Photomicrograph No. 99b is open-hearth iron which is entirely made up of free ferrite. In No. 3b there is considerable pearlite, here appearing black, though the sample of steel yet contains but .10% of carbon. In No. 5, which is of a steel containing .30% of carbon, we have more pearlite and in No. 22c with .50% carbon we have yet more. Manifestly at this rate the comparative pearlite areas are growing so that there will soon be room for no ferrite at all. In No. 23g this has occurred. This, the photomicrograph of a steel containing .86% of carbon is one of the steels in which we found that the point of recalescence, loss of magnetism, decrease in electrical conductivity and rate of expansion take place all at the one point. [Illustration: No. 99b. CARBONLESS IRON ] [Illustration: No. 3b. STEEL WITH .1 PER CENT CARBON ] [Illustration: No. 5. STEEL WITH .3 PER CENT CARBON ] [Illustration: No. 22c. STEEL WITH .5 PER CENT CARBON ] (_Magnification 60 Diameters._) Now as we go still farther on up in percentage of carbon content, i.e. (beyond .86%, we have a white constituent beginning to appear as cell walls around the grains of the pearlite and this increases with increase of carbon until, with alloys having carbon around 3%, we have a proportionately small amount of pearlite while the white areas have so increased that it appears that the more or less round patches of pearlite float in a lake of white. This white which appears first as cell walls, and later in greater and greater quantity is free cementite.) [Illustration: No. 23g. STEEL WITH .9 PER CENT CARBON ] [Illustration: No. 24a. STEEL WITH 1.25 PER CENT CARBON ] [Illustration: No. 36b. STEEL WITH 2 PER CENT CARBON ] [Illustration: No. 109. WHITE CAST IRON WITH 3 PER CENT CARBON ] (_Magnification 60 Diameters._) Such are illustrated in photomicrographs Nos. 24a, 36b and 109 which contain 1.25%, 1.98% and 3.00% of carbon respectively. While steels with the typical white, free ferrite areas are so soft that a needle-point will plow furrows across them, those with over 1.25% of carbon have such excess of free cementite that they are very hard to scratch and too brittle to use except for special purposes. [Illustration: AUSTENITE (WHITE) AND MARTENSITE (DARK) MAGNIFIED 1,000 TIMES THEIR ACTUAL SIZE ] So during ordinary cooling from the molten alloy or the slower cooling of the steel during the annealing process, the martensitic structure breaks down at the recalescent temperature into pearlite and ferrite (soft iron) if the carbon content of the steel is lower than about .90%, or pearlite and the other and very hard constituent, “cementite,” if the steel has more than .90% of carbon. If the carbon content happens to be just .90%, or thereabouts, there is exactly sufficient pearlite to make up the total area of the field shown under the microscope.