In discussing the equilibria in systems containing a solid solution, it is of essential importance to remember that a solid solution constitutes only _one_ phase, a phase of varying composition, as in the case of liquid solutions. Solution of Gases in Solids.--Comparatively little work has been done in this connection, the investigations being limited chiefly to the phenomena of occlusion or adsorption of gases by charcoal.[257] We shall, therefore, indicate only briefly {177} and in a general manner, the behaviour which the Phase Rule enables us to foresee.[258] In dealing with the systems formed by the two phases gas--solid, three chief cases call for mention:-- I. _The gas is not absorbed by the solid, but when the pressure reaches a certain value, combination of the two components can result._ [Illustration: Fig. 46.] The graphic representation of such a system is shown in Fig. 46, the ordinates being the pressures of the gas, and the abscissæ the concentrations of the gaseous component in the solid phase. Since there is no formation of a solid solution, the concentration of gas in the solid phase remains zero until the pressure has increased to the point A. At this point combination can take place. There will now be three phases present, viz. solid component, compound, and vapour. The system is therefore univariant, and if the temperature is maintained constant, the vapour pressure will be constant, irrespective of the amount of compound formed, _i.e._ irrespective of the relative amounts of gas and solid. This is indicated by the line AB. When the solid component has entirely disappeared, the system ceases to be univariant, and if no absorption occurs, the pressure will increase again, as shown by BC. If a second compound can be formed, then a second _pc_-line will be obtained, similar to the preceding. To this group belong the salt hydrates (Chap. VII.). II. _The gas may be absorbed and may also form a compound._ If absorption of gas occurs with formation of a solid solution, then, as the system consists of two phases, solution--vapour, it is bivariant. At constant temperature, therefore, the pressure will still vary with the concentration of the gaseous component in the solid phase. This is represented by the curve AB in Fig. 47. When, however, the pressure has reached a certain value, combination can take place; and since there are now three phases present, the system is {178} univariant, and at constant temperature the pressure is constant, as shown by the line BC. III. _Absorption of gas occurs, but at a certain concentration the solid solution can separate into two immiscible solid solutions._