RADIANT ENERGY 407 



startling and valuable medical use of ultraviolet light is in the 

 cure of rickets, a use that can be demonstrated by the following 

 experiment. One young chicken is allowed to run under normal 

 conditions out of doors, and two are kept within a greenhouse. 

 The glass of the greenhouse does not permit ultraviolet light to 

 penetrate. One of the chickens in the greenhouse receives the 

 ultraviolet light of the sun or of a sun lamp for fifteen minutes 

 each day. In time, it will be found that the chicken running 

 free outside will be a normal one; so also the one in the greenhouse 

 treated to ultraviolet light, while the untreated one in the green- 

 house is a weak, undeveloped dwarf. The undeveloped chicken 

 is a victim of rickets. 



There are two cures for rickets — sunlight (ultraviolet) and 

 cod-liver oil. It is possible to "bottle up" sunlight in a number 

 of ways and make medical use of its beneficial effects. The 

 greatest advance in the collecting of sunshine for medical use 

 has been in the treatment of ergosterol with ultraviolet light. 

 A highly efficient substance for the cure and prevention of rickets 

 is thus produced (page 464). Whether it is ergosterol, milk, 

 or another substance that is irradiated, the energy resident in 

 ultraviolet light has caused certain changes in molecular orienta- 

 tion which impart curative powers to the treated substance. 

 The antirachitic portion of the spectrum extends from 3,100 to 

 2,400 A. U. Because of poor adsorption by the skin, the extremes 

 are not very effective. This is essentially the range of the adsorp- 

 tion spectrum of ergosterol. 



Such a specialized use of ultraviolet rays is for the specialist 

 to handle; we, who are laymen, can make good use of the health- 

 giving powers of ultraviolet light by letting the rays of the sun 

 play upon our bodies and penetrate every corner of our homes. 

 As a disinfectant, ultraviolet light has hardly an equal. Exces- 

 sive and intense ultraviolet radiation may cause considerable 

 harm. The indiscriminate use of sun lamps is inadvisable. 



The spectrum limits of ultraviolet light are from the visible 

 at 3,900 to nearly 1,000 A. U. It is the longer part of this range, 

 just beyond the visible region, which is most beneficial for plant 

 and animal life. According to Withrow and Benedict, plants 

 do best (grow tallest) in ultraviolet light of 2,970 (2,900 to 

 3,100) A. U. Very short wave lengths (shorter than 2,900 A. U.) 

 are harmful to both plants and animals. Experimenters, 



