FERMENTS, ENZYMES, TOXINS AND PTOMAINS 59 



Influence' of Light. Light is not only unnecessary to the 

 performance of functions by bacteria but appears to be 

 in varying degrees inhibitory. 



Direct sunlight is destructive to many species. It is a 

 matter of common experience that cultures of particularly 

 important species retain their type characteristics better 

 and longer if cultivated in the dark than in diffuse daylight. 



Electric light has likewise a depressing influence upon 

 the viability of bacteria. Beyond the fact that bacteria in 

 vacuo are unaffected by light we have no knowledge of the 

 mechanism of its action. Presumably it has something to 

 do with oxidation processes. 



The germicidal action of the direct rays of the sun may 

 be easily demonstrated by preparing a plate of colon bacillus, 

 shading a portion and allowing the sun to shine upon it for 

 a time, varying with the intensity of its light. Growth will 

 occur in the shaded part, none or only relatively little in 

 the illuminated part of the plate. 



Influence of Pressure. The influence of pneumatic pres- 

 sure on the viability of bacteria appears to depend upon the 

 character of the gas used. Ordinary air, or its constituents, 

 oxygen and nitrogen, whenever pressed heavily (600 to 

 2000 atmospheres) upon cultures of bacteria, have a slight 

 inhibitory effect; Carbon dioxide under five to ten atmos- 

 pheres pressure is shown by Park and his associates to 

 destroy almost all of the typhoid, dysentery, diphtheria 

 and colon bacilli exposed to it within twenty-four 

 hours. 



Effect of Moisture. As is the case with all living plants 

 a degree of moisture is essential to life. Certain species of 

 bacteria are killed by ordinary drying, and many of them by 

 absolute drying. The spores (to be described later) of bac- 



