GROWTH OF BACTERIA 



non-conducting material. On the inside, some little way 

 above the bottom, is a perforated partition upon which 

 the glass flasks, tubes, and other vessels containing the 

 media to be sterilized can rest. 



When used water is poured into the sterilizer, and the 

 latter then placed over a Bunsen burner; steam soon 

 forms, and fills' the space surrounding the tubes of cul- 

 ture media. 



An inexpensive sub- 

 stitute for this appar- 

 atus is an ordinary 

 large saucepan fitted 

 with a potato steamer. 

 For the sterilization 

 of glass tubes, flasks, 

 and other laboratory 

 apparatus, which are 

 uninjured by high tem- 

 peratures, a hot-air 

 sterilizer (Fig. 15) is 

 generally used. 



This is a cubical 



FIG. xs.-Hot.air sterilizer. sheet-iron box, With a 



door in front, the whole having double walls, with an air 

 space of about an inch between them. 



A temperature of 150 C. or higher is easily main- 

 tained in the interior by means of a Bunsen burner 

 placed beneath it ; all tubes, flasks, Petri dishes, and 

 similar apparatus should be subjected to this or a higher 

 temperature for not less than half an hour before being 

 used for bacteriological work. 



7. Pasteurization, or heating for about twenty minutes 

 at 60 to 85 C., followed by rapid cooling to about 10 



