The farther end of the model (Fig. 116) corresponds, as already mentioned, to the temperature -30°, so that the outline evidently represents the isothermal curve for that temperature. Fig. 117 does not show this. We can, however, follow the isothermal for -20°, which is the extreme curve on the right in Fig. 117. Point A represents the solubility of 2FeCl_{3},12H_{2}O in water. If hydrogen chloride is added, the concentration of ferric chloride in the solution first decreases and then increases, until at point 34 the ternary double salt 2FeCl_{3},2HCl,12H_{2}O is formed. If the addition of hydrogen chloride is continued, the ferric chloride disappears ultimately, and only the ternary double salt remains. This salt can coexist with solutions of the composition represented by the curve which passes through the points 173, 174, 175. At the last-mentioned point, the ternary salt with 8H_{2}O is formed. The composition of the solutions with which this salt is in equilibrium at -20° is represented by the curve which passes through a point of maximal concentration with respect to HCl, and cuts the curve SN at the point 112, at which the solution is in equilibrium with the two solid phases 2FeCl_{3},4H_{2}O and 2FeCl_{3},2HCl,8H_{2}O. The succeeding portion of the isotherm represents the solubility curve at -20° of 2FeCl_{3},4H_{2}O, which cuts the dark line OS at point 113, at which the solution is in equilibrium with the two solid phases 2FeCl_{3},4H_{2}O and 2FeCl_{3},2HCl,4H_{2}O. Thereafter comes the solubility curve of the latter compound. The other isothermal curves can be followed in a similar manner. If the temperature is raised, the region of existence of the ternary double salts becomes smaller and smaller, and at temperatures above 30° the ternary salts with 12H_{2}O and 8H_{2}O are no longer capable of existing. If the temperature is raised above 46°, only the binary compounds of ferric chloride and water and the anhydrous salt can exist as solid phases. The isothermal curve for 0° represents the solubility curve for 2FeCl_{3},12H_{2}O; 2FeCl_{3},7H_{2}O; 2FeCl_{3},5H_{2}O; and 2FeCl_{3},4H_{2}O. {296} Finally, in the case of the system FeCl_{3}--HCl--H_{2}O, we find _closed_ isothermal curves. Since, as already stated, the salt 2FeCl_{3},2HCl,8H_{2}O has a definite melting point, the temperature of which is therefore higher than that at which this compound is in equilibrium with solutions of other composition, it follows that the line of intersection of an isothermal plane corresponding with a temperature immediately below the melting point of the salt with the cone-shaped surface of its region of existence, will form a closed curve. This is shown by the isotherm for -4.5°, which surrounds the point Q, the melting point of the ternary salt. The following table gives some of the numerical data from which the curves and the model have been constructed:--