454. Dr. W. W. J. Nicol on 
2nd. That which manifests itself only in the absence of 
undissolved salt. 
Of the above, the former occurs with all salts, hydrated or 
anhydrous in the solid form. Its existence is dependent 
solely on the fact that a finite time is necessary to permit of 
the establishment of equilibrium or saturation in a solution. 
When a hot saturated solution is cooled to the temperature 
of the air, the first portions of the excess of salt crystallize out 
at once, but more slowly as the excess becomes less and less, 
until when equilibrium is nearly perfect separation of solid 
salt is.extremely slow. This has been commented on by 
Kremers under the name of “ Inertia,’’* and isa fertile source 
of error in determinations of solubility. J have myself de- 
tected traces of this form of supersaturation even in the case 
of solutions which have been prepared at a high temperature 
and, after cooling, agitated for twenty-four hours with the 
solid salt; the percentage dissolved was frequently slightly 
greater than in the case of solutions prepared from solid salt 
and water without previous heating f. 
This is, I hope to show, true supersaturation; it is never 
well marked, and is not permanent, the excess of salt thus 
remaining in solution being exceedingly small, and becoming 
less and less with time. 
The phenomenon usually termed supersaturation is the 
second of the two kinds I have distinguished, and is quite 
distinct from the former. It occurs only with hydrated salts, 
and is well marked and permanent. The amount of salt thus 
retained may be very large, and crystallization may, under 
certain conditions, be delayed indefinitely. It is manifested 
whenever a strong warm solution of a hydrated salt is allowed 
to cool out of contact with the air or in contact with air 
which has been heated or filtered through cotton-wool. In 
nearly all cases such solutions remain permanently liquid, de- 
positing crystals on strong cooling, which dissolve again on 
the application of a gentle heat, such as that of the hand. No 
crystallization is caused by shaking or by the passage of elec- 
tricity; but crystallization takes place at once on the addition 
of a crystal of the hydrated salt. 
It is with such cases of supersaturation that this paper has 
to deal, and I hope to be able to show that, though a solution 
may deposit salt under the above conditions, yet it is not 
really supersaturated. I have confined myself strictly to the 
state of the solutions, and have no remarks to offer on the 
causes which bring about eine 
* Poge. Ann. lxxxy. p. 4 
+ Phil. Mag. June eee) 1884. 
