PRESENT STATE OF THE PROBLEM 63 



chemically extremely active. On reaching the outer layers 

 of the atmosphere they are absorbed by molecular oxygen, 

 as a result of which the oxygen is converted into ozone. At 

 a height of about 30 kilometres above the surface of the 

 Earth there is a layer of ozone in the atmosphere called the 

 * ozone screen ' which shields us from the short-wave radia- 

 tions of interplanetary space (Fig. 2). It was noticed as long 

 ago as 1877 that sunshine has a harmful effect on many 

 bacteria. It was later established that this effect is mainly 



Fig. 3. The action of ultraviolet radiations on bacteria. 

 Living bacteria on the left. 



due to the ultraviolet part of the spectrum ^vhich has a wave- 

 length of less than 3,100 A. Using artificial ultraviolet light 

 from a mercury lamp, it was shown that the bactericidal 

 activity of ultraviolet radiations increases as the wavelength 

 decreases. It reaches a maximtun at a wavelength of about 

 2,700-2,800 A, and then falls off somewhat till the wavelength 

 is about 2,600-2,400 A, after which it again increases strongly 

 on passing to still shorter wavelengths. In the course of a 

 few minutes, or even seconds, light of this sort will destroy 

 not only the bacteria known to us, but also their spores 



(Fig. 3)-^* 



Arrhenius knew^ about the bactericidal effect of sunlight 



but he considered that it was not the light itself that killed 

 the bacteria but the oxygen which had been activated by 

 it. This idea seemed to be fully confirmed by the experi- 

 ments of Roux and Duclos, who kept spores in glass test 

 tubes without oxygen under intense illumination for months. 

 A considerable proportion of the spores retained their viabil- 

 ity under this treatment. 



These experiments suffered from a technical fault in that 

 all the ultraviolet radiation was absorbed by the glass walls 



