THE BIOLOGY OF BACTERIA 25 



1877, and though many of their results appear contradictory, 

 we may be satisfied to adopt the following conclusions re- 

 specting the matter: 



(i) Sunlight has a deleterious effect upon bacteria, and to 

 a less extent on their spores. 



(2) This inimical effect can be produced by light irre- 

 spective of rise in temperature. 



(3) The ultra-violet rays are the most bactericidal, and the 

 infra-red the least so, which indicates that the phenomenon 

 is due to chemical action. 



(4) The presence of oxygen and moisture greatly increases 

 this action. 



(5) The sunlight acts prejudicially upon the culture me- 

 dium, and thereby complicates the investigation and after- 

 growth. 



(6) The time occupied in the bactericidal action depends 

 upon the heat of the sun and the intrinsic vitality of the 

 organism. 



(7) With regard to the action of light upon pathogenic 

 organisms, some results have recently been obtained with 

 Bacillus typhosus. Janowski maintains that direct sunlight 

 exerts a distinctly depressing effect on typhoid bacilli. At 

 present more cannot be said than that sunlight and fresh 

 air are two of the most powerful agents we possess with 

 which to combat pathogenic germs. 



A very simple method of demonstrating the influence of 

 light is to grow a pure culture in a favourable medium, 

 either in a test-tube or upon a glass plate, and then cover 

 the whole with black paper or cloth. A little window may 

 then be cut in the protective covering, and the whole exposed 

 to the light. Where it reaches in direct rays it will be found 

 that little or no growth has occurred ; where, on the other 

 hand, the culture has been in the dark, abundant growth 

 occurs. In diffuse light the growth is merely somewhat in- 

 hibited. It has been found that the electric light has but 



