142 MEDICAL BACTERIOLOGY 



Boiling for i hour, or 20 minutes, in an autoclave at 15 pounds pressure 

 kills bacillus subtilis spores. 



Four or five daily exposures at 8oC. for 10 minutes is sometimes required 

 when sterilization is to be accomplished by the fractional or intermittent method. 



When a pure culture of bacillus subtilis is desired for study and none is at 

 hand, some hay dust is placed in sterile water, gradually heated to the boiling 

 point and then boiled for 20 minutes. 



If tubes of agar are planted with several loopsful of this decoction imme- 

 diately before heating, when it reaches 6oC., 8oC. and iooC., and again when 

 it has boiled for i minute, 2 minutes, 5 minutes, 10 minutes, 15 minutes and 20 

 minutes a pure culture of subtilis will be obtained and a study of the culture 

 made before and after exposure to different degrees of heating will show the 

 variations in the heat-resisting power and different bacteria and spores. 



