1124 BIOLOGICAL EFFECTS OF RADIATION 



ingly, the incident light intensities in the regions of the spectrum on 

 the plate were different. With incubation of the plates, the extent of 

 colony development gave a picture of the influence of the various wave- 

 lengths. Under such conditions killing occurred in the ultra-violet, 



o 



but X2960 A was the longest wave-length lethal for these organisms, 

 and even at that wave-length an exposure of 1 hr. or more was required. 

 Questioning this wave-length limit, they proceeded to use sunlight as a 

 direct source, merely interposing glass screens of determined absorption 

 (using a Watkins actinometer) between the incident light and the expo- 

 sure vessel. With such an arrangement it was clear that at 3500 A 

 killing occurred in 3 hr., whereas with a blue filter transmitting X5200 to 



o 



3600 A, no killing occurred in 6 hr. 



Recent Work. — A very careful radiometric study of the effects of 

 ultra-violet on B. coli was made by Coblentz and Fulton (29). They 

 used a quartz-mercury-vapor lamp. The several broad wave-length 

 regions investigated were defined by means of glass screens of differing 

 transmission qualities and by mica screens of different thicknesses. 

 Absolute intensity values were obtained by the use of a thermopile and 

 galvanometer, exposure to the radiation source, and reference to a 

 standard of radiation affording a basis of computation of the experimental 

 intensities. Their work was facilitated by the use of the mica screens 

 referred to, having an absorption band with a maximum at 2600 A, 

 so that variations in transmission were obtained with the different 

 thicknesses. 



From their data it appeared that the effective spectral range varied 



o 



from about 3650 A to the shortest used, i.e., in the range of the Schumann 

 region, and it was concluded that the shortest wave-lengths have the 

 most violent lethal action. In the limiting region of longer ultra-violet, 

 killing occurred under effective screens when the intensities were adequate, 

 and they believed that this intensity explains the bactericidal action of 



o 



sunlight. With this source, wave-lengths less than 2800 A are estimated 

 to be at least 10 times more rapid in killing action than wave-lengths 



o 



greater than 3050 A, in spite of the intensity differences in favor of the 

 latter. It appeared also from their results that continuous and inter- 

 mittent exposures were equally effective, so that intermittent irradiation 

 of the organism does not induce a latent effect, during the intervals of 

 rest, "either in stimulating growth or in continuing the lethal action." 

 At low intensities killing action is greatly retarded relatively, i.e., at 7/50 

 the killing ratio of the standard to low intensity is 50:70 to 50:80. The 

 energy value of the most active germicidal radiation from the mercury 

 arc is defined as follows: "Assuming that all the radiation wave-lengths 

 170 van to 280 m/i are intercepted, conserved, and utilized in lethal action, 

 then the total energy required to kill a bacterium amounts to ... 19 X 

 10-'2 watt or 4.5 X 10-'^ gm. cal." 



