20 
BACTERIOLOGY. 
Frac¬ 
tional 
sterili¬ 
zation 
ings in this way to live steam will kill pathogenic bac¬ 
teria in thirty minutes but not their spores. 
Certain kinds of culture media, particularly those 
containing sugars are sterilized by the Arnold method. 
In order to destroy the spores the media is exposed 
to the steam for thirty minutes on three successive 
days. After each exposure the media is exposed to 
room temperature to permit the spores to develop into 
bacteria. At the end of the third exposure it is pre¬ 
sumed that all spores have developed into bacteria and 
all bacteria destroyed by the steam. Live steam is also 
used for killing bacteria in milk, and will be considered 
under the subject of pasteurization. 
The most effective method of sterilizing by heat 
is the use of steam under pressure. The action of the 
steam is intensified and its penetrating power increased 
by the pressure. The instrument used is called an 
autoclave. It consists of a double-walled cylinder or 
globe made of metal, with a steam gauge and vent at 
the top. The materials to be sterilized are placed in 
the inner chamber, the door closed, and the steam 
allowed to enter the outer jacket. The vent at the 
top is left open until all of the air has been forced out 
of the inner chamber. The vent is now closed and 
steam is allowed to enter the inner chamber until the 
gauge registers a pressure of 15 pounds, or one atmos¬ 
phere, and allowed to remain so for twenty to thirty 
minutes. This exposure will kill all bacteria and 
spores. If any fluid contained in flasks or test-tubes 
is being sterilized, care must be taken that the steam 
