164 LIGHT AND PROTOPLASM [Ch. VII 



to show the important chemical effects of sunlight in the dis- 

 integration of organic compounds. 



3. Substitution Effects of Light. — The principal substitu- 

 tion effect of light is the replacement of hydrogen in an organic 

 compound by either chlorine or bromine. This occurs so fre- 

 quently that examples are superfluous. The substitution takes 

 place most rapidly and completely in direct sunlight, and it has 

 been shown that the rays at the blue end of the spectrum are 

 the most active in this process. The compounds affected 

 belong to the most varied groups of both the fatty and aro- 

 matic series — carbohydrates, acids, aldehydes, ketones, and 

 sulphides. 



4. The Isomerismic and Polymerismic Changes produced by 

 Light are among the most interesting. I will cite some exam- 

 ples. In the first place, it may be said that the changes in the 

 elements phosphorus and sulphur by which they assume their 

 red form have been ascribed to sunlight. Elgeomargin acid, 

 Ci^HggOg, is a compound found in connection with glycerine 

 in the oil of the seeds of Elseococca (Aleurites) vernica — 

 Chinese oil tree — one of the Euphorbiacese. This acid crys- 

 tallizes in rhombic plates which melt at 48°. When an alco- 

 holic solution of this acid is placed in a bright light, leaf -like 

 crystals of its isomere, elseostearin acid, which melt at 71°, 

 are produced (B. I, 535). Again, thymochinon forms yellow 

 crystals, which are soluble in alcohol. Subjected to a strong 

 light, opaque, whitish-yellow crystals are produced, which are 

 insoluble in alcohol. This substance, which does not arise in 

 the dark, and is hence not merely the result of oxidation, is 

 called polythymochinon (B. Ill, 180). Again, among the 

 derivatives of ethylene, CjH^, is chlorethylene, CjHgCl, a gas. 

 When placed in the sunlight this passes into a polymere, which 

 forms a viscous, amorphous, insoluble mass (B. I, 158). In 

 like fashion, bromethylene, C2HgBr, a fluid, is rapidly trans- 

 formed in the sunlight into a polymere, which is solid, amor- 

 phous, and insoluble in water, alcohol, or ether (B. I, 181), and 

 bromacetylen, CjHBr, a gas, is gradually transformed, in the 

 light, into a solid polymere. Finally, very many substances 

 undergo a gradual change of color in the sunlight, but the 

 nature of the accompanying molecular change is unknown. 



