NO. 2 EFFECT OF ULTRAVIOLET ON ALGAE MEIER 5 



To prove this assumption, experiments were performed with rela- 

 tive intensities of i, ^, and 3-fold, as shown in table 2'. The results 

 are recorded in tables 3, 4, and 5. As it is difficult to obtain algal plates 

 coated precisely alike over the entire surface, some variation must be 

 expected in observations of the reaction time or period between ir- 

 radiation time and the first appearance of each radiotoxic region. 

 Table 6 gives representative cases for each of the rays at the relative 

 intensities studied. They show that for a range of 12-fold the pro- 

 portionality holds within 40 or 50 percent in practically all cases and 

 often much closer, thereby confirming the assumption that the lethal 

 effect, as measured by the reciprocal of the reaction time, is pro- 

 portional to the product of the exposure time and intensity. (See pi. 2.) 

 As in many photochemical phenomena in the purely physical world, 

 the total effect may depend on the total number of light quanta ab- 

 sorbed by the individual. If this is true, then any algal cell will die 

 when the necessary number of quanta of ultraviolet irradiation have 

 been absorbed. 



THE LETHAL RADIOTOXIC THRESHOLD 



A Study of tables i and 3 indicates that the lethal radiotoxic thresh- 

 old or minimum amount of radiotoxicity required to produce lethal 

 effect for wave length 2250 A lies at 720 seconds for an intensity of 

 280 ergs/sec. cm'-'. Since the radiotoxic effect is proportional to the 

 product of the intensity and the duration of irradiation, then for 

 1,000 ergs/sec. cm- the exposure required for 2250 A may be set as 

 .280x720 = 202 seconds. The reciprocal of this threshold value, 202, 

 gives the radiotoxic spectral sensitivity value, which, with the other 

 computations given above, is recorded in table 7. For the sake of 

 convenience the reciprocals are multiplied by 100. In a similar man- 

 ner the radiotoxic spectral sensitivity value for each wave length was 

 determined, as shown in table 7, and the smooth curve in figure i was 

 drawn. 



The determination of the lethal radiotoxic threshold factors as 

 given in table 7 and the location of the radiotoxic spectral sensitivity 

 value for each wave length in the radiotoxic spectral sensitivity curve 

 in figure i are discussed in the following paragraphs. 



2250 A. No lethal region appeared for this wave length at the 

 exposure of 600 seconds. The algae were killed by this ray in two 

 plates at an exposure of 720 seconds, which gives a radiotoxic spectral 

 sensitivity value of .495. The lethal regions, though visible, are not 

 very distinct, and as the intensity of the ray is so small that multiply- 

 ing by so large a factor as 3.6 gives a doubtful result, it does not seem 



