ACTION OF LIGHT ON MICRO-ORGANISMS 389 



and Blunt's original experiments on casual mixtures of 

 micro-organisms, but also by numerous experiments in 

 which pure cultivations of the most diverse microbes 

 have been employed. 



2. This deleterious effect can be produced by light 

 irrespectively of the rise in temperature which must 

 accompany direct insolation unless special precautions 

 be taken. It is, moreover, the most highly refrangible 

 rays of the spectrum that are the most injurious to 

 bacterial life, the ultra-violet being the most, and the 

 infra-red the least, powerful in this respect, a circum- 

 stance which clearly indicates that the phenomenon is 

 due to chemical action. 



3. It was already shown by Downes and Blunt, and 

 has been abundantly confirmed by Eoux and Moment, 

 that the action of light is greatly increased by the 

 simultaneous presence of air and moisture ; indeed, so 

 pronounced is the influence of oxygen in this action 

 that there can be no doubt that the effect is due to a 

 process of oxidation, possibly brought about through 

 the agency of ozone or peroxide of hydrogen, or both. 

 This view is supported by some recent experiments of 

 Eichardson ( 4 Proc. Chem. Soc.,' 1893, p. 121), in which 

 it is shown that peroxide of hydrogen is formed in urine 

 during insolation, and that the sterilising action of light 

 can be -counteracted by the addition of substances, 

 e.g. peroxide of manganese, which destroy hydrogen 

 peroxide. 



The formation of peroxide of hydrogen during in- 

 solation naturally suggests the question whether the 

 whole bactericidal effect of light is due to this material, 

 or whether it only partially accounts for the pheno- 

 menon. Eichardson has shown that the formation of 

 peroxide of hydrogen is due to the presence of some 

 ingredient or ingredients in the urine, and that it is not 



