lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



Applying the analogy to the consideration of radiotoxic virulence, 

 the determination of the speed with which the toxic doses of the 

 several rays produce lethal effects has heen made hy computing 

 the brevity of time required to produce lethal effect for a standard 

 radiotoxic quotum exceeding the lethal radiotoxic threshold. 



The radiotoxic quotum is the amount of radiotoxicity applied, and 

 it is apparently proportional ( i ) to the time during which the algae 

 are exposed to it, (2) to the intensity of the ray, and { 3 ) to the. 

 radiotoxic spectral sensitivity. The radiotoxic virulence is evidently 

 inversely proportional (i) to the radiotoxic quotum applied and (2) 

 to the time required to produce a toxic effect. The determination has 

 been made for eight ultraviolet rays, and as plotted against wave 

 length gives a curve of radiotoxic virulence (the reciprocal of the 

 product of the radiotoxic quotum for each ray Ijy the time of re- 

 sponse ) . ( See table 6 and fig. 2. ) 



DISCUSSION 



The curve in figure I does not disagree beyond reasonable error 

 with the one shown in figure i of Meier (1932), although the earlier 

 curve was determined by a different method. It is questionaljle 

 whether the wave length for the maximum radiotoxic effect has yet 

 been determined, since effects at 2600 A and shorter wave lengths 

 have not been sufificiently studied in these experiments. Experi- 

 ments including regions 2300 to 2700 A should be made for the pur- 

 pose of finding further information on this subject. Additional 

 experiments should also be performed to check the assumption made 

 that the radiotoxic effect is proportional to the intensity of irradiation 

 and to the time of irradiation jointly. It is also possible that weaker 

 irradiations would produce a stimulation of growth which is not 

 apparent in these plates because of the luxuriant green growth 

 covering the entire surface of each culture before irradiation. Further 

 experiments are being planned to investigate this point. 



SUMMARY 



The radiotoxic spectral sensitivity has been determined for eight 

 wave lengths in the ultraviolet ranging from 2652 to 3022 A as applied 

 to a unicellular green alga, Chlorclla vulgaris. Although all the rays 

 from 2652 to 3022 A killed the algae eventually, death ensued much 

 more (juickly in some of the regions than in others. The radiotoxic 

 virulence or speed of effectiveness of each lethal ray in killing the 

 algal cells for a radiotoxic quotum at eight wave lengths ranging from 

 2652 to 3022 A has been calculated. 



