786 
LORD RAYLEIGH OH THE CLARK CELL AS 
with the paste. The object is to leave but little air, and at the same time to avoid 
squeezing up the paste between the cork and the glass. The whole is now made 
tight by pouring marine glue over the cork high enough to cover the zinc and 
soldering, and leave only the wire projecting. The tube should rise high enough to 
receive the glue, and thus secure a good adhesion. 
The marine glue is melted over the gas flame in a small pot or basin, and stirred, 
until uniform, with a small stick. It should be fluid enough to pour by its own 
weight. If necessary, a little benzole may be added, but the cement should be pretty 
hard when cold. 
In the operation of pouring in the marine glue the glass is heated by the glue 
sufficiently for adhesion ; but this heat does not extend appreciably below the cork. 
Neither in this, nor any other stage of the process of charging, is heat applied to the 
paste. 
§ 44. The earlier cells, prepared with paste, which was doubtless strongly acid, 
frequently gave irregular results for several weeks. Extreme cases are afforded by 
15 and 16, which are shown by Table VIII. of the former paper to have been at first 
more than 2 per cent, too strong. Moreover, as appears from the continuation of this 
table in the notes (p. 459), it took nearly two months for these cells to settle down to 
their normal values. The cause of irregularity is to be sought rather at the mercury 
than at the zinc (or amalgam) electrode. 
In order to examine this question, H-cells were charged with pure mercury and 
paste in both legs, and filled up as usual with saturated zinc sulphate solution. There 
should, of course, have been no E.M.F.; but the value of one of the cells w'as ’0041 
Clark, and remained tolerably constant for several days. By stirring with a glass 
rod, the E.M.F. could be either increased or diminished. After some weeks the cells 
had come sensibly to zero, and would bear stirring (in one or both legs) without much 
disturbance. To another cell, which still show T ed irregularity, zinc carbonate was 
added. The E.M.F. was much reduced, and in a few days was scarcely sensible even 
on stirring. 
When the paste is neutralised in the first instance with zinc carbonate, the irregu¬ 
larities are much reduced. Two cells thus prepared had an E.M.F. less than '0001 
Clark, and were scarcely affected by stirring. On Jan. 27, 1885, a piece of zinc wire 
was poked through the paste, so as momentarily to touch the mercury in one leg of 
one of these cells. A large E.M.F. was thus developed, which remained operative 
for half-an-hour or more ; but on Jan. 2S the E.M.F. was only ‘0003 Clark, and on 
Jan. 31 '0002 Clark. It is clear that the mercurous sulphate lias the property of 
freeing the mercury from the smallest contamination with zinc. 
§ 45. In consequence of these observations more recent cells have been prepared 
with neutralised paste. This course has the advantage that the cells attain their 
normal values in a few days, sometimes within one day, of charging. So far as I can 
judge, however, there is no difference in the ultimate value whether the paste be acid 
