The solubility of solids in liquids is invariably limited. As a rule it increases with the temper ature, but cases are known (e.g. that of sodium sulphate, with respect to water) in which an elevation of temperature may cause a decrease in solubility. A fact important to remember is that if a solid is capable of existing in two or more different modifications (e.g,. in different allo tropic forms, in an anhydrous form and one or more forms containing water of crystallization, etc.), each modification has its own solubility, and a solution exactly saturated with the more soluble modification is more or less 'supersatu rated' with the less soluble one. Bearing in mind that the supersaturation of a sohition is destroyed, with rapid separation of the excess of dissolved substance, when a trace of the latter is introduced into the solution, the following experiment may serve to illustrate the point under consideration: Let ordinary Glauber's salt, Le. sodium sulphate containing 10 mole cules of water of crystallization be heated to boiling with about one-half its weight of water, in a flask whose mouth is loosely closed with a plug of cotton (to keep out particles of Glauber's salt that may he floating in the air). If the solution thus obtained be
cooled to —10° C., a sodium sulphate containing seven molecules of water of crystallization will separate out, and when the separation is complete the mother-liquor will be exactly saturated with respect to this salt. Now, has a greater solubility than Hence, the saturated mother liquor of .Na,..S0.,.7H,G must evidently be super saturated with respect to Na.,S0,.10110. As a matter of fhet, if a trace of the latter be now introduced into our mother-liquor, a new crystal lization will set in, a mass of sep arating out and leaving the solution exactly sat urated with respect also to this form of the salt. Such, as well as a host of other phenomena, com plicate exceedingly the problem of discovering a precise relationship between the solubility of substances in various solvents and their nature.