372 PHYSIOLOGY OF BACTERIA 



case, irradiated water should be toxic. This has been 

 found not to be true (Browning and Russ, 1917, Coblentz 

 and Fulton, 1924). Consequently, the light acts upon 

 the cells directly. The observation of a very low tem- 

 perature coefficient, and the fact that the pH is of practi- 

 cally no influence upon the death rate by ultraviolet 

 light (Bayne-Jones and van der Lingen, 1923, Gates, 

 1929) point in the same direction. 



A large number of experiments have been carried 

 out with ultra-violet light. The results by Gates (1929) 

 are among the few derived from monochromatic light. 

 The test organism (usually Micr. pyogenes) was spread 

 on agar surfaces, exposed to the rays, and the survivors 

 counted by the colonies developing. With MicrococcuSj 

 the logarithmic order was not established (see p. 296). 



Exposure for thirty minutes to wave lengths of 3340 

 and 3660 A.u. did not decrease the number of living cells 

 noticeably, and even 3130 A.u. seemed^ to have little 

 effect. Wave lengths of 2500 to 3000 A.u. were quite 

 efficient, and even at 2250 A.u., bactericidal action was 

 observed, though the intensity of this wave length was 

 very low with the lamp used by Gates. 



The temperature coefficient is very nearly 1, indicating 

 that death is primarily due to a photochemical reaction. 



Another group of rays has been frequently proved 

 to be harmful to bacteria, namely the X-rays. Holweck 

 (1929) and Lacassagne (1929) computed from their 

 experiments with Ps. pyocyanea that this organism 

 must have a sensitive zone of about 0.5/z diameter. If 

 this zone is hit by one ''quantum,'^ the cell dies. If 

 the cell is hit outside of this zone, it does not die. 

 Wyckoff (1930) made computations for Bad. coli from 

 experiments with wave lengths from 0.56 to 3.98 A.u., 

 and came to the conclusion that the measured zone is 



