298 THE VITAMINS 



290 and 313 nfi in wave length. These shorter rays penetrate dark cloth- 

 ing or a heavily pigmented skin less readily than light clothing or a 

 white skin. This indicates one factor in the exceptional susceptibility 

 of negro infants to rickets (Hess, Unger and Pappenheimer, 1922a). 



Within a few years (1921-24) evidence was accumulated in many 

 laboratories which showed that irradiation of the body with light rays 

 of short wave length from sunlight, mercury vapor quartz lamps, 

 or from carbon arc lamps promoted growth and improved ossification 

 in children, rats, chicks and pigs, particularly when the diets were 

 decidedly unsatisfactory in the relations of calcium and phosphorus. 

 (As more fully explained elsewhere, this is not to be construed as 

 indicating that irradiation, or vitamin D in any form, can take the place 

 of the needed calcium.) X-ray studies, analyses of the blood for calcium 

 and inorganic phosphorus and bone analyses were the chief methods 

 employed to study these effects. 



Steenbock and Nelson (1923) found 10 minutes of irradiation daily 

 with ultra-violet light to be as effective as 2 per cent aerated cod-liver 

 oil in promoting growth which had failed before the incidence of 

 ophthalmia in their rats on a diet free from fat-soluble vitamins, but 

 otherwise adequate. 



Hart, Steenbock and Elvehjem (1924) reported that ultra-violet 

 light can influence the storage of calcium and phosphorus and the 

 equilibrium of these elements in the blood stream of mature animals 

 in a way similar to its effect upon growing animals. Two mature lac- 

 tating goats and one mature dry goat were brought into distinctly nega- 

 tive calcium balances or calcium equilibrium on a diet deficient in the 

 antirachitic factor; irradiation with a mercury vapor quartz lamp 

 brought them into positive balance. The inorganic phosphorus of the 

 blood was also increased. 



Hart, Steenbock, Elvehjem, Scott and Humphrey (1926) found 

 that June sunlight (Wisconsin) could lessen but not obviate the losses 

 of body calcium suffered by cows giving 45 to 60 pounds of milk per 

 day while receiving a ration which contained 70 or 80 grams of cal- 

 cium oxide. But by increasing the intake of calcium oxide to 200 grams, 

 calcium equilibrium could be established even when the metabolism 

 experiment was conducted indoors. 



This work indicates that vitamin D or ultra-violet light can help 

 the organism to utilize advantageously the available calcium or phos- 

 phorus, but cannot replace an actual deficiency. 



Meanwhile investigators asked if light has also the property of 

 producing, or functioning as an equivalent of, vitamin A. 



