As a result of some unexpected facts observed during an investigation on the etching properties of the carbides in alloy steels, the study of the changes which are well known to occur in iron carbide was commenced. Since pure iron carbide was not to be obtained in bulk, the material used Was a Swedish white pig iron which contained about 60 per cent. Fe
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C. The remainder consisted of more or less pure iron, and in view of the complexity of the curves expressing the relationship of the electrical resistance to temperature it was necessary to add to the high carbon material as pure a sample of iron as could be obtained. This was a remarkably pure sample of American ingot iron containing 99⋅9 per cent. iron. To link up these two materials, two steels with about 0⋅15 and 1 per cent, of carbon were also examined. The full analyses of the materials used are recorded in Table I. It is now well known that carbide of iron, Fe
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C, has a change point at about 200°C., but the fact that there are also equally important changes in pure iron has not, hitherto, been clearly realised, although there have, from time to time, been found values for various physical and mechanical properties which did not fit well on curves showing the general relationship of the property and temperature.
On the changes in iron and steel at temperatures below 280° C