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 contact 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 constant, 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 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.