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. 



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 



