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The lattice spacings of high-purity iron have been measured accurately between 20 and 1502°C. The results, together with previous low-temperature data, give data between the absolute zero and the melting-point (1534°C). The thermal expansion of the lattice and of bulk metal are in good agreement over the temperature range for which the latter has been studied accurately. At the A 2 point, there is no discontinuity in the lattice spacing. In contrast to the work of some previous investigators, we have found no observ­able difference between the coefficients of expansion of the lattice immediately above and below the A 2 point, and the results suggest that the forces responsible for ferromagnetism do not affect the interatomic distances by more than 1 part in 10000. At the A 3 point, the α → γ transformation is accompanied by an increase in the interatomic distance which, when expressed as a percentage change, is slightly less than the decrease in interatomic distance during the γ → δ change at the A 4 point. The change in bond length calculated by the Pauling equation for the change (f. c. c.⇌b. c. c.) lies between the observed values for the A 3 and A 4 points, and the accuracy of the equation is confirmed.

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