the less stable modification is produced, appear to be rather various.[[145]]
Although a number of at least apparent exceptions to Ostwald's law have been found, it may nevertheless be accepted as a very useful generalization which sums up very frequently observed phenomena.
CHAPTER V
SYSTEMS OF TWO COMPONENTS—PHENOMENA OF DISSOCIATION
In the preceding pages we have studied the behaviour of systems consisting of only one component, or systems in which all the phases, whether solid, liquid, or vapour, had the same chemical composition (p. [13]). In some cases, as, for example, in the case of phosphorus and sulphur, the component was an elementary substance; in other cases, however, e.g. water, the component was a compound. The systems which we now proceed to study are characterized by the fact that the different phases have no longer all the same chemical composition, and cannot, therefore, according to definition, be considered as one-component systems.
In most cases, little or no difficulty will be experienced in deciding as to the number of the components, if the rules given on pp. [12] and [13] are borne in mind. If the composition of all the phases, each regarded as a whole, is the same, the system is to be regarded as of the first order, or a one-component system; if the composition of the different phases varies, the system must contain more than one component. If, in order to express the composition of all the phases present when the system is in equilibrium, two of the constituents participating in the equilibrium are necessary and sufficient, the system is one of two components. Which two of the possible substances are to be regarded as components will, however, be to a certain extent a matter of arbitrary choice.
The principles affecting the choice of components will best be learned by a study of the examples to be discussed in the sequel.