24 LABORATORY GUIDE IN BACTERIOLOGY 



6. Adjust the weight, and filter through paper or 

 absorbent cotton previously moistened with hot water, 

 and distribute into thirty culture-tubes. 



7. Sterihze in the autoclav for 5 minutes at 120°. 

 Care is necessary not to overheat this medium, because 

 gelatin easily loses the property of sohdifying when cool. 

 In order to avoid this, the discontinuous or intermittent 

 method of sterilization is often used. By this method 

 the medium is exposed in a steam steriUzer to steam at 

 the temperature of 100° for 20 minutes on three con- 

 secutive days. The first day all vegetative forms are 



Fig. 13 



Arnold Steam Sterilizer 



The hood is taken off and the door 



opened, showing inside arrangement 



Fig. 14 

 Arnold Steam Sterilizer 



a. Inner water compartment 



b. Outer water compartment 



c. Perforated bottom 



d. SteriHzing chamber 

 c. Sheet-copper walls 



killed. By keeping the medium at room or incubator 

 temperature, spores, which may be present, will develop 

 into the vegetative form and be killed by the second day's 

 exposure. If after this any spores should happen to 



