The absorption spectra of the cyclic dienes in the vacuum ultra-violet

Abstract

The oxidation of iron, previously freed from oxide by hydrogen treatment, has been studied at 175 to 350 degrees C; five methods (gravimetric, electrometric, film-transfer followed by chemical or microscopic examination, X-rays and electron diffraction) have been used to identify and estimate the different oxides. If specimens are exposed to air at room temperature without subsequent hydrogen treatment, complications are introduced, magnetite being formed where otherwise it would be absent. On hydrogen-reduced iron powder, ferrous oxide, magnetite and $\alpha $-ferric oxide appear when oxygen is admitted, but the oxidation is not isothermal, since a glow appears on admission of oxygen even when the initial temperature is only 40 degrees C. On hydrogen-reduced Swedish sheet, $\alpha $-ferric oxide alone appears at 175 and 225 degrees C, while at 300, 325 and 350 degrees C, a duplex film of magnetite (not $\gamma $-ferric oxide) overlaid with $\alpha $-ferric oxide is formed; at 250 degrees C, the film consists of a single layer ($\alpha $-ferric oxide) for 8 h, but then magnetite appears below it at certain places (distinguishable by the more advanced interference colours) and spreads laterally, the rate of magnetite formation increasing rapidly with time. Most of the films studied display interference tints in the usual sequence, when they are on the metal; after transfer to glass, they show different colours, which in the case of the duplex films are themselves different according as they are viewed from the magnetite side or the $\alpha $-ferric oxide side. This can easily be explained. Some experiments on pure iron give results similar to those on Swedish iron. The growth law on sheet is parabolic (W$^{2}$ = Kt + K$^{\prime}$) at 325 and 350 degrees C but logarithmic (W = K$^{\prime}$ ln (Kt + K$^{\prime \prime}$)) at 175, 225, 250, 275 and 300 degrees C. The parabolic law is generally recognized to be connected with the growth of a continuous oxide film, the thickening of which is controlled by migration or diffusion through the film, giving dW/dt = k/W or $\frac{1}{2}$W$^{2}$ = kt + k$^{\prime}$. At low temperature such movement through the film substance will be negligibly slow, but leakage of oxygen through definite discontinuities can continue; if the volume increase accompanying oxygen at each discontinuity exerts a certain chance of blocking other leakage points in the neighbourhood, the logarithmic law is arrived at.