38 THE FUNDAMENTAL PRINCIPLES. 



within a few minutes for the ordinary non- spore-bearing bacteria. 

 Spores are able to resist for a long time the effects of cold and 

 excessive heat. Some have developed after having been im- 

 mersed for a long time in liquid air (temperature, 200 C.j, 

 and also after exposure to dry heat of above 150 F. for 

 sixty minutes and moist heat for thirty or forty minutes. 



II. Moisture. 



A certain amount of moisture is absolutely indispensable 

 for the growth of bacteria, desiccation being fatal within a 

 few minutes for nearly all the fully formed bacteria. 



Spores, however, are capable of developing after being kept 

 dry for an indefinite period. 



III. Decomposable Organic Material. 



A certain amount of decomposable organic matter is indis- 

 pensable for the development of the bacteria. This they 

 decompose into simple elements, and are so able to obtain the 

 nitrogen and carbon necessary for their sustenance. At the 

 same time they set free carbon dioxide and nitrogen from the 

 remainder, and so provide for the nourishment of the higher 

 vegetables, which must have these substances free in order to 

 support life. And in so doing bacteria render a service of 

 incalculable value, as without- it life in the animal kingdom 

 would soon be extinct. 



Different bacteria require a greater or lesser proportion of 

 the proteid substances for their nutrition ; and according as 

 substances contain these in a more or less favorable condition 

 for absorption they are said to be more or less good culture- 

 soils or media. 



Some few species seem to be able to live in saline solution 

 and in other media where there is no appreciable amount of 

 organic matter, but in these cases they probably obtain their 

 nutrition from the decomposition of slight traces of ammonia 

 and the carbon dioxide of the air contained in the water. 



Again, some bacteria seem to have the power of decomposing 



