CHAPTER XXIX 



THE ACTION OF ULTRA-VIOLET LIGHT ON BACTERIA 

 AND THEIR PRODUCTS 



JOHN F. NORTON 



University of Chicago 



Light rays at varying positions in the spectrum produce different effects on both 

 inorganic and organic substances — on dead material or on living cells. The visible 

 spectrum extends from a wave-length of about 6,500 A at the red end to about 4,000 A 

 in the violet. (An Angstrom unit equals 10-' mm. or o.i mm-) The near ultra-violet 

 region includes wave-lengths from 4,000 to about 3,000 A, the far ultra-violet extends 

 to about 2000 A, and the extreme ultra-violet (sometimes known as the Schumann 

 region) includes wave-lengths down to about 1,200 A — the lower limit of the metallic 

 arc. X-rays have wave-lengths below 600 A. As will be shown later, these divisions 

 of the wave-lengths of light have a physiological interest. 



The action of light on micro-organisms was first described by Downes and Blunt 

 in 1877.^ Their conclusions are of some interest. 



1. Light is inimical to the development of bacteria and the microscopic fungi associated 

 with putrefaction and decay, its action on the latter organisms being apparently less rapid 

 than upon the former. 



2. Under favorable conditions it wholly prevents that development, but under less favor- 

 able it may only retard. 



3. The preservative quality of light, as might be expected, is most powerful in the direct 

 solar ray, but can be demonstrated to exist in ordinary diffused daylight. 



4. So far as our investigation has gone it would appear that it is chiefly, but perhaps not 

 entirely, associated with the actinic rays of the spectrum. 



5. The fitness of a cultivation-liquid to act as a nidus is not impaired by insolation. 



6. The germs originally present in such a liquid may be wholly destroyed and a putresci- 

 ble fluid perfectly preserved by the unaided action of light. 



Downes and Blunt apparently believed that oxygen was essential for the germi- 

 cidal action of light, in spite of an experiment recorded by them in which bacteria in 

 evacuated tubes were killed by exposure to light. 



Nine years later Arloing^ and, independently, DuclauXj^'i studied the action of 

 light on both vegetative cells and spores. Arloing showed that anthrax spores ger- 

 minated more slowly when exposed to gaslight and that vegetative cells gradually 

 lost their virulence. Duclaux noticed a marked difference in the rapidity of action of 

 strong sunlight as compared with diffuse light. Arloing attributed the action to the 



' Downes, A., and Blunt, T. P.: Proc. Roy. Soc, 26, 488. 1877. 

 ^Arloing, S.: Compl. rend. Acad, de Sci., loo, 378. 1885; loi, 511. 1885. 

 3 Duclaux, E.: ibid., 100, 119. 1885. 

 ■t Duclaux, E.: An7i. de Vlnst. Pasteur, i, 88. 1887. 



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