LIGHT 65 



but is very much lower when grown in the absence of oxygen. 

 ]\Iany other soil organisms exhibit very little difference in 

 rate or amount of growth when grown at temperatures 

 which may vary as much as 10° or 15°, apparently an 

 adaptation to their normal environment. The disease-pro- 

 ducing organisms show much narrower limits for growth, 

 especially those which are difBcult to cultivate outside the 

 body. For example, the bacterium of tuberculosis in man 

 scarcely develops beyond the limits of 2° or 3° from the 

 normal body temperature of man (37°), while the bacterium 

 of tuberculosis in birds grows best at 41° to 45°, the normal 

 for birds, and the bacterium of so-called tuberculosis of cold- 

 blooded animals at 14° to 18°. 



Those bacteria whose optimum temperature is above 40° 

 are sometimes spoken of as the "thermophil" bacteria. The 

 fixing of the "thermal death-point," that is, the tem- 

 perature at which bacteria are killed is a matter of great 

 practical importance in many ways and numerous deter- 

 minations of this have been made with a great many organ- 

 isms and by different observers. The factors which enter 

 into such determinations are so many and so varied that 

 unless all the conditions of the experiment are given, the 

 mere statements are worthless. It may be stated that all 

 young, actively growing (non-spore-containing) disease-pro- 

 ducing bacteria, when exposed in watery liquids and in small 

 quantities are killed at a temperature of 60° within half an 

 hour. It is evident that this fact has very little practical 

 application, since the conditions stated are rarely, if ever, 

 fulfilled except in laboratory experiments. (See Sterilization 

 and Pasteurization, Chapter XIII.) 



LIGHT. 



Speaking generally, it can be said that light is destructive 

 to bacteria. Many growing forms are killed in a few hours 

 when properly exposed to direct sunlight and die out in 

 several dayS in the diffuse daylight of a well-lighted room. 

 Even spores are destroyed in a similar manner, though the 

 exposure must be considerably longer. Certain bacteria 

 5 



