Even below the solidification point, however, changes can take place. As has been said, the solid phase which finally separates out from the molten mass is a solid solution represented by the point D; and the curve DE represents the change in the composition of this solid solution with the temperature. As indicated in the figure, DE forms a part of a curve representing the mutual solubility of graphite in iron and iron in graphite; the latter solutions, however, not being shown, as they would lie far outside the diagram. As the temperature falls below 1130°, more and more graphite separates out, until at E, when the temperature is 1000°, the solid solution contains only 1.8 per cent. of carbon. At this temperature cementite also begins to be formed, so that as the temperature continues to fall, separation of cementite (represented by the line E'F') occurs, and the composition of the solid solution undergoes alteration, as represented by the curve EF. Below the temperature of the point F (670°) the martensite becomes heterogeneous, and forms pearlite. From the above description, therefore, it follows that if we start with a molten mixture of iron and carbon, the composition of which is represented by any point between D and C (from 2 to 4.3 per cent. of carbon), we shall obtain, on cooling the mass, first of all solid solutions, the composition of which will be represented by points on the line AD; that then, after the mass has completely solidified at 1130°, further cooling will lead to a separation of graphite and a change in the composition of the martensite (from 2 to 1.8 per cent. of carbon). On cooling below 1000°, however, the martensite and graphite will give rise to cementite and solid solutions {227} containing less carbon than before, until, at temperatures below 670°, we are left with a mixture of pearlite and cementite. We have already said that iron consists in three allotropic modifications, the regions of stability of which are separated by definite transition points. The transition point for [alpha]- and [beta]-ferrite (780°) is represented in Fig. 75 by the point H; and the transition point for [beta]- and [gamma]-ferrite (870°) by the point I. Since neither the [alpha]- nor the [beta]-ferrite dissolves carbon, the transition point will be unaffected by addition of carbon, and we therefore obtain the horizontal transition curve HG. In the case of the [beta]- and [gamma]-ferrite, however, the latter dissolves carbon, and the transition point is consequently affected by the amount of carbon present. This is shown by the line IG.