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Among a number of diamonds supplied to us by Professor W. T. Gordon, of King’s College, London, one, by a fortunate chance, was found to differ from the rest in its infra-red spectrum. Having confirmed by various methods th at a large absorption band at 8 g. present in the spectrum of all the other diamonds, was absent in this particular one, we explored photographically the ultra-violet spectrum of all the diamonds then available, and found th at the stone which was transparent at 8 p. in the infra-red was also transparent from about X 3000 to X 2250 in the ultra-violet, the other diamonds being opaque beyond X 3000. At this stage, between two and three hundred diamonds were examined visually by means of a simple ultra-violet spectroscope with fluorescent eye-piece without another diamond transparent beyond X 3000 being found. Among other physical and optical properties examined in comparison, little difference was found between diamonds of the usual and the transparent type : their waterwhiteness, density, refractive index, dielectric constant, Raman frequency and the earlier X-ray patterns appeared the same. A difference in the crystalline condition was, however, noted, for the transparent diamond was made up of a large number of parallel laminae, and it was also more nearly isotropic when examined by polarized light than the others.

It has been realized by several investigators that with some exceptions compounds containing the hydroxyl group have a sharp absorption band close to 2.75 μ , and frequently another much wider band about 3 μ . Erreta and Mollet (1936, 1937) and Erreta (1937) have shown that the latter is an "association" band which diminishes on dilution in a non-polar solvent, or by raising the temperature, when the association complexes split up and the hydroxyl band at 2.75 μ becomes more pronounced. (Compare also Barchewitz (1937) and Freymann (1937). In the present investigations of the infra-red absorption spectra of certain hydroxyl compounds in the 3 μ region we have made observation of the effect of the structure of the molecule on both the above-mentioned bands, as well as on the CH vibration bands. The OH bands in the region about 1.4-1.6 μ have been investigated by Wulf and his co-workers (1935, 1936) for the most part with solutions of about 0.01 mol./l/ in which the association band had practically disappeared.

Sklar's for benzene, etc.) is th at the shorter wave-length transitions are interpreted as being the more strongly antibonding. The reverse is observed to be the case as judged from the vibration frequencies appearing in the spectra. R eferences Carr and Stiicklen 1938

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