RADIATION AND VITAMINS 325 



esterification became definitely antirachitic upon irradiation, but that 

 phytosterols from old vegetable oils acquired only slight, if any, activity. 

 Rosenheim and Webster (67) activated cholesterol. 



Thus it became evident that in foodstuffs the sterol fraction contains 

 the acceptor of the activating rays. The course of investigation now 

 turned to the elucidation of the chemical changes induced by irradiation 

 in the sterols. 



Hess and Weinstock (33, 34) introduced the use of the quartz spectro- 

 graph for investigating the chemistry of activation. They found that 

 ordinary cholesterol was somewhat opaque to ultra-violet light, and that 

 irradiation decreased its opacity. On the other hand, dihydrocholesterol 

 and dihydrositosterol, which were not activatable, were practically 

 transparent, and the transmission of ultra-violet light was not altered 

 by irradiation. Unfortunately, a mercury arc was used as the light 

 source for the spectrograms. Since this gives a discontinuous spectrum, 

 nothing could be learned of the spectral structure in the region of 

 absorption. 



Schlutz and Morse (75), with better technique, determined that the 

 absorption spectrum of ordinary cholesterol is banded. Their densitom- 

 eter tracings showed two maxima of absorption at approximately 2940 

 and 2830 A. A trained observer, biased, perhaps, by later knowledge, 

 can see in these original tracings two additional faint inflections. In 

 retrospect it thus appears that Schlutz and Morse were the first to record 

 what is now recognized as the provitamin absorption spectrum. They 

 noticed that after brief irradiation the inflections gave way to general 

 absorption, and, considering Beer's law, they postulated that either the 

 cholesterol had been at least half metamorphosed, or else "the substance 

 in which the absorption spectrum is changed may be a small amount of 

 impurity in the cholesterol which is not removed by repeated crystalliza- 

 tions from alcohol, and which is exceedingly absorptive." 



Such are the coincidences of research that, on December 10, 1926, 

 reports from three separate laboratories confirmed the contamination 

 hypothesis. Rosenheim and Webster (69) found that cholesterol which 

 had been regenerated from cholesterol dibromide was so pure that it no 

 longer showed the characteristic absorption spectrum, and was no longer 

 activatable. They had previously noted (68) that ergosterol is activa- 

 table, but having made no quantitative determination of the degree of 

 activatability, they did not now recognize "ergosterol" as the con- 

 taminant removed by bromination. 



Heilbron, Kamm, and Morton (28) reported that fractional crystal- 

 lization of cholesterol led to the accumulation, in the least soluble fraction, 

 of the substance responsible for the characteristic absorption spectrum. 

 They recognized a third absorption maximum, 2690 A. Irradiation 

 destroyed the three bands, leaving only general absorption. 



