Note 63 — Chapter I · ON WATER AND ITS COMPOUNDS

Supersaturated solutions give an excellent proof in this respect. Thus a solution of copper sulphate generally crystallises in penta-hydrated crystals, CuSO4 + 5H2O, and its saturated solution gives such crystals if it be brought into contact with the minutest possible crystal of the same kind. But, according to the observations of Lecoq de Boisbaudran, if a crystal of ferrous sulphate (an isomorphous salt, see note 55), FeSO4 + 7H2O, be placed in a saturated solution of copper sulphate, then crystals of hepta-hydrated salt, CuSO4 + 7H2O, are obtained. It is evident that neither the penta- nor the hepta-hydrated salt is contained as such in the solution. The solution presents its own particular liquid form of equilibrium.