ACTION OF COLOURED LIGHT 21 



fungi. Similar though less marked results were obtained 

 with an electric arc light, so much so that Professor Ward 

 thinks that this form of light may prove to be an effective 

 disinfecting agent. As with sunlight, however, its action 

 is necessarily confined to organisms directly exposed to the 

 rays, and not protected by media which absorb them, such 

 as even shallow water. 



Action of Coloured Light. — When a plate culture of 

 anthrax is exposed to the solar spectrum, the germicidal 

 action is found to be the strongest at the blue-violet end 

 (Ward). Janowski exposed cultures under screens of various 

 coloured glasses and analine dyes, and found that no action 

 took place under brown or yellow ; whereas solutions of 

 fuchsine (which transmits violet rays), gentian violet, and 

 methyl blue, had but little more effect than colourless 

 fluids. 



The action of light on micro-organisms supports the 

 opinion of specialists in hygiene, that free access of light 

 is a great factor in relation to the health of a community. 



(2) Action of Heat. — As we have already seen, the ' opti- 

 mum ' temperature for the growth of most of the bacteria 

 is between 20° to 40° C. ; while some of them can grow at 

 the freezing-point of water, and others can grow at as high 

 a temperature as 60° to 70° C. Generally speaking, the 

 pathogenic organisms requii-e a temperature of 35° to 40° C. 

 In considering the influence of heat on the bacteria, we 

 must take into account the very great difference in the 

 resisting power of the vegetative cells and the spores ; and 

 the different destructive powers of dry and moist heat, as 

 well as the time of exposure and other conditions. 



Dry Heat. — If bacteria, or their spores, when in a well- 

 dried condition, are exposed to the action of heated dry 

 air, the temperature required for their destruction is much 

 higher than when moist heat is employed. 



