On following the solubility curve of the hexahydrate from the ordinary temperature upwards, it is seen that at a temperature of 29.8° represented by the point H, it cuts the solubility curve of the [alpha]-tetrahydrate. This point is therefore a quadruple point at which the four phases hexahydrate, [alpha]-tetrahydrate, solution, and vapour can coexist. It is also the transition point for these two hydrates. Since, at temperatures above 29.8°, the [alpha]-tetrahydrate is the stable form, it is evident from the data given before (p. 146), as also from Fig. 38, that the portion of the solubility curve of the hexahydrate lying above this temperature represents _metastable_ equilibria. The realization of the metastable melting point of the hexahydrate is, therefore, due to suspended transformation. At the transition point, 29.8°, the solubility of the hexahydrate and [alpha]-tetrahydrate is 100.6 parts of CaCl_{2} in 100 parts of water. The retroflex portion of the solubility curve of the hexahydrate extends to only 1° below the melting point of the hydrate. At 29.2° crystals of a new hydrate, [beta]-tetrahydrate, separate out, and the solution, which now contains 112.8 parts of CaCl_{2} to 100 parts of water, is saturated with respect to the two hydrates. Throughout its whole extent the solubility curve EDF of the [beta]-tetrahydrate represents metastable equilibria. The upper limit of the solubility curve of [beta]-tetrahydrate is reached at 38.4° (F), the point of intersection with the curve for the dihydrate. Above 29.8° the stable hydrate is the [alpha]-tetrahydrate; and its solubility curve extends to 45.3° (K), at which temperature it cuts the solubility curve of the dihydrate. The curve of the latter hydrate extends to 175.5° (L), and is then succeeded by the curve for the monohydrate. The solubility curve of the anhydrous salt does not begin until a temperature of about 260°. The whole diagram, therefore, shows a succession of stable hydrates, a metastable hydrate, a metastable melting point and retroflex solubility curve. {150}