METHODS OF CULTURE. 



43 



the steam, c, and a lower one for water, 6. Two or more 

 gas-burners placed underneath the chest, which stands on a 

 tripod, supply the heat. In the cover is an opening for a ther- 

 mometer. The chest is usually covered with felt. When the 

 thermometer registers 100 C. the culture-medium or other sub- 

 stance to be sterilized is placed in the steam and kept there 

 from 10 to 15 minutes, or longer, as required. 



Arnold's Steam-sterilizer will answer every purpose of the 

 Koch steam-chest. It is cheaper, requiring also less fuel to keep 

 it going. The steam does not escape, but is condensed in the 

 outer chamber. (Fig. 16.) 



FIG. 16. 



Arnold's Steam-sterilizer. 



The autoclave of Chamberland allows a temperature of 120 

 C. to be obtained, and is much used in Pasteur's laboratory. 



Instead of sterilizing for a long time at once, successive steri- 

 lization is practised with nutrient media, so that the albumen 

 will not be too strongly coagulated. Fifteen minutes each day 

 for three days in succession. 



