[61] The essence of the matter may he thus represented. A gaseous or easily volatile substance _A_ forms with a certain quantity of water, _n_H_{2}O, a definite complex compound _An_H_{2}O, which is stable up to a temperature t° higher than 100°. At this temperature it is decomposed into two substances, _A_ + H_{2}O. Both boil below _t_° at the ordinary pressure, and therefore at _t_° they distil over and re-combine in the receiver. But if a part of the substance _An_H_{2}O is decomposed or volatilised, a portion of the undecomposed liquid still remains in the vessel, which can partially dissolve one of the products of decomposition, and that in quantity varying with the pressure and temperature, and therefore the solution at a constant boiling point will have a slightly different composition at different pressures. [62] For solutions of hydrochloric acid in water there are still greater differences in reactions. For instance, strong solutions decompose antimony sulphide (forming hydrogen sulphide, H_{2}S), and precipitate common salt from its solutions, whilst weak solutions do not act thus. Many compunds containing water of crystallisation are solid substances (when melted they are already solutions--_i.e._ liquids); furthermore, they are capable of being formed from solutions, like ice or aqueous vapour. They may be called _crystallo-hydrates_. Inasmuch as the direct presence of ice or aqueous vapour cannot be admitted in solutions (for these are liquids), although the presence of water may be, so also there is no basis for acknowledging the presence in solutions of crystallo-hydrates, although they are obtained from solutions as such.[63] It is evident that such substances present one of the many forms of equilibrium between water and a substance dissolved in it. This form, however, reminds one, in all respects, of solutions--that is, aqueous compounds which are more or less easily decomposed, with separation of water and the formation of a less aqueous or an anhydrous compound. In fact, there are not a few crystals containing water which lose a part of their water at the ordinary temperature. Of such a kind, for instance, are the crystals of soda, or sodium carbonate, which, when separated from an aqueous solution at the ordinary temperature, are quite transparent; but when left exposed to air, lose a portion of their water, becoming opaque, and, in the process, lose their crystalline appearance, although preserving their original form. This process of the separation of water at the ordinary temperature is termed the _efflorescence_ of crystals. Efflorescence takes place more rapidly under the receiver of an air pump, and especially at a gentle heat. This breaking up of a crystal is dissociation at the ordinary temperature. Solutions are decomposed in exactly the same