As we have already seen (p. 122), water and succinic nitrile can form two liquid layers between the temperatures 18.5° and 55.5°; while alcohol and nitrile can form two liquid layers between 13° and 31°. If, then, between these two temperature limits, alcohol is added to a heterogeneous mixture of water and nitrile, or water is added to a mixture of alcohol and nitrile, two heterogeneous ternary systems will be formed, {250} and two boundary curves will be obtained in the triangular diagram, as shown in Fig. 90.[328] On changing the temperature, the boundary curves will also undergo alteration, in a manner similar to that just discussed. As the temperature falls, the two curves will spread out more and more into the centre of the triangle, and might at last meet one another; while at still lower temperatures we may imagine the curves still further expanding so that the two heterogeneous regions flow into one another and form a _band_ on the triangular diagram (Fig. 91). This, certainly, has not been realized in the case of the three components mentioned, because at a temperature higher than that at which the two heterogeneous regions could fuse together, solid separates out. [Illustration: FIG. 92.] The gradual expansion of a paraboloid into a band-like area of heterogeneous ternary systems, has, however, been observed in the case of water, phenol, and aniline.[329] In Fig. 92 are shown three isothermals, viz. those for 148°, 95°, and 50°. At 148°, water and aniline form two layers having the composition-- Water, 83.5 per cent. } { water, 20 per cent. } and { Aniline, 16.5 " } { aniline, 80 " {251} and the critical point _k'_ has the composition-- Water, 65; phenol, 13.2; aniline, 21.8 per cent. At 95°, the composition of the two binary solutions is-- Water, 93 per cent. } { water 8 per cent. } and { Aniline, 7 " } { aniline, 92 " while the point _k"_ has the composition Water, 69.9; phenol, 26.6; aniline, 3.5 per cent. At 50°, the region of heterogeneous states now forms a band, and the two layers formed by water and aniline have the composition-- Water, 96.5 per cent. } { water, 5.5 per cent. } and { Aniline, 3.5 " } { aniline, 94.5 " while the two layers formed by water and phenol have the composition-- Water, 89 per cent.} { water, 38 per cent. } and { Phenol, 11 " } { phenol, 62 " All mixtures of water, phenol, and aniline, therefore, the composition of which is represented by any point within the band _abcd_, will form two ternary solutions; while if the composition is represented by a point outside the band, only one homogeneous solution will be produced. 3. _The three components form three pairs of partially miscible liquids._ [Illustration: FIG. 93.]