Cryohydrates.[195]--On cooling a solution of common salt in water to a temperature of -3°, Guthrie observed that the hydrate NaCl,2H_{2}O separated out. This salt continued to be deposited until at a temperature of -22° opaque crystals made their appearance, and the liquid passed into the solid state without change of temperature. A similar behaviour was found by Guthrie in the case of a large number of other salts, a temperature below that of the melting point of ice being reached at which on continued withdrawal of heat, the solution solidified at a constant temperature. When the system had attained this minimum temperature, it was found that the composition of the solid and the liquid phases was the same, and remained unchanged throughout the period of solidification. This is shown by the following figures, which give the composition of different samples of the solid phase deposited from the solution at constant temperature.[196] {118} --------------------------------- No. | Temperature of | NaCl. | solidification. | Per cent. ----|-----------------|---------- 1 | -21° to -22° | 23.72 2 | -22° | 23.66 3 | -22° | 23.73 4 | -23° | 23.82 5 | -23° | 23.34 6 | -23° | 23.35 --------------------------------- Mean 23.6 --------------------------------- Conversely, a mixture of ice and salt containing 23.6 per cent. of sodium chloride will melt at a definite and constant temperature, and exhibit, therefore, a behaviour supposed to be characteristic of a pure chemical compound. This, then, combined with the fact that the solid which was deposited was crystalline, and that the same constant temperature was attained, no matter with what proportions of water and salt one started, led Guthrie to the belief that the solids which thus separated at constant temperature were definite chemical compounds, to which he gave the general name _cryohydrate_. A large number of such cryohydrates were prepared and analysed by Guthrie, and a few of these are given in the following table, together with the temperature of the cryohydric point:[197]-- CRYOHYDRATES.