STERILIZATION BY HEAT. 71 



uniformly in the soil that the customary designation for 

 them is that of " the soil bacteria." This group includes 

 a number of species that are endowed with remarkable 

 resistance to heat. Some of them are probably ther- 

 mophilic by nature, which would account not only for 

 the failure to destroy their spores by the ordinary ex- 

 posures to steam, but also for their slow and incomplete 

 development from the spore to the less resistant vege- 

 tative stage during the intervals between the heatings, 

 for, as a rule, the materials containing them are kept at 

 a temperature during these intervals that is too low to 

 favor the rapid germination of the species having ther- 

 mophilic tendencies. 



As a result of the presence of these species, media 

 that have been subjected to the customary discontin- 

 uous method of sterilization may, after having been 

 kept for a time, reveal the presence of isolated col- 

 onies of bacteria distributed through them in such a 

 way as to preclude all likelihood of their having fallen 

 upon it from the air after sterilization was supposedly 

 complete. 



Theobald Smith l has called attention to an instruc- 

 tive personal experience. He finds that when media 

 are present in vessels in only thin layers the spores of 

 anaerobic species do not develop into the vegetative 

 forms during the interval between the heatings, for the 

 reason that the shallow layer of medium does not suf- 

 ficiently exclude free oxygen to permit it ; and by sub- 

 jecting such materials, apparently sterilized by the inter- 

 mittent method, to strictly anaerobic conditions a devel- 

 opment of anaerobic species will often occur. On the 



1 Theobald Smith : Journal of Experimental Medicine, vol. iii. No. 

 6, p. 047. 



