76 BACTERIOLOGY. 



may be formed in the presence of sun-light is hydrogen 

 peroxide. Insolated urines not only become sterile but 

 remain so, as a result of the formation of this substance. 



The action of direct sun-light is especially marked in 

 the presence of air. It must not, however, be inferred that 

 oxygen is necessary. Insolation will kill bacteria which 

 are kept in a vacuum, though more slowly than if air was 

 present. Oxygen, therefore, favors or assists the action 

 of sun-light. 



Diffuse sun-light has very little or no action on ordinary 

 bacteria. Some pathogenic bacteria, like the tubercle 

 bacillus, are weakened by mere exposure to day-light. By 

 far the best results are obtained when bacteria are grown 

 in the dark. 



Sun-light, the same as heat, first weakens or attenuates 

 the organism and finally destroys it. Thus, any one of the 

 bright pigment-producing bacteria on exposure to sun- 

 light becomes so altered physiologically that it will no longer 

 produce a pigment. Colorless varieties can thus be readily 

 obtained. In the case of disease-producing bacteria the 

 weakened condition is seen in the fact that the insolated 

 organisms will no longer kill animals. 



High Pressure. 



When bacteria are grown under a high pressure they 

 develop under unfavorable conditions. A weakening or 

 attenuation results especially if a number of successive 

 generations are made under such conditions. Thus, the 

 anthrax bacillus, when grown for several generations under 

 a pressure of 9 atmospheres, became so attenuated that it 

 had no effect upon the most susceptible animal, the white 

 mouse. That bacteria can live under considerable pressure 

 is seen in the fact that certain species are known to grow 

 on the ocean's bottom at a depth of several thousand feet. 



