IT has been fully proved by various investigators that although cholesterol itself exhibits no antirachitic properties prior to irradiation with ultra-violet light, these are developed on comparatively short exposure to these rays. This brings the reaction within the sphere of photochemistry and since photochemical changes do not occur without the absorption of light and since it follows from the Grotthus-Draper law that only the rays absorbed are active, it would immediately become a matter of urgency to ascertain whether chole- sterol showed well-defined absorption bands in the ultra-violet. The hitherto published results on this aspect of the problem are essentially preliminary in character. Hess and Weinstock [1925] observed that when cholesterol was irradiated by ultra-violet rays a change in the absorption spectrum occurred. The activated material absorbed light of certain wave-lengths to a less degree than ordinary cholesterol, a difference in the absorption over the entire range of wave-lengths (integrated) being detected by -the use of a thermopile and galvanometer set. Schlutz and Ziegler [1926], carrying the matter further, observed that carefully recrystallised cholesterol, melting at 148.50 or very near that point, showed selective absorption "of wave-lengths between 294-296 ,u and 279-294 ,u,u with a great deal of general absorption beyond 294 ,u." The absorption bands, which were evidently very shallow, could not be detected in alcoholic solution but showed up in ether or chloroform. Moreover, the bands were only observed in the first crop (five fractions col- lected) from a fractional crystallisation from alcohol of cholesterol (M.P. 148.50) which had previously been recrystallised seven times from the same solvent.
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Heilbron et al. (1927) studied this question.