i66 



PATHOGENIC BACTERIA. 



has introduced little paper caps with which the cotton 

 stoppers are protected from the dust. These are easily 

 made by curling a small square of paper into a ' ' cornu- 

 copia, ' ' fastening by turning up the edge or putting in a pin. 

 The paper is placed over the stopper before the sterilization, 

 after which no contamination of the cotton can occur. 



Sterilization of Culture-media. — As almost all of the 

 culture-media contain about 80 per cent, of water, which 

 would be evaporated in the hot-air closet, so that the 

 material would be destroyed, hot-air sterilization is not 

 appropriate for them. Sterilization by streaming steam 

 is the best and surest method. The prepared media are 



placed in flasks or tubes care- 

 fully plugged with cotton and 

 previously sterilized with dry 

 heat, and then sterilized in what 

 is known as Koch's steam appa- 

 ratus (Fig. 10) or in Arnold's 



Fig. 10. — Koch's steam sterilizer. 



Fig. 11. — Arnold's steam sterilizer. 



steam sterilizer (Fig. 11), which is more convenient and 

 more generally useful. 



The temperature of boiling water, ioo° C, does not 



