ACTION OF LIGHT 19 



Agents. The resistance of micro-organisms to these in- 

 fluences is very high, especially in the spore stage. 



(1) Light. — Messrs. Downes and Blount, in a communica- 

 tion to the Eoyal Society in 1881, first called attention to 

 the fact that light had an injurious effect upon bacteria, 

 and that cultures may be destroyed by exposure to sunlight. 

 About 1885, Duclaux and others took up this subject, and 

 with various pure cultures of micro-organisms it was found 

 that by exposure to sunlight the spores of various bacteria 

 lose their capacity to germinate. It was also found that 

 cultures lost their power of reproduction in diffused light, 

 and that they also became ' attenuated ' in their pathogenic 

 power. In his address before the International Medical 

 Congress at Berlin, in 1890, Koch stated that the tubercle 

 bacillus was killed by the action of sunlight in a time vary- 

 ing from a few minutes to several hours, depending upon 

 the thickness of the layer exposed. Diffused daylight 

 had the same effect, although a considerably longer time 

 was required ; when placed close to the window, about a 

 week was required. Similar results have been obtained by 

 Janowski (1891) on the typhoid bacillus. 



In the experiments of Momont (1892), dry anthrax spores 

 were found to resist the action of light for a long time, but 

 moist spores freely exposed to the air failed to grow after' 

 forty-four hours' exposure to sunlight. In the absence of 

 spores, anthrax bacilli in a moist condition freely exposed 

 to the air failed to grow after from half an hour to two 

 hours' exposure to sunlight ; but in the absence of air 

 these same bacilli were not destroyed at the end of fifty 

 hours of exposure. Buchner found that broth cultures of 

 typhoid. Bacillus coli communis, Bacillus pyocyaneus, and 

 the Vibrio cholerce Asiaticce yielded no growth after one 

 hour's exposure to direct sunlight. 



The most recent and conclusive experiments of all, how- 



