GENERAL BACTERIOLOGY 
43 
which has a shallow double bottom. The reservoir is 
filled with water which is heated by a gas flame; this 
rapidly vaporizes the thin layer of water contained in 
the double bottom of the reservoir, the steam rises rapid¬ 
ly and passes into the inner chamber containing the 
articles to be sterilized, through its perforated bottom; 
the steam is condensed under the outer chamber and 
drops back into the reservoir. 
Fig. 6.—The Arnold sterilizer. 
An exposure from fifteen to thirty minutes destroys 
all vegetative forms of bacteria. In order to get rid 
of spores a “fractional’’ method of sterilization is 
used; this consists in exposing the substances to be 
sterilized (usually culture media) from fifteen to thirty 
minutes on three successive days; the first exposure de¬ 
stroys the vegetative forms; by next day any spores 
which may have been present will have developed with 
the vegetative stage, and are killed by the second ex- 
