JOHN F. NORTON 373 



While, in general, the results obtained agree fairly c-'iosely, certain discrepancies 

 should be noted. The active geimicidal region of the s^pectrum extends from about 

 2,950 to 1,850 A. P>om 3,000 to 3,500 A activity exis'ts but is greatly diminished as 

 compared with shorter rays. Bazeoni' believed that t/.hese longer rays were really the 

 active ones but were stimulated by short light wav/es. There is no evidence to sup- 

 port this contention. Other investigators have reported maximal germicidal action 

 at various wave-lengths between 2,950 and 2,200 A, but, as Coblentz and Fulton^" 

 point out, rapidity of abiotic action is depenchent upon two factors — light intensity 

 and wave-length. In most of the experimenftal work reported in the hterature Httle or 

 no attention has been given to the inten^sity of the radiation, and it is evident that 

 wave-lengths of, for example, 2,600 A^ of high intensity might exert a more rapid ef- 

 fect than those of 2,200 A of low intensity. On the basis of all the data available, it is 

 fair to state that germicidal actioni of light increases with decreasing wave-lengths in 

 the region through which quart'z will transmit light waves. 



Coblentz and Fulton^ have calculated the energy required to kill bacteria. For 

 ultra-violet light containing ^A^ave-lengths of 1,700-2,800 A this amounted to 19X10"" 

 watts per bacterium. When light waves below 2,200 A were excluded, seven times 

 this amount of energy wrxs required. This would indicate a high activity for the short 

 radiations. Regardless of the accuracy of these calculations, it is evident that only a 

 minute amount of ligh.t energy is required for the destruction of bacterial cells. 



The Schumann re.'gion has been the subject of several investigations. Lyman^ has 

 criticized some of this work from a physical standpoint. These very short waves are 

 not found in light from the mercury arc. Furthermore, they are absorbed by air and 

 by quartz. Experimental work must be done in vacuo and exposure must be made 

 through fluorite windows. Bovie-" has made an extensive study of these radiations, 

 using organisms higher than the bacteria, and has found that they exert a more rapid 

 destructive acticm on living cells than the longer light waves. 



THE INFLUENCE OF VARIOUS FACTORS 



Bacterial species. — The extremely rapid germicidal action of ultra-violet light may 

 account for reports showing that but little difference exists in regard to the suscepti- 

 bility of various? bacteria toward ultra-violet light. Even the tubercle bacillus, one of 

 the most resistant of the bacteria to external agents, appears to be almost as easily 

 destroyed by light as are the organisms usually regarded as more sensitive to physical 

 and chemical agents. Reported differences in sensitivity are not consistent. Fluores- 

 cent strains are much more difficult to kill than non-fluorescent organisms of the same 

 species.s Bacterial spores have been generally reported as no more resistant than veg- 

 etative cells. Higher organisms, such as Paramecium, yeasts, and molds, are consider- 

 ably more resistant than bacteria. Colored mold spores are highly refractory. 



Medium. — The various menstrua used to prepare bacterial suspensions for ex- 

 posure to ultra-violet light may have a marked influence on the results. Broth-pep- 

 tone media absorb the rays and thus exert a protective action. Pure water, on the 



• Bazzoni, C. B.: loc. cit. 3 Lyman, T.: Nature, 84, 71. 1910. 



^ Coblentz, W. W., and Fulton, H. R.: loc cit. " Bovie, W. T.: Bot. Gaz., 61, i. 1916. 



5 Burge, W. E., and Neill, S. J.: Am. J. Physiol., 38, 401. 1915. 



