In the most quickly quenched high carbon steels, austenite commonly forms the ground mass which is interspersed with martensite, a large field of which is illustrated in Fig. 50. Martensite is usually considered to be a solid solution of cementite in beta iron. It represents an unstable condition in which the metal is caught during rapid cooling. It is very hard, and is the chief constituent of hardened high-carbon steels, and of medium-carbon nickel-steel and manganese-steel. Troostite is of doubtful composition, but possibly is an unstable mixture of untransformed martensite with sorbite. It contains more or less untransformed material, as it is too hard to be composed entirely of the soft alpha modification, and it can also be tempered more or less without changing in appearance. Its normal appearance as rounded grains is given in Fig. 51; larger patches show practically no relief in their structure, and a photograph merely shows a dark, structureless area. [Illustration: FIG. 49.--Coarse-grained martensite, polished and etched with nitric acid and magnified 50 times. Made by Prof. Chas. Y. Clayton.] Sorbite is believed to be an early stage in the formation of pearlite, when the iron and iron carbide originally constituting the solid solution (austenite) have had an opportunity to separate from each other, and the iron has entirely passed into the alpha modification, but the particles are yet too small to be distinguishable under the microscope. It also, possibly, contains some incompletely transformed matter. Sorbite is softer and tougher than troostite, and is habitually associated with pearlite. Its components are tending to coagulate into pearlite, and will do so in a fairly short time at temperatures near the lower critical, which heat will furnish the necessary molecular freedom. The normal appearance, however, is the cloudy mass shown in Fig. 52. Pearlite is a definite conglomerate of ferrite and cementite containing about six parts of the former to one of the latter. When pure, it has a carbon content of about 0.95 per cent. It represents the complete transformation of the eutectoid austenite accomplished by slow-cooling of an iron-carbon alloy through the transformation range. (See Fig. 46.) [Illustration: FIG. 50.--Quenched high-carbon steel, polished, etched and viewed at 100 magnifications. This structure is called martensite and is desired when maximum hardness is essential. Photograph by H. S. Rawdon.] [Illustration: FIG. 51.--Martensite (light needles) passing into troosite (dark patches). 130 X. From a piece of eutectoid steel electrically welded.] [Illustration: FIG. 52.--Sorbite (dark patches) passing into pearlite (wavy striations). Light Areas are Patches of Ferrite. 220 X. From a piece of hypo-eutectoid steel electrically welded.]