328 BIOLOGICAL EFFECTS OF RADIATION 



vitamin D is formed with equal efficiency by any wave-lengths within 

 the region of the principal absorption of ergosterol. Strictly with 

 reference to the formation of vitamin, and apart from side reactions and 

 decompositions, such a conclusion may be correct. It is known, however, 

 that the irradiation of ergosterol produces several substances in succession 

 and/or in parallel. This fact was established by the early investigations 

 of Rosenheim and Webster (71), Webster and Bourdillon (85), Smakula 

 (78), Bills, Honeywell, and Cox (12), Fabre and Simonnet (25), Webster 

 and Bourdillon (86), Delaplace and Rebiere (22), Bourdillon, Fischmann, 

 Jenkins, and Webster (17), Windaus, Westphal, v. Werder, and Rygh 

 (102), Lahousse and Gonnard (50), Reerink and van Wijk (65), and 

 others. It has been confirmed in many later investigations. Inasmuch 

 as the several products of irradiation have absorption spectra different 

 from ergosterol, they are themselves susceptible to different wave- 

 lengths. The net result is that the sequence of products obtained by 

 irradiation varies with the wave-lengths of light employed (Reerink and 

 van Wijk, 65; Windaus, 89; Askew, Bourdillon, Bruce, Jenkins, and 

 Webster, 4). 



The temperature coefficient of activation is small. Bills and Brick- 

 wedde (10) found that cholesterol containing 1.2 parts of "ergosterol" 

 per 1000 parts was readily activated at -183°C., although the product 

 was somewhat less potent than the product of similar irradiation at room 

 temperature. Webster and Bourdillon (85) irradiated ergosterol at 

 temperatures between —195° and +78°C. and again observed the small- 

 ness of the temperature coefficient. Since bimolecular reactions have a 

 large temperature coefficient and are generally inhibited at very low 

 temperatures, the conclusion from these experiments was that the change 

 involved is monomolecular. All subsequent experience, including the 

 analysis of crystalline preparations of vitamin D, has borne out the view 

 that activation consists in an isomerization of the ergosterol molecule. 



The composition of the irradiation product is influenced by the purity 

 and nature of the solvent in which the ergosterol is dissolved. Ergosterol 

 in the dry state takes activation poorly, presumably because the first 

 irradiation products which form on the exposed surfaces act as filters to 

 prevent light from reaching the under layers until the surface products 

 are destroyed. This principle may explain the observation of Windaus, 

 Westphal, v. Werder, and Rygh (102) that agitation of the solution 

 during exposure enhances the formation of vitamin D. To the extent 

 that the solvent itself is opaque to short waves, it necessarily exerts a 

 a filtering action on the activating rays. 



The presence of dissolved oxygen in the solvent markedly affects 

 the spectral picture of activation (Smakula, 78; Bills, Honeywell, and 

 Cox, 12; Reerink and van Wijk, 65). It does so by altering the by-prod- 

 ucts of activation, rather than by affecting to any large extent the forma- 



