From these curves now, it will be evident that it will be possible, in any given case, to decide whether or not an inactive body is a mixture or a racemic compound. For this purpose, {283} two solubility determinations are made, first with the inactive material alone (in excess), and then with the inactive material plus excess of one of the optically active forms. If we are dealing with a mixture, the two solutions thus obtained will be identical; both will have the composition corresponding to the point _c_, and will be inactive. If, however, the inactive material is a racemic compound, then two different solutions will be obtained; namely, an inactive solution corresponding to the point _d_ (Fig. 111), and an _active_ solution corresponding either to _e_ or to _f_, according to which enantiomorphous form was added. _Partially racemic compounds._[364] In this case we are no longer dealing with enantiomorphous forms, and the solubility of the two oppositely active isomerides is no longer the same. The symmetry of the solubility curves therefore disappears, and a figure is obtained which is identical in its general form with that found in the case of ordinary double salts (Fig. 112). In this case there is a transition interval. [Illustration: FIG. 112.] The curves _acb_ belong to a temperature at which the partially racemic compound cannot be formed; _a'dfb'_, to the temperature at which the compound just begins to be stable in contact with water, and _a"ed'f'b"_ belongs to a temperature at which the partially racemic compound is quite stable in contact with water. Suppose now solubility determinations, made in the first case with the original material alone, and then with the original body plus each of the two compounds, formed from the enantiomorphous substances separately, then if the original body was a mixture, identical solutions will be obtained in all three cases (point _c_); if it was a partially racemic compound, three different solutions (_e_, _d'_, and _f'_) will be obtained if the temperature was outside the transition interval, and two solutions, _d_ and _f_, if the temperature belonged to the transition interval. {284} _Representation in Space._ Space Model for Carnallite.--Interesting and important as the isothermal solubility curves are, they are insufficient for the purpose of obtaining a clear insight into the complete behaviour of the systems of two salts and water. A short description will, therefore, be given here of the representation in space of the solubility relations of potassium and magnesium chlorides, and of the double salt which they form, carnallite.[365] [Illustration: FIG. 113.]