vni CONDITIONS OF LIFE 93 



processes could not go on, from some of the compounds 

 contained in the fluid in which they Uve. Bacteria are for 

 this reason divided into aerobic species which require free 

 oxygen, and anaerobic species which do not. 



As to temperature, common observation tells us that 

 Bacteria flourish only within certain limits. We know for 

 instance that organic substances can be preserved from 

 putrefaction by being kept either at the freezing-point, or at 

 or near the boiling-point. One important branch of modern 

 industry, the trade in frozen meat, depends upon the fact that 

 the putrefactive Bacteria, like other organisms, are rendered 

 inactive by freezing, and every housekeeper knows how easily 

 putrefaction can be staved off by roasting or boiling. Simi- 

 larly it is a matter of common observation that a moderately 

 high temperature is advantageous to these organisms, the 

 heat of summer or of the tropics being notoriously favourable 

 to putrefaction. In the case of Bacterium termo, it has been 

 found that the optimum temperature is from 30° to 35° C, 

 but that the microbe will flourish between 5° and 40° C. 



Although fully-formed Bacteria, like other organisms, are 

 usually killed by exposure to heat several degrees below 

 boiling-point, yet the spores of some species will withstand, 

 at any rate for a limited time, a much higher temperature — 

 even one as high as 130° C. On the other hand, putrefactive 

 Bacteria retain their power of development after being 

 exposed to a temperature of -111° C, although during the 

 time of exposure all vital activity is of course suspended. 



Bacteria also resemble other organisms in being unable 

 to carry on active life without a due supply of water : no 

 perfectly dry substance ever putrefies. The preservation for 

 ages of the dried bodies of animals in such countries as 

 Egypt and Peru depends at least as much upon the moisture- 

 less air as upon the antiseptics used in embalming. 



