OXYGEN 31 



temperatures for a considerable time without losing 

 their germinating power, those of the hay bacillus (Bs. 

 subtilis} being uninjured by boiling in water for half an 

 hour or longer. The spores of certain forms isolated 

 from soil have been found to be capable of development 

 after exposure for an hour to a dry heat of 130 C., but 

 a few minutes at 150 C. is sufficient to destroy all kinds 

 of spores. 



3. Water. Water is essential for the manifestation 

 and continuance of the phenomena of life in all forms of 

 living beings ; without it active life is suspended, and 

 prolonged dryness results in death. The power of resist- 

 ing drought varies with the kind of organism and with 

 its stage of development ; many kinds of bacteria in a 

 vegetative state become weakened or lose their vitality 

 altogether when dried for a few days, while their spores 

 will often withstand drying for many months, or in some 

 cases even for several years. 



4. Oxygen. Bacteria differ very much from each 

 other in regard to the need of free oxygen for their 

 growth and maintenance of their vital functions. Some 

 of them are dependent upon a supply of free oxygen in 

 the medium in which they are grown, and are spoken of 

 as aerobic species ; examples of this class are Bs. subtilis 

 and Bact. fluorescens. Forming another group are those 

 species which thrive only when free oxygen is excluded, 

 their vital powers being suspended or destroyed by 

 exposure to air for a few hours ; these are termed 

 anaerobic bacteria, good examples of which are the 

 organisms responsible for butyric acid fermentations. 

 Between these obligate aerobes, which soon die unless 

 well supplied with air, and the obligate anaerobes, which 

 are checked by the presence of less than .5 per cent, of 



