54 STERILIZATION OF CULTURE MEDIA. 
to be sterilized. The form shown in Fig. 23 is commonly used in 
bacteriological laboratories. 
It must be remembered that a much higher temperature is re- 
quired for the destruction of microédrganisms when dry heat is em- 
ployed than is the case with moist heat. The experiments of Koch 
and Wolffhiigel (1881) show that a temperature of 120° to 128° C. 
(248° to 262° F.) is required to destroy the spores of mould fungi, and 
micrococci or bacilli in the absence of spores. For the spores of ba- 
cilli a temperature of 140° C. (284° F.), maintained for three hours, 
was required. 
In practice we usually maintain a temperature of about 150° C. 
(302° F.) for an hour or more; and it is customary to sterilize all 
test tubes and flasks, which are to be used as receptacles for culture 
media, in the hot-air sterilizer. This procedure could no doubt, how- 
ever, be dispensed with in many cases and reliance be placed upon 
the sterilization of the flask, together with its contents, in the steam 
sterilizer, especially with such culture media as are not injured by 
long exposure to a boiling temperature—e.g., bouillon and agar-agar. 
When we propose to cultivate aérobic bacteria, or such as require 
oxygen for their development, a cotton air filter is placed in the 
mouth of each test tube and flask before it is sterilized in the hot-air 
oven. This is a loose plug of cotton, pushed into the neck of the 
flask for an inch or more, and projecting from its mouth for a short 
distance. These cotton filters should fill the tube completely and 
