118 A MANUAL OK BACTERIOLOGY 



Experience lias shown that objects must be exposed to dry heat 

 for a longer time than to moist heat. In practice it is necessary to 

 maintain a temperature of 140-150 C. for one hour, or of 125 0. 

 for two hours. Objects placed in the hot-air sterilizer should be per- 

 fectly clean and dry. After the heat is turned off, the door should 

 be kept closed until the temperature of the oven has fallen to near 

 the temperature of the room. 



The Lautenschlager oven is heated by a tubular gas burner extend- 

 ing around three sides of the base. If possible, the burner should 

 connect directly with the pipe supplying the gas, and entirely avoid 

 connections of rubber tubing. The special handling of various objects 

 in the hot-air sterilizer is described below. Needles, forcep tips, and 

 scalpel blades may be sterilized directly in the Bunsen flame. 



Steam sterilization at 100 C. Moist heat in the form of steam is 

 the most efficient sterilizing agent for the culture media usually 

 employed. 



At the temperature of boiling water most forms of germ life are 

 killed, although, as previously stated, numerous spores may success- 

 fully resist this temperature. Sterilization may thus be effected in 

 a boiler over an ordinary water bath. For economical and effective 

 work, however, the Arnold type of sterilizer is preferred and, in 

 some form, is in use in most laboratories. 



The type of sterilizer which, is heated over a gas burner is pro- 

 vided with a double bottom. The lower bottom contains only a thin 

 layer of water, which consequently boils quickly and the steam passes 

 up through a short chimney into the chamber of the sterilizer. Water 

 enters the lower bottom through small holes from the reservoir above, 

 which will hold a supply sufficient for several hours. 



The chamber of the Arnold sterilizer has double walls, which are 

 so arranged that the condensed steam drips back into the reservoir. 

 The chamber of the sterilizer is fitted with one or more shelves upon 

 which media may be placed. 



If the laboratory is supplied with steam, it will be found more 

 economical and satisfactory to install a sterilizer fitted for steam. 



The Arnold sterilizer and all other steam sterilizers should be 

 installed, if possible, in a room separate from the main laboratory, 

 and should stand under a hood which will carry off escaping steam. 



The sterilizer should always be provided with a thermometer 

 whose bulb extends about 5 cm. into the chamber ; otherwise it is 

 difficult to know the exact time at which the contents reach the 



