 
waterewere used, and for every part of the bichromate of potash 
eight parts of water.* 
The liquid surrounding the zine was a mixture of twelve 
parts of water to one of sulphuric acid. 
Constancy.—VTo answer the question of the relative merits of 
the two bichromates in regard to their constancy or powers 
of depolarizing, I obtained a series of photographic records of 
cells set up with these salts. The method of obtaining these 
records was, in brief, the substitution of a sheet of sensitive 
paper—driven by clock-work and properly protected from ex- 
traneous light—for the ground-glass scale of the ordinary re- 
flecting galvanometer ; the place of the zero point on sucha 
scale being taken by a base line traced out by the reflection 
from a fixed mirror.t 
The records thus obtained give a history of the current 
strength, showing its fluctuations and gradual weakening, from 
* The result of later work indicates that a much less quantity of water would 
be preferable with the soda salt. 
+ For details, this Journal, vol. xxix, p. 374, May, 1885. 
