STERILIZATION BY HEAT 81 



have been subjected to the customary discontinuous method 

 of sterihzation may, after having been kept for a time, 

 reveal the presence of isolated colonies of bacteria distrib- 

 uted 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^ has called attention to an instructive 

 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 sufficiently exclude free oxj-gen to per- 

 mit it; and by subjecting such materials, apparently steril- 

 ized by the intermittent method, to strictly anaerobic 

 conditions a development of anaerobic species will often 

 occur. On the other hand, if the vessels be nearly filled with 

 media, and especially if the area of the surface be small, the 

 conditions are much more favorable to the germination of 

 anaerobic spores, and sterilization by this process after such 

 precautions is usually perfect. 



Fortunately, these undesirable experiences are rare, but 

 that they do occur, and result in no small degree of annoy- 

 ance, will be admitted by most bacteriologists. 



It must be borne in mind that this method of sterilization 

 is only applicable in those cases which present conditions 

 favorable to the germination of the spores into mature 

 vegetative cells. Dry substances, such as instruments, 

 bandages, apparatus, etc., or organic materials in which 

 decomposition is far advanced, where conditions of nutrition 

 favorable to the germination of spores are not present, do 

 not offer the conditions requisite for the successful operation 



1 Journal of Experimental Medicine, iii, No. 6, p. 647. 



