ACTION OF HEAT 21 



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 require 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. 



Koch and WoUffhiigel, in 1881, made a thorough investi- 

 gation of this subject. A large number of pathogenic and 

 non-pathogenic organisms were tested, with the following 

 results : 



A temperature of 78° to 123° C. ( = 172° to 253° F.), main- 

 tained for over an hour, was found not to kill, and it was 

 found necessary to employ a temperature of 120° to 128° C. 

 ( = 248° to 262° P.) "for at least an hour and a half to insure 

 the complete destruction, in the absence of spores, of all of 

 the species tested. The spores of Bacillus anthracis and 

 Bacillus subtilis resisted this temperature, and required to 

 insure their destruction a temperature of 140° C. ( = 284° F.) 

 maintained for three hours. This temperature is injurious 

 to most articles requiring disinfection, such as bedding and 



