As examples of the solution of gases in solids there may be cited (in addition to the phenomena of occlusion already mentioned), the hydrated silicates and the zeolites. During dehydration these crystalline substances remain clear and transparent, and the pressure of the water vapour which they emit varies with the degree of hydration or the concentration of water in the mineral.[[256]] As examples of the solution of solids in solids we have the cementation of iron by charcoal, the formation of glass, and the crystallization together of isomorphous substances.
Although we have here spoken of the glasses as "solid solutions," it should be mentioned that the term "solid" is used in its popular sense. Strictly speaking, the glasses are to be regarded as supercooled liquids (see also p. 53, [footnote]).
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
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.
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.