1130 BIOLOGICAL EFFECTS OF RADIATION 



Meanwhile considerable evidence has gradually accumulated in 

 support of the view that the presence of oxygen may influence the 

 alleged destructive action of visible light, and likewise the effects of 

 fluorescing substances on the bactericidal effect at visible wave-lengths. 

 This evidence has come more particularly from the side of studies on 

 the action of light on enzymes, toxins, and other products usually requir- 

 ing high intensities in order to effect inactivation (cf. the discussions in 

 Papers X and XXXVII of this work). 



The view that irradiation of aqueous media with ultra-violet light 

 may produce peroxide, and that the injurious effect should be attributed 

 to this product, has also been strongly maintained by Bedford (13). 

 He attempts to show that the destructive effect of radiation is propor- 

 tional to the production of peroxide, and that relative susceptibility of 

 the organisms to radiation is of the same order as their susceptibility 

 to peroxide. Results of this type do not appear to be supported by the 

 general evidence from the side of photochemistry (cf. Paper VII of this 

 work) . 



Wholly aside from the influence of oxygen, or the production of 

 peroxide in the media, is the question that has been raised frequently 

 in the literature regarding the effect of visible or ultra-violet light on the 

 medium or substrate in which the organisms are irradiated. A continuing 

 influence through the substrate can apply, of course, only when the 

 irradiated material becomes a part of the environment during the 

 further growth of the irradiated organism, as in the case of irradiation 

 on agar surfaces, but not with the usual procedure in suspension irradia- 

 tion. The evidence from the practical experience of many indicates 

 clearly that the irradiation of the usual bacteriological media with the 

 dosages commonly employed induces no changes resulting in toxicity (cf . 

 Ward, 155; Browning and Russ, 19; Mashimo, 100; Coblentz and Fulton, 

 29; Gates, 59, and many others). 



In general, the evidence is further substantiated by the many data 

 that have accumulated in regard to the high intensities required for the 

 production of changes leading to toxicity in such products as the simple 

 proteins and peptones, or in such complex media as milk. On the other 

 hand, caution should be observed and controls invariably included in 

 any series where high intensities are employed, since it is equally certain 

 that changes are induced in proteinaceous and other media by intensities 

 considerably greater than those needed for destruction of microorganisms. 



ABSORPTION OF RADIANT ENERGY 



Owing to the extent of studies on the lethal effect of ultra-violet 

 radiation and the significance of energies in the ultra-violet in producing 

 the bactericidal effect, absorption studies with bacteria have been con- 

 cerned more particularly with the shorter wave-lengths of sunlight and 



