1896-97.] Mr W. W. J. Nicol on Super saturation. 
I H Q 
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On Supersaturation and its Dependence on Crystalline 
Form. By W. W. J. Nicol, M.A., D.Sc., F.R.S.E., F.I.C. 
(Read May 3, 1897.) 
The phenomenon of Supersaturation in certain well-marked 
instances is familiar to every student, and may briefly be defined 
as consisting in the retention in solution at a low temperature of a 
quantity of a substance sufficient to form a saturated solution at a 
higher temperature. So long as the temperature is not allowed to 
fall below a certain point, and the solution is not brought in con- 
tact with a particle of the solid substance or with a substance 
isomorphous with it, the solution is in perfectly stable equilibrium. 
The researches of Gernez and many others have established this 
now generally accepted fact, that, the temperature remaining con- 
stant, the sole cause which is able to disturb the equilibrium of the 
solution is contact with a crystal of the same form as that which 
will crystallise out from the .solution. 
In a paper communicated to this Society nearly twelve years 
ago,* I described a series of experiments on supersaturation which 
had forced me to the conclusion that in the strict sense of the word 
no solution is ever supersaturated, a conclusion which I found had 
been reached by Loewel f nearly fifty years before, as the result 
of his investigations on the solubility of sodium sulphate. Shortly 
stated, the views of Loewel and myself were that a supersaturated 
solution is a saturated or non-saturated solution of the anhydrous 
salt, the term anhydrous being used by me as a convenient means 
of indicating that no definite hydrate exists in the solution, but 
that the whole of the water is in the same relation to the salt. 
In the paper referred to above, and in two .t subsequent ones, I 
showed that a supersaturated solution was able to dissolve more 
salt even when it was as concentrated as the solution obtained by 
* Phil. Mag., June 1885, p. 453. 
t Ann. d. Chim. et Phys., (3), xlix. p. 51. 
X Phil. Mag., September 1885, p. 295 ; Journ. Chem. Soc., 1887, p. 389. 
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