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BOTANICAL GAZETTE 



[JANUARY 



Photochemical action necessitates light absorption, although all 

 light absorption is not accompanied by photochemical action. 

 Since, for substances in general, light absorption increases as the 

 wave length decreases, chemical action also increases as the wave 

 length decreases; and, as pointed out in the pioneer work of 

 Downes and Blunt, the destructive action of light upon proto- 

 plasm increases as the wave length decreases. 



There is evidence for the supposition that the chemical charac- 

 ters of some of the elements are changed when they are acted on by 

 ultra-violet light. For example, when oxygen is acted on by light 

 of short wave length ozone is formed; that is, ozone is more stable 

 in such oxygen than it is in ordinary oxygen. In passing, it may 

 be pointed out that this fact is of particular interest to the biologist, 

 for ozone is more opaque to short light waves than is molecular 

 oxygen, and it seems that life on earth is possible only because 

 the ozone formed in the upper layers of the atmosphere by 

 the ultra-violet of sunlight serves as a light-filter and protects the 

 organisms on the surface of the earth from these shorter and 

 more destructive rays. Chlorine may be mentioned as another 

 example. In this case, not only are the chemical characters of the 

 atom changed, but according to Trautz (32) the specific heat as 

 well. 



In consequence of the fundamental nature of the changes pro- 

 duced by the light, it is often found that many compounds contain- 

 ing the same element are photosensitive. For example, light 

 affects many of the compounds containing silver. Protoplasm 

 contains many photosensitive elements, and it is found that 

 protoplasm and a large number of the substances elaborated by 

 protoplasm (sugar, starch, cellulose, chitin, hair, rubber, etc) are 

 decomposed when exposed to ultra-violet light. 



The temperature coefficient of light reactions is very low. For 

 photochemical reactions, therefore, temperature has but little 

 influence upon the speed of the reaction. Photochemical changes 

 may take place in dry materials or in a vacuum. The writer has 

 found that the time required to kill spores of fungi was the same, 

 whether the spores were exposed while in a very high vacuum or 

 while in the air and turgid with imbibed water. This result is most 



