8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



5.3X1,920=10,176 seconds. From the above rough determinations 

 and the computation of the lethal factors for each wave length as 

 compared with 3022 A, table 5 has been compiled and the smooth 

 curve in figure i has been drawn. 



Table 5. — Lcihal Rad'wioxic Threshold and Radiotoxic Spectra! Sensitivity 



(Based on Table 4) 



RADIOTOXIC SPECTRAL SENSITIVITY AND RADIOTOXIC VIRULENCE 



The lethal response of the algae to the ultraviolet rays may be 

 considered from two points of view, as to the radiotoxic spectral 

 sensitivity and the radiotoxic virulence. The term " radiotoxic spectral 

 sensitivity " relates to the certainty of the lethal action, while the 

 term " radiotoxic virulence " may be used to describe the quickness 

 of the attack. To make the matter clearer by analogy, let the behavior 

 of algae with respect to three different ultraviolet rays be compared 

 to tlie behavior of a human being toward three poisons, namely, 

 radium in watch-face paint, cyanide of potassium, and rattlesnake 

 venom. 



With respect to sensitivity, each one of the three poisons is fatal 

 if administered in a sufificient dose. Probably in order of minimum 

 lethal dosage or sensitivity, they would rank : radium, cyanide, 

 snake venom. But in order of toxic virulence, or the time required 

 for lethal eiTect, they would rank very differently, probably : cyanide, 

 snake venom, radiuuL 



Applying this analogy to the selected ultraviolet rays, the deter- 

 mination of the radiotoxic spectral sensitivity, that is, the relative 

 radiotoxicity of rays of different wave lengths when applied with 

 equal intensity and duration, has been measured as described in 

 the preceding section. The determinations for each of the eight ultra- 

 violet rays when plotted against wave lengths gives a curve of radio- 

 toxic spectral sensitivity. (See table 5 and fig. i.) 



