STEAM. 



49 



sterilization. After the third sterilization the medium can 

 safely be said to be absolutely sterile. If the cotton plugs are 

 not removed, the medium will remain sterile forever, but 

 unfortunately it will, in the course of time, dry up. If the 

 tube is hermetically sealed, the medium will remain intact. 

 Any period of sterilization should not be continued too long, 

 as it may spoil the medium by preventing its solidification ; and 



FIG. 14. 



Boston Board of Health steam sterilizer. 



undue prolongation of the first period may also inhibit the 

 development of the spores, which will thus escape the effect 

 of the later sterilizations and develop afterward. Agar-agar 

 is sterilized only twice, because it cannot withstand repeated 

 liquefying. 



Sterilization by steam may also be effected by exposing the 

 article to be sterilized to the action of streaming or live steam 

 for one hour, or for fifteen minutes to the action of steam 



4 Bact, 



