Since a two-component system may undergo three possible {78} independent variations, we should require for the graphic representation of all the possible conditions of equilibrium a system of three co-ordinates in space, three axes being chosen, say, at right angles to one another, and representing the three variables--pressure, temperature, and concentration of components (Fig. 18). A curve (_e.g._ AB) in the plane containing the pressure and temperature axes would then represent the change of pressure with the temperature, the concentration remaining unaltered (_pt_-diagram); one in the plane containing the pressure and concentration axes (_e.g._ AF or DF), the change of pressure with the concentration, the temperature remaining constant (_pc_-diagram), while in the plane containing the concentration and the temperature axes, the simultaneous change of these two factors at constant pressure would be represented (_tc_-diagram). If the points on these three curves are joined together, a surface, ABDE, will be formed, and any line on that surface (_e.g._ FG, or GH, or GI) would represent the simultaneous variation of the three factors--pressure, temperature, concentration. Although we shall at a later point make some use of these solid figures, we shall for the present employ the more readily intelligible plane diagram. The number of different systems which can be formed from two components, as well as the number of the different phenomena which can there be observed, is much greater than in the case of one component. In the case of no two substances, however, have all the possible relationships been studied; so that for the purpose of gaining an insight into the very varied behaviour of two-component systems, a number of different examples will be discussed, each of which will serve to give a picture of some of the relationships. Although the strict classification of the different systems according to the Phase Rule would be based on the variability of the systems, the study of the many different phenomena, and the correlation of the comparatively large number of different systems, will probably be rendered easiest by grouping these different phenomena into classes, each of these classes being studied with the help of one or more typical examples. The order of treatment adopted here is, of course, quite arbitrary; {79} but has been selected from considerations of simplicity and clearness. PHENOMENA OF DISSOCIATION.