METHODS OF STUDYING BACTERIA 25 



that an ordinary kitchen oven may be used, the temperature 

 being gauged by a thermometer or in some other way. An 

 exposure of an hour at this temperature is practically certain 

 to kill off all organised life. Flasks and test-tubes are first 

 plugged with dry cotton-wool, as it has been found by experi- 

 ence that this material acts as a filter for air, but prevents the 

 entrance of organisms into the interior of vessels so plugged. 



FIG. 3. Hot Air Steriliser, strongly made in Sheet Iron or 

 Copper, covered with Asbestos to prevent loss of heat. 

 Fitted with Hearson's Patent Automatic Gas Valve and 

 Capsule, capable of maintaining any temperature up to 156* 

 C. (=250 F.). Electricity may also be used as a source of 

 heat. 



It should be remembered here, however, in passing, that wet 

 cotton- wool does not possess this property, and that moulds, 

 etc., falling upon moist cotton-wool can grow down through it, 

 and so obtain access to the contents of the vessel. The in- 

 terior of tubes, flasks, bottles, etc., plugged with dry wool and 

 sterilised in this manner, remain free from living organisms for 

 an indefinite period (see Figs. 11 and 12). 



Again, moist heat may be employed for objects \vhich will 

 withstand its action. This, as in ordinary domestic cooking, 



