THERE is considerable evidence as to the existence T of lag in the crystallisation of pearlite, particularly in hypoeutectoid steels. Cooling curves show it in the observed temperature of the change, which depends on the rate of cooling. Microscopical observations testify to it in that the presence of carbide nuclei within the austenite (gamma iron) areas leads to crystallisation of globular pearlite at a temperature considerably higher than that at which growth occurs in the absence of such nuclei. Moreover, the growth of lamellar pearlite when once started does not occur simultaneously throughout the specimens. The change proceeds gradually, and there is no difficulty in quenching a specimen so that it contains areas both. of pearlite (transformed) and martensite (partially transformed) intermixed.