374 PHYSIOLOGY OF BACTERIA 



cell, perhaps much smaller. For a coccus of 1/x diameter, 

 the sensitive zone would correspond to a sphere of 

 0.215m diameter. 



The logarithmic order is well established, as may be 

 seen from Table 118. The same is true for cathode 

 rays, as observed by Wyckoff and Rivers (1930) (see Fig. 

 35, p. 297). 



If death is caused by the hit of a single quantum, 

 the death rate by a uniform source of X-rays should 

 be the same for all species of bacteria having the same 

 sensitive zone. Wyckoff found it to be identical for the 

 two species he tried, i.e.. Bad. coli and Bad. aertryke. 



SUMMARY 



The exposure of bacteria to light will cause them to 

 die if the intensity of the light is strong enough. Ultra- 

 violet light is very efficient in wave lengths between 

 2500 and 3000 A.u. 



The effect of light is not brought about by a chemical 

 change of the medium through the light ; the fundamental 

 reaction which causes the death is produced inside of 

 the cell by the light. 



The temperature coefficient is very low, indicating 

 a physical or photochemical reaction as the cause of 

 death. 



Death by X-rays and cathode rays is caused by the 

 bombardment of the bacteria with electrons which will 

 kill the cell if they hit its sensitive zone. This sensitive 

 zone has a volume about one-hundredth of the cell. 



The order of death by rays is logarithmic. 



X. DEATH THROUGH AGE 



In the chapter on the endpoint of growth, the cause of 

 death by old age of bacterial cultures has already been 



