PHYSIOLOGY OF THE MICROBES 97 



specimen of rock salt cuts off all visible light without cutting 

 off the active ultra-violet portion, and rays which traverse it 

 destroy bacteria. 



The action of ultra-violet rays is practically equally rapid in 

 the presence as in the absence of oxygen. They produce a 

 little peroxide of hydrogen in the medium of suspension, but 

 in quantities 400 times too weak to be active ; hence the action 

 is not due to the peroxide. By putting in the path of the 

 ultra-violet rays from a mercury lamp a plate of white glass of 

 one millimetre thickness, all the ultra-violet spectrum is cut off 

 beyond the rays 3027-3022 ; the latter only penetrate the glass 

 very much weakened, and in this case the bactericidal action 

 is much diminished. By far the most powerfully bactericidal 

 rays are those which have a wave-length below 2*800 units. 

 " Protoplasm (albumin, gelatine, and serum) absorbs the ultra- 

 violet rays below 2*900 units : it is therefore the rays absorbed 

 by the cells which exert the destructive action." 



The ultra-violet rays have been studied with a view to the 

 destruction of cancer cells. Exposed to the ultra-violet rays the 

 tubercle bacillus loses its property of taking on a stain which is 

 acid-fast. An exposure of ten minutes kills them. An exposure 

 of one minute attenuates them, and, inoculated in guinea-pigs, 

 they now produce a slow lingering infection ; the animal lives 

 for months, whereas the controls die within forty days at most. 

 After an exposure of three minutes the bacilli no longer grow 

 on potatoes. The toxin of tubercle, tuberculin, which stands 

 heating at 134 C. for half an hour, is destroyed by five hours' 

 exposure to ultra-violet rays. The solutions should be exposed 

 n a layer of two or three millimetres and kept shaken. Tuber- 

 culin exposed to the rays in vacua is destroyed much more 

 slowly than tuberculin exposed in air (M. and Mme. Henri 

 and V. Baroni). 



Certain coloured and fluorescent substances such as eosin 

 erythrosin, and bengal-rose, are injurious to bacteria ; and still 

 more so to infusoria, in presence of light, but are quite or 

 almost harmless in the dark. These have been called photo- 

 dynamic substances. Several have been employed in photography 



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