48 



STERILIZATION AND DISINFECTION. 



The Arnold steam sterilizer (Fig. 13), or one patterned 

 after this, (Fig. 14) is used for this purpose. It is simple in 

 construction, easily used, and inexpensive. An exposure of 

 one hour is sufficient to destroy bacteria, but bacteria which 

 are in the resting-spore stage may resist the action of steam 

 for hours. 



As such a prolonged steaming would spoil the culture- 

 medium, this should not be exposed to its action for such a long 



FIG. 13. 



STERILIZING CHAMBER 



Ju A Ai 



Arnold steam sterilizer. 



time ; and for that reason the intermittent or fractional method 

 of sterilizing is used. The culture-medium is exposed to the 

 action of the steam for fifteen minutes on each of three suc- 

 cessive days. The first sterilization will kill all the fully de- 

 veloped bacteria. Any spores which may have survived this 

 sterilization will develop into bacteria in the course of the suc- 

 ceeding twenty-four hours, and these are killed by the next 



