The different modifications of iron also possess different properties. Thus, [alpha]-ferrite is magnetic, but does not possess the power of dissolving carbon; [beta]-ferrite is non-magnetic, and likewise does not dissolve carbon; [gamma]-ferrite is also non-magnetic, but possesses the power of dissolving carbon, and of thus giving rise to solid solutions of carbon in iron. Various alloys of iron and carbon, also, have to be distinguished. First of all, there is _hard steel_, which contains varying amounts of carbon up to 2 per cent. Microscopic examination shows that these mixtures are all homogeneous; and they are therefore to be regarded as solid solutions of carbon in iron ([gamma]-ferrite). To these solutions the name _martensite_ has been given. _Pearlite_ contains about 0.8 per cent. of carbon, and, on microscopic examination, is found to be a heterogeneous mixture. If heated above 670°, pearlite becomes homogeneous, and forms martensite. Lastly, there is a definite compound of iron and carbon, iron carbide or _cementite_, having the formula Fe_{3}C. A short description may now be given of the application of the Phase Rule to the two-component system iron--carbon; and of the diagram showing how the different systems are related, and with the help of which the behaviour of the different mixtures under given conditions can be predicted. Although, with regard to the main features of this diagram, the different areas to be mapped and the position of the frontier lines, there is general agreement; a final decision has not yet been reached with regard to the interpretation to be put on all the curves. [Illustration: FIG. 75.] The chief relationships met with in the case of the {225} iron-carbon alloys are represented graphically in Fig. 75.[309] The curve AC is the freezing-point curve for iron,[310] BC the unknown freezing-point curve for graphite. C is an eutectic point. Suppose, now, that we start with a mixture of iron and carbon, represented by the point _x_. On lowering the temperature, a point, _y_, will be reached at which solid begins to separate out. This solid phase, however, is not pure iron, but a solid solution of carbon in iron, having the composition represented by _y'_ (cf. p. 185). As the temperature continues to fall, the {226} composition of the liquid phase changes in the direction of _y_C, while the composition of the solid which separates out changes in the direction _y'_D; and, finally, when the composition of the molten mass is that of the point C (4.3 per cent. of carbon), the whole mass solidifies to a heterogeneous mixture of two solid solutions, one of which is represented by D (containing 2 per cent. of carbon), while the other will consist practically of pure graphite, and is not shown in the figure. The temperature of the eutectic point is 1130°.