Note 37 — Chapter X · SODIUM CHLORIDE—BERTHOLLET'S LAWS—HYDROCHLORIC ACID

This crystallo-hydrate (obtained by Pierre and Puchot, and investigated by Roozeboom) is analogous to NaCl,2H2O. The crystals HCl,2H2O at -22° have a specific gravity 1·46; the vapour tension (under dissociation) of the solution having a composition HCl,2H2O at -24° = 760, at -19° = 1,010, at -18° = 1,057, at -17° = 1,112 mm. of mercury. In a solid state the crystallo-hydrate at -17·7° has the same tension, whilst at lower temperatures it is much less: at -24° about 150, at -19° about 580 mm. A mixture of fuming hydrochloric acid with snow reduces the temperature to -38°. If another equivalent of water be added to the hydrate HCl,2H2O at -18°, the temperature of solidification falls to -25°, and the hydrate HCl,3H2O is formed (Pickering, 1893).