As a rule, we select for the form in which a given ion is to be precipitated, a form which, in a saturated aqueous solution, shows the ‹smallest concentration of the ion in question›. But if no form is [p154] known which is sufficiently insoluble to give satisfactory quantitative results, then we have recourse to a ‹change› in the ‹solvent›. «Solubility and Solvent.»—For instance, a mixture of alcohol and water may be used, or water be excluded altogether; and either absolute (water-free) alcohol or a mixture of alcohol and ether may be employed. In the quantitative treatment of potassium chloroplatinate, the last-named mixture is used in place of water. The change of solvent affects the solubility by a change both in the solubility of the ionized portion of a salt and in that of the nonionized salt. An important quantitative relation between the solubility of a given ionogen in different solvents and the ionizing powers of the solvents, as determined by their dielectric constants (p. 63), was predicted, on the basis of theoretical considerations, by Malström[321] and by Baur.[322] Walden[323] has furnished experimental confirmation of the relation: ‹The degree of ionization of a salt is found to be the same in its saturated solutions in different solvents›, when the solutions are saturated at the same temperature. If this relation is combined with that discussed on page 63, according to which the degree of ionization of a given salt, in different solvents, ‹is the same›, when the cube roots of its concentrations are directly proportional to the dielectric constants of the solvents (‹e›_{1} : ∛‹c›_{1} = ‹e›_{2} : ∛‹c›_{2} = a constant), then we find, that in ‹saturated solutions of a given salt, in different solvents, the cube roots of the concentrations, or solubilities, are directly proportional to the dielectric constants of the solvents›, or, ‹the solubilities are proportional to the third powers of the dielectric constants›. ‹e›_{1} : ∛‹c›_{1} = ‹e›_{2} :∛‹c›_{2} = a constant, or ‹e›_{1}^3 : ‹e›_{2}^3 = ‹c›_{1} : ‹c›_{2}, ‹c›_{1} and ‹c›_{2} representing the solubilities, in molar concentrations, in two solvents of dielectric constants ‹e›_{1} and ‹e›_{2}. The following table illustrates the relations for a salt examined by Walden, a derivative of ammonium iodide, namely tetraethyl ammonium iodide (C_{2}H_{5})_{4}NI. The first column gives the name of the solvent, the second the solubility or concentration in the saturated solution, in terms of the proportion of moles of the solute to the total number of moles present[324] [p155] (solute + solvent); the third column gives the dielectric constant, under comparable conditions, and the last column gives the relation ‹e› : ∛‹c›.