ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 
443 
filled up above the level of the cross tube with saturated zinc sulphate (E), and a few 
crystals are added. Evaporation is prevented by corks (F), closing the upper ends of the 
tubes. Electrical contact with the amalgam and with the pure mercury is made by 
platinum wires (A), sealed into the glass. 
A preliminary experiment in which both legs of a cell were charged with amalgam 
(the mercurous sulphate being dispensed with) having shown that the E.M.F. was inde¬ 
pendent of the excess of undissolved zinc, two cells, H 2 , were set up on February 12, 
1884, and submitted to various tests, such as stirring up the amalgam with a glass rod. 
The amalgam was prepared from pure mercury and the same zinc as before. Sub¬ 
sequently, on March 6, six more cells were charged with a somewhat different 
treatment. The sulphate of zinc was from another sample and contained appreciable 
quantities of iron. Moreover, the amalgam was differently prepared. The mercury 
and zinc were shaken up together in a bottle with a little acid, after which the acid 
was washed out by shaking with several changes of water, until litmus paper was no 
longer reddened. Into each cell, in addition to the fluid amalgam, there was dropped 
a piece of solid zinc from the bottle. The same mercurous sulphate as before was 
employed, but the washing with distilled water was dispensed with. The three 
remaining cells of this pattern H 9 , H 10 , H n , were charged on March 12 , 1884, with a 
third gample of zinc sulphate. 
The agreement among themselves and the constancy of the H-cells has been all 
that could be wished ; but some modification in preparation will be desirable, for it 
has been found that the amalgam tends to harden into compact lumps, the expansion 
of which is liable to burst the cells. From this cause H 3 , H^, H 7 , succumbed at a 
comparatively early stage. It is probable that the addition of solid zinc to the fluid 
amalgam had better be omitted, but on this and other points we hope to make 
further investigation. The H pattern lends itself conveniently to experiment, as it is 
possible by withdrawing the corks to make any desired addition to the contents. On 
more than one occasion the contents of each leg have been vigorously stirred, without 
the slightest change in the E.M.F. 
Since the first draft of this memoir was written two new batches of cells of the 
ordinary pattern have been prepared with different materials. In this case the zincs 
were used as supplied, without re-casting,' 25 and the mercurous sulphate, though 
distinctly acid, was not washed. The first batch (10, 11, 12, 13) were set up on 
May 7, and the second batch (14, 15, 10, 17, 18, 19) on May 26. 
* The surface of the metal was brightened with file and sand paper. 
