VITAMIN D 303 



the depth of penetration of ultra-violet rays through animal tissue and 

 found that light waves of 280 //// pass through the skin of an anesthe- 

 tized rabbit, where the skin is 1 to 2 millimeters thick ; and rays of 

 300 nfi pass through the abdominal wall, 3 to 4 millimeters thick. Hume, 

 Lucas and Smith (1927) did not succeed in obtaining any indication of 

 penetration of light waves of 280 fifi through skin after 3 hours' ex- 

 posure to a mercury vapor lamp at 2 feet. Calculating from Hasselbach's 

 (1911) observations, they estimated that 1.56 per cent of the incident 

 light of a wave length of 297 fi^ penetrates 0.1 millimeter epidermis, 

 and one three-thousandth penetrates 0.2 millimeter epidermis. They 

 found that "vitamin D in an irradiated cholesterol can be absorbed 

 from a small area of undamaged skin in sufficient amounts to supply 

 the needs of the animal" in rats on a diet deficient in fat-soluble vita- 

 mins. Further it appeared that rickets could be prevented "in rabbits 

 fed on a rickets-producing diet (McCollum 3143) and almost normal 

 calcification of the bones produced when an area of skin 2.5 X 3.5 

 cm. was irradiated for 10 minutes 3 times a week." 



Nature of Vitamin D 



Stability. — The early studies on the nature of vitamin D were con- 

 ducted on cod-liver oil or fractions thereof, for this oil was the richest 

 known source of the vitamin. As has been pointed out, McCollum 

 and his collaborators (1922) were able to differentiate vitamins A 

 and D in cod-liver oil on the basis of their differing stability toward 

 oxidation. Cod-liver oil oxidized for 12 to 20 hours (at 100° C.) did 

 not cure xerophthalmia in rats ; it did, however, cause renewed deposi- 

 tion of calcium in bones of young rats which were suffering from 

 rickets. Zucker, Pappenheimer and Barnett (1922) showed the stability 

 of vitamin D to saponification, by demonstrating that the non-saponi- 

 fiable fraction of cod-liver oil diluted with 90 parts of cottonseed oil 

 was as potent as the original oil. This was confirmed by Steenbock, 

 Jones and Hart (1923). Southgate (1925) found that the antirachitic 

 vitamin of cod-liver oil was destroyed partially by heating for 2 hours 

 at 200° C. in absence of air, and to a still greater degree by heating 

 for 4 hours at 200° C. 



Dubin and Funk (1923) have found vitamin D stable to hydroge- 

 nation at 55° C. for 36 hours with colloidal platinum as a catalyst. 

 Bills recorded in 1925 that "it is not destroyed by hydrogen dioxide, 

 hydrogen sulfide, sulfur dioxide or formaldehyde, but is readily de- 

 stroyed by nitrous fumes and slowly by direct steam or contact with 

 mineral acids." Hart and Steenbock and Lepkovsky (1925) found that 



