From the example which has just been discussed, it might {13} appear as if the choice of the components was rather arbitrary. On examining the point, however, it will be seen that the arbitrariness affects only the _nature_, not the _number_, of the components; a choice could be made with respect to which, not to how many, constituents were to be regarded as components. As we shall see presently, however, it is only the number, not the nature of the components that is of importance. After the discussion of the conditions which the substances chosen as components must satisfy, another method may be given by which the number of components present in a system can be determined. Suppose a system consisting of several phases in equilibrium, and the composition of each phase determined by analysis. If each phase present, regarded as a whole, has the same composition, the system contains only one component, or is of the first order. If two phases must be mixed in suitable quantities in order that the composition of a third phase may be obtained, the system is one of two components or of the second order; and if three phases are necessary to give the composition of a fourth coexisting phase, the system is one of three components, or of the third order.[16] Although the examples to be considered in the sequel will afford sufficient illustration of the application of the rules given above, one case may perhaps be discussed to show the application of the method just given for determining the number of components. Consider the system consisting of Glauber's salt in equilibrium with solution and vapour. If these three phases are analyzed, the composition of the solid will be expressed by Na_{2}SO_{4}, 10H_{2}O; that of the solution by Na_{2}SO_{4} + _x_H_{2}O, while the vapour phase will be H_{2}O. The system evidently cannot be a one-component system, for the phases have not all the same composition. By varying the amounts of two phases, however (_e.g._ Na_{2}SO_{4}, 10H_{2}O and H_{2}O), the composition of the third phase--the solution--can be obtained. The system is, therefore, one of _two components_.