STERILISATION BY DRY HEAT 27 



or anaerobes. Each of these methods will be considered in 

 turn. 



The Methods of Sterilisation. 



To exclude extraneous organisms, all food materials, glass 

 vessels containing them, wires used in transferring bacteria from 

 one culture medium to another, instruments used in making 

 autopsies, etc., must be sterilised. These objects being so 

 different, various methods are necessary, but underlying these 

 methods is the general principle that all bacteria are destroyed 

 by heat. The temperature necessary varies with different 

 bacteria, and the vehicle of heat is also of great importance. 

 The two vehicles employed are hot air and hot water or steam. 

 The former is usually referred to as " dry heat," the latter as 

 "moist heat." As showing the different effects of the two 

 vehicles, Koch found, for • instance, that the spores of bacillus 

 anthracis, which were killed by moist heat at 100° C, in one 

 hour, required three hours' dry heat at 140° C. to effect death. 

 Both forms of heat may be applied at different temperatures — 

 in the case of moist heat above 100° C, a pressure higher than 

 that of the atmosphere must of course be developed. 



A. Sterilisation by Dry Heat. 



A (1). Red Heat or Dull Red Heat. — Red heat is used for 

 the sterilisation of the platinum needles which, it will be found, 

 are so constantly in use. A dull heat is used for cauteries, the 

 points of forceps, and may be used for the incidental sterilisation 

 of small glass objects (cover-slips, slides, occasionally when neces- 

 sary even test-tubes), care of course being taken not to melt the 

 glass. The heat is obtained by an ordinary Bunsen burner. 



A (2). Sterilisation by Dry Heat in a Hot-Air Chamber. — 

 The chamber (Fig. 2) consists of an outer and inner case of 

 sheet iron. In the bottom of the outer there is a large hole. 

 A Bunsen is lit beneath this, and thus plays on the bottom of 

 the inner case, round all the sides of which the hot air rises 

 and escapes through holes in the top of the outer case. A 

 thermometer passes down into the interior of the chamber, half- 

 way up which its bulb should be situated. It is found, as a 

 matter of experience, that an exposure in such a chamber for 

 one hour to a temperature of 160° C, is sufficient to kill all the 

 organisms which usually pollute articles in a bacteriological 

 laboratory, though circumstances^might arise where this would 



