The work described below has been done in an attempt to isolate vitamin D from the complex mixture formed by the ultra-violet irradiation of ergosterol. It is evident that vacuum distillation might be of use for this purpose, but, except for a brief statement by Windaus and Holtz (1927), we are unaware of previous work on the distillation of vitamin D.We have distilled various products obtained from ergosterol, and most frequently have used resinous products prepared as follows. A solution of ergosterol in ether was exposed to the unfiltered radiation from a mercury vapour lamp while flowing through a narrow silica tube at a rate such that about 40 per cent, of the ergosterol was destroyed. The unchanged ergosterol was then removed by precipitation with digitonin as described in a previous paper (Webster and Bourdillon, 1928) and the ethereal solution of the products of radiation was evaporated in vacuo to a dry resin. Construction of Still.

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In two earlier papers in these ‘Proceedings’ (Askew et al ., 1930, a ; Angus et al ., 1931) a crystalline antirachitic substance was described, obtained by vacuum distillation of the products of irradiation of ergosterol. In the second paper, the physical properties and antirachitic activities of a number of successive preparations of this product were recorded, and it was shown that these were sufficiently constant to suggest that it consisted mainly of one substance. We avoided the conclusions that it was yet pure, the properties showing distinct variations which suggested that other, and presumably inactive, substances were present to a variable extent. For convenience, and with reference to its conspicuous biochemical activity, we provisionally applied the name “calciferol” to this crystalline product of distillation. We realised, however, that the substance to which that name could be properly and definitely applied, as being the pure antirachitic vitamin, might differ to some extent from the product then described. Among other indications in that direction was the fact biological activity of this product was not greatly in excess of that shown by many samples of the crude irradiation product. It was indeed suggested that the irradiation mixture might contain more than one active substance ( cp . Bills et al. , 1931). Since that second publication, progress has been rapid in more than one laboratory. A few weeks later Windaus (1931) gave, at a discussion meeting of the Royal Society, a preliminary description of a crystalline product which he had separated after partial condensation of the mixed products of irradiation with maleic or citraconic anhydride (Diels’ reaction). Its melting-point, absorption and antirachitic activity were practically identical with those of our product, but its optical dextrorotation was so much lower that the two preparations could not be identical.