790 
LORD RAYLEIGH ON THE CLARK CELL AS 
in general tlie anhydrous salt must tend either to leave or to enter the solution, and 
the former alternative is excluded by the observed behaviour on first contact. 
§ 50. In view of the above facts we can hardly doubt that a Clark cell, heated 
nearly to the temperature of boiling water and then cooled, would be likely to 
become supersaturated ; but I thought it would be satisfactory actually to try the 
experiment. A normal cell of my own preparation and containing an excess of 
undissolved salt, was maintained for several hours at an elevated temperature, and 
tested after cooling. A temperature of 38° C. did not permanently alter the cell; 
neither did a temperature of 49°, nor one of 60°. But after an exposure to about 
80° a permanent change set in. Immediately after cooling the value in terms of the 
standards was ’9914, but after one day’s standing it settled to ’9943, close to which 
value it has since remained. It appears from the above that the cell probably 
requires to be heated sufficiently to decompose the normal hydrate, and not merely 
to bring all the immersed salt into solution. In the latter case there may well be 
solid particles within reach, which re-determine normal crystallisation on cooling. 
A second experiment was tried with an old cell (e of Table XIII.) which contained 
a large excess of undissolved salt. This was of the H-pattern, which lends itself 
more conveniently to observation and experiment. On November 13 the cell w r as 
heated for several hours nearly to 100° C. After cooling a solid mass of crystals was 
to be seen over tbe metals in both legs, and it might have been supposed that the 
operation had been unsuccessful. On November 16, however, the E.M.F. was found 
to have changed (from about 1'0005) to *9949, at which value it remained until 
November 25. On that day, at 5 k p.m. the corks were drawn. At 6 k no effect had 
been produced, and a fragment of the normal hydrate was dropped into each leg. 
At 6 h 45 ra new crystals had formed, and the E.M.F. had risen to '9996. A few 
hours later the E.M.F, was l - 0000, at which value it has since remained within 2 or 3 
parts in 10,000. It was remarked that the crystalline deposit on contact with the 
normal hydrate was much less in amount than had been met with in experimental 
tubes with simple zinc sulphate. The explanation is probably that during the 
heating no complete diffusion of the salt was effected, so that after cooling super¬ 
saturation was limited to the lower layers. Both metals being at the bottom in this 
form of cell, the E.M.F. is independent of the condition of the upper parts. 
It is worthy of note that all the “ supersaturated ” cells which I have tested are 
about 5 parts per 1000 too low. That they should give a definite E.M.F. is to be 
expected whenever the lower hydrate is formed. For the solution in contact with 
the lower hydrate is just as definite in composition as when it is in contact with the 
normal hydrate. It is, however, possible to have “ supersaturated ” solutions without 
formation of the lower hydrate, and then the E.M.F. would be indefinite. The 
deficiency may certainly be less than the 5 parts per 1000, and may probably be 
more in certain cases. 
§ 51. In view of the possibility of error from under and over-saturation, the reader 
