A STANDARD OF ELECTRO-MOTIVE FORCE. 
789 
There was now little doubt but that the solution had been supersaturated, in the 
sense with which we are concerned. The presence of crystals is no evidence to the 
contrary, unless it can be proved that the crystals were those of the normal, hepta- 
hydrated salt. 
At this stage I wrote to Mr. M. Evans to inquire whether there was anything in 
the history of the cells that would account for the separation into two classes, and I 
was informed that all the cells had been prepared originally in the same manner. The 
mercury was twice distilled in vacuo, and in other respects the greatest care had been 
taken. When, however, the cells came to be tested, it was found that owing to 
contraction all but two were wanting in proper contact between the mercury and 
platinum, and that this contact could only be restored by remelting over a water-bath 
the whole of the paraffin wax in which the cells were imbedded. It was by this 
operation, no doubt, that the solution became supersaturated. The agreement with 
mine of the two cells which were not heated (one of them (T 3 ) is referred to later) is 
very satisfactory as showing that the great precautions exercised by Mr. Evans lead 
to the same E.M.F. as 1 have obtained with far less trouble. I may add that a second 
abnormal cell (T 8 ) moved to equality with my standards on being opened. 
§ 49. With a view to the better understanding of this matter I made myself 
acquainted with the beautiful researches of M. Gernez # upon supersaturation, 
conducted principally with solutions of sulphate and of acetate of soda; and have 
performed parallel experiments upon sulphate of zinc. A very strong solution of this 
salt prepared hot, and sealed up in a glass tube, will sometimes cool without any 
deposit. More often it throws down an abnormal (lower) hydrate. If in this condition 
the tube be heated pretty rapidly in boiling water, some of the salt dehydrates further 
to a powder (presumably mono-hydrated) ; if it be allowed to cool again the inferior 
crystalline hydrate reforms. However long the solution stands cold over this hydrate, 
it is still supersaturated as regards the normal hydrate, the minutest addition of which 
causes the supernatant liquid to become almost solid, with needles penetrating it in all 
directions. The experiment has been repeated many times with less strong solutions 
standing in open test-tubes, charged and preserved with the simple precautions 
indicated by M. Gernez. 
It is evident that “ supersaturation ” is a term without definite meaning until 
further explained. Gernez has shown that a solution may be supersaturated with 
respect to one or both of two different hydrates, i.e , will crystallise similarly on 
contact with the smallest fragment, and not supersaturated at all with respect to the 
anhydrous salt, the addition of which causes no effect. De CopPETt has proved 
that the so-called supersaturated solution may be disposed to take up a further por¬ 
tion of anhydrous salt; as may, indeed, be inferred from previously known facts, since 
* “ Annales de l’ecole normale,” t. iii., 1866, p. 163 ; t. v., 1876, p. 1. 
t C.R. LXXIII., p. 1324, 1871. See also Nicol, Phil. Mag., June, 1885. 
MDCCCLXXXV. 5 I 
