66 ETERNITY OF LIFE 



intense. We can deduce a curve relating intensity with wave- 

 length for the ultraviolet radiation at the limit of the atmo- 

 sphere of the Earth (Figs. 4 and 5).'*^ 



In his review D. E. Lea'*^ also presented a wide range of 

 material showing that all forms of microbes and spores which 

 have been investigated in this respect are destroyed by the 

 action of short-wave ultraviolet light. We now possess con- 

 siderably greater factual material but it completely confirms 

 the earlier work on the destruction by ultraviolet light of all 

 forms of micro-organisms whatever their species.^^ 



Thus the earlier findings of R. Wiesner*^ that there exist 

 forms of bacteria which are resistant to ultraviolet light were 

 not confirmed by later workers. On the contrary, it is now 

 asserted that the various species only differ very slightly from 

 one another in their resistance. This effect of light is quite 

 different from that of temperature, for we know many very 

 thermostable bacteria. This difference is particularly notice- 

 able where spores are concerned. Thus, for example, the 

 spores of Bacillus anthracis and B. suhtilis are very resistant 

 to high temperatures and, in contrast to their vegetative 

 forms, can even undergo more or less prolonged boiling. 

 However, the difference in resistance between the vegetative 

 forms and spores does not exist in respect of the effect of 

 ultraviolet light, which destroys both forms of these organ- 

 isms almost equally easily. 



Another difference between the effects of temperature and 

 light is that the presence of water is not necessary for the 

 effect of light. It has now been established that completely 

 dried cultures and spores of various microbes always exhibit 

 considerable radiosensitivity.^" Neither does oxygen seem 

 necessary for the bactericidal activity of light. The earlier 

 view that the effect of ultraviolet light depended on an 

 oxidative activity seems to be untenable. It has been shown 

 experimentally that short-wave radiations can have a destruc- 

 tive effect on micro-organisms even in the absence of gaseous 

 oxygen in the surrounding medium. Ultraviolet radiation 

 is bactericidal by virtue of its direct action on the substance 

 of the bacteria. 



