40 BACTERIOLOGY. 



ences in resistance toward heat which the spore-forming 

 organisms are seen to possess at their different stages of 

 development, are taken advantage of in the process of 

 sterilization by steam which is known as the fractional 

 or intermittent method, and form the principle on which 

 the method is based. 



The object aimed at in this method is to destroy the 

 organisms in the shortest time and with the least amount 

 of heat. It is accomplished by subjecting them to the 

 elevated temperature at the time when they are in the 

 vegetating or growing stage i. e., the stage at which 

 they are least resistant. In order to accomplish this it 

 is necessary that there should exist conditions of tem- 

 perature, nutrition, and moisture which favor the vege- 

 tation of the bacilli, and the germination of any spores 

 that may be present. When these surroundings aiv 

 found, the spore-forming organisms are not only less 

 likely to go into the spore stage than when their en- 

 vironments .are less favorable to their vegetation, but 

 spores which may already exist develop very quickly 

 into mature cells. 



It is plain, then, that with the first application of the 

 steam to the substance to be sterilized, the mature vege- 

 tative forms of these organisms are destroyed, while cer- 

 tain spores which might have been present resist this 

 treatment, providing the sterilization has not been con- 

 tinued for too long a time. If now the sterilization is 

 discontinued, and the material which presents conditions 

 favorable to the germination of the spores is allowed to 

 stand for a time, usually for about twenty-four hours, 

 at a temperature of from 30-35 C., those spores which 

 resisted the action of the steam will in the course of this 

 time germinate into the less resistant vegetative cells- 



