Phases.--Before proceeding farther we shall first consider what exactly is meant by the terms _phase_ and _component_. We have already seen (p. 5) that a heterogeneous system is made {9} up of different portions, each in itself homogeneous, but marked off in space and separated from the other portions by bounding surfaces. These homogeneous, physically distinct and mechanically separable portions are called _phases_. Thus ice, water, and vapour, are three phases of the same chemical substance--water. A phase, however, whilst it must be physically and chemically homogeneous, need not necessarily be chemically simple. Thus, a gaseous mixture or a solution may form a phase; but a heterogeneous mixture of solid substances constitutes as many phases as there are substances present. Thus when calcium carbonate dissociates under the influence of heat, calcium oxide and carbon dioxide are formed. There are then _two_ solid phases present, viz. calcium carbonate and oxide, and one gas phase, carbon dioxide. The _number of phases_ which can exist side by side may vary greatly in different systems. In all cases, however, there can be but one gas or vapour phase on the account of the fact that all gases are miscible with one another in all proportions. In the case of liquid and solid phases the number is indefinite, since the above property does not apply to them. The number of phases which can be formed by any given substance or group of substances also differs greatly, and in general increases with the number of participating substances. Even in the case of a single substance, however, the number may be considerable; in the case of sulphur, for example, at least eight different solid phases are known (_v._ Chap. III.). It is of importance to bear in mind that equilibrium is _independent of the amounts_ of the phases present.[13] Thus it is a familiar fact that the pressure of a vapour in contact with a {10} liquid (_i.e._ the pressure of the saturated vapour) is unaffected by the amounts, whether relative or absolute, of the liquid and vapour; also the amount of a substance dissolved by a liquid is independent of the amount of solid in contact with the solution. It is true that deviations from this general law occur when the amount of liquid or the size of the solid particles is reduced beyond a certain point,[14] owing to the influence of surface energy; but we have already (p. 5) excluded such cases from consideration.