800 
LORD RAYLEIGH ON THE CLARK CELL. 
§ 57. In Table X., § 32, are given some comparisons with No. 1 Clark of a standard 
Daniell with equi-dense solutions of Raoult’s pattern. As a mean at 16° C., 
Daniell =7703 Clark= 1T046 true volt. 
I have never succeeded in obtaining really good results with Dantells of any con¬ 
struction, variations in the conditions of the copper* rapidly altering the E.M.F. by 
two or three thousandths. The mean of a rather large number of comparisons in 
August, 1885, gives about '7715 Clark at 16°, and this number was confirmed by 
further observations in December, 1885. The sp. gr. of the solutions was 1T01 ; the 
zinc was amalgamated, and the copper was freshly-coated electrolytically. The effect 
of variations in the condition of the metals and of the solutions are discussed by 
Alder Wright and by Fleming. 
Some observations have also been made with apparatus and solutions lent me by 
Dr. Fleming, and arranged nearly according to the description published by him.t 
The variations observed with this form of Daniell were smaller than with Raoult’s, 
perhaps in consequence of the better protection of the copper solution from the 
atmosphere.]; The mean value for 1 6° was about 
•7674 Clark = 1T004 volt, 
in pretty good agreement with the value found by Dr. Fleming himself. In cor¬ 
recting for the effect of a small variation of temperature, I have allowed only for the 
variations of the Clark, assuming that of the Daniell to be insensible. 
[P.S.—May, 1886.—I have lately had the opportunity of comparing with mine five 
cells set up by Dr. A. Muirhead, who has had great experience in these matters. 
All five agreed with my No. 1 to about one part in ten thousand. Dr. Muirhead 
informs me that other cells, including one prepared by himself seven or eight years 
ago, agree closely with these.] 
* It seems not unlikely that the greater part of the uncertainty of ordinary Daniell’s might be got 
rid of by substitution of silver for copper. Silver sulphate , however, is not sufficiently soluble. But I 
have not made experiments in this direction, believing that the mercury of the Clark cell is better still. 
t Phil. Mag., August, 1885. 
f In my experience the copper found its way to the zinc more readily than one would have expected. 
The amalgamated zinc became coated with a furry deposit, which struck a blue colour on solution in 
nitric acid and snpersaturation with ammonia. In using Raoult’s cells I have endeavoured to hold the 
copper in check by placing a loose strip of zinc in the connecting Y-tube. 
