STERILISERS 51 



registering to 200 C., is inserted in a cork, while through 

 the other some form of mercurial regulator can be intro- 

 duced if required, but is not usually needed. In the 

 hot-air steriliser all thin-glass vessels and cotton-wool 

 are sterilised by heating to a temperature of about 

 150 C. by means of a Bunsen or a small ring burner 

 under the steriliser, which is supported on a suitable 

 iron stand. If the steriliser is 

 placed on a table or other 

 wooden support, a piece of 

 sheet iron, asbestos cardboard 

 or uralite should be laid over 

 the wood to protect it from 

 the heat. An inexpensive sub- 

 stitute for the hot-air steriliser 

 may readily be devised, any 

 iron box or even a biscuit-tin 

 being used for the purpose. 



Steam steriliser (Fig. 3). 

 This consists of a cylindrical or 

 rectangular vessel of tinplate, 

 galvanised iron, or copper, 

 covered on the outside with a 

 layer of felt or asbestos, having 

 a false perforated bottom sup- 

 ported a few inches above the 



true bottom, and provided with a movable lid. In the 

 steam steriliser or " steamer " the culture media, and 

 thick glass vessels and other apparatus which would 

 crack or be damaged by the high temperature of the 

 hot-air steriliser, are sterilised by steam. The lower 

 chamber of the steamer, below the false bottom, is 

 partly filled with water, which is boiled by means of 

 a Bunsen or ring burner. Above the false bottom 

 the culture media or apparatus are placed, and are 



42 



