In addition to the large amount of heat liberated when a metal solidifies, there are other changes indicated by the thermal analysis of many alloys which occur _after_ the body has become entirely solidified. These so-called transformation points or ranges may be caused by chemical reactions taking place within the solid, substances being precipitated from a "solid solution," or a sudden change in some physical property of the components, such as in magnetism, hardness, or specific gravity. It may be difficult to comprehend that such changes can occur in a body after it has become entirely solidified, owing to the usual conception that the particles are then rigidly fixed. However, this rigidity is only comparative. The molecules in the solid state have not the large mobility they possess as a liquid, but even so, they are still moving in circumscribed orbits, and have the power, under proper conditions, to rearrange their position or internal configuration. In general, such rearrangement is accompanied by a sudden change in some physical property and in the total energy of the molecule, which is evidenced by a spontaneous evolution or absorption of latent heat. Cooling curves of the purest iron show at least two well-defined discontinuities at temperatures more than 1,000 deg.F., below its freezing-point. It seems that the soft, magnetic metal so familiar as wrought iron, and called "alpha iron" or "ferrite" by the metallurgist, becomes unstable at about 1,400 deg.F. and changes into the so-called "beta" modification, becoming suddenly harder, and losing its magnetism. This state in turn persists no higher than 1,706 deg.C., when a softer, non-magnetic "gamma" iron is the stable modification up to the actual melting-point of the metal. These various changes occur in electrolytic iron, and therefore cannot be attributed to any chemical reaction or solution; they are entirely due to the existence of "allotropic modifications" of the iron in its solid state. [Illustration: FIG. 45.--Inverse Rate Cooling Curve of 0.38 C Steel.]