36 GROWTH OF BACTERIA 



bacteria, their spores included. Even the boiling of 

 liquids, such as water, milk, and broth at 100 C., kills 

 all living things in them if continued long enough, but 

 certain spores resist this temperature for several 

 hours. Usually it is only needful to heat most liquids 



to 1 00 C. for fifteen or twenty 

 minutes to destroy all bacteria 

 in active growth, and a great 

 many of their spores also. 



In the case of many liquids 

 it is not possible to heat them 

 in an open vessel above 1 00 C., 

 for when this temperature is 

 reached steam forms, and 

 evaporation occurs without a 

 further rise in temperature. 



If, however, steam is pre- 

 vented from escaping, the liquid 

 may be heated above 100 C. 

 An apparatus for effecting this 

 is the autoclave (Fig. 13). It 

 consists of a strong gun-metal 

 or brass boiler fitted with a lid 

 which can be screwed down. 

 In the latter a safety-valve is 



FIG. 13. Autoclave. 



provided, as well as a stopcock, 



by means of which the steam can be allowed to escape 

 when necessary. Attached to the apparatus is a gauge 

 to indicate the pressure of the enclosed steam. 



In using the autoclave for the sterilisation of water, milk, 

 or other media used in the cultivation of bacteria, the 

 following instructions may be adopted : Introduce a 

 small quantity of water into the autoclave, and after the 



