VITAMIN D 313 



lengths by means of absorption spectra and feeding tests and made the 

 apparently important observation that "irradiation of ergosterol with 

 light of wave-length 254 nix ("short wave" irradiation) gives rise to a 

 series of reaction products different from that caused by irradiation 

 with light of wave-length >275 /im ("long-wave" irradiation). In the 

 case of long-wave irradiation, the reaction proves to be comparatively 

 simple, in so far as only one product is directly formed from the 

 ergosterol. This product itself, which appears to be vitamin D, is on 

 longer irradiation gradually transformed into a new and much more 

 stable product. As the rate of destruction of the first product is rather 

 small, it is possible to convert about 60 per cent of the ergosterol into 

 vitamin D before the secondary reaction becomes important. The 

 short-wave irradiation causes changes of a more complex character. 

 In this case there are two reaction-products present during the first 

 period of irradiation. It is made probable that one of these products, 

 which is very quickly destroyed on prolonged irradiation, and thus 

 does not accumulate to an appreciable amount is identical with vita- 

 min D. The other product, into which the main portion of the ergo- 

 sterol is transformed, is much more stable in respect to this irradia- 

 tion." 



The product formed by the long-wave irradiation, about 50 per cent 

 of which is considered to be vitamin D, was capable of curing a rat of 

 rickets in 14 days in daily doses of 0.00001 milligram. Another sample 

 estimated to contain about 30 per cent of vitamin D cured infantile 

 rickets within 14 days by a daily dose of 0.04 milligram, an amount said 

 to be 100 times smaller than the customary dose of irradiated ergosterol. 

 Contrary to previous suggestions that the maximum absorption of 

 vitamin D is at 280 ///^ the very active substance showed maximum 

 absorption at about 270 fifi. 



Marshall and Knudson (1930), using monochromatic light of various 

 wave-lengths obtained from several intense sources, first determined the 

 effect of varying light intensities on the minimum daily dose of irradiated 

 ergosterol necessary to show the first incidence of healing in rats follow- 

 ing the technique of Knudson and Moore (1929) and then varying the 

 wave-lengths (from 302.2 ///i to 230 nn) and the time of exposure 

 calculated the rate of production of vitamin D with results which they 

 summarize as follows : 



"The rate of production of vitamin D from ergosterol is propor- 

 tional to the first power of the light intensity. The rate of production of 

 vitamin D is directly proportional to the number of light quanta absorbed 

 by ergosterol and independent of the wave-length of the light used. 



