THE METHODS OF STERILIZATION. 



77 



destroy all known bacteria and their spores within fifteen or 

 twenty minutes. The sterilization of culture-media,, then, is 

 usually done by the means of steam, with or without pressure. 



4. In some instances, when exposure to steam for one hour 

 would be prejudicial to the medium, what is known as dis- 

 continuous or fractional sterilization is resorted to. The 

 medium is steamed during three consecutive days for twenty 

 minutes each time ; during the interval it is kept in favorable 



FIG. 38. 



FIG. 39. 



Laboratory hot-air sterilizer. 



Rose-burner. 



conditions for the development of bacteria. In this manner 

 the first heat destroys all the fully formed bacteria that may 

 exist. The favorable temperature in the interval between 

 the first and second heatings allows all the spores contained 

 in the medium which may not have been destroyed in the 

 first heating to develop into fully formed germs, which are 

 destroyed by the second application of steam on the ensuing 

 day. The application of heat on the third day is to make 



