VLriiA- VIOLET RAYS 165 



foriiRT is cxtcnsnc'ly fxploitcd. C'liicl' aiiionii' tlif pliott-t'licniical 

 actions is that of forming a j)rotective pigment in accordance with 

 the principle of Le ChateHcr, Some of the other chemical results 

 of exposure to ultra-violet rays are of interest, but can only l)e 

 mentioned here. 



(1) Ergosterol has two well-defined absorption bands betw^een 

 2,500 and 3,000 AU. After irradiation these bands vanish and 

 new bands appear further in the ultra-violet region (about 2,100 

 and 2,400 AU). Irradiated ergosterol is extraordinarily effective 

 in curing rickets, and is thus of use where effective sunlight cannot 

 be directly applied to the body. 



(2) Hydrophilie colloids on exposure to radiation " take on " 

 some extra electrons, and so become internally more mobile. If 

 they are incorporated in a membrane that membrane becomes 

 more permeable. 



(3) Certain syntheses are accelerated by the irradiation of the 

 reacting substances, e.g. exposure to rays of 2,000-2,500 AU of a 

 solution of anmionium carbonate for two or more hours causes the 

 formation of urea. This synthesis is probably brought about by 

 alternate reduction and oxidation of carbonic acid. 



Further Reading 



Russell. " Ultra- Violet Radiation." Messrs. Livingstone. 

 MiLLiKAN. " The Electron." University of Chicago Press. 

 Crowther. "Molecular Phvsics." Messrs. J. & A. Churchill. 



