THE DESTRUCTION OP BACTERIA 



67 



mined. Exposures to hot air and to steam were then made for com- 

 parison, and the results were as tabulated:' 



This great penetrating power of steam is due presumably to its com- 

 paratively low specific gravity which enables it to displace air from the 

 interior of porous materials, and also to the fact that as the steam comes 

 in contact with the objects to be disinfected a condensation takes place 

 with the consequent liberation of heat. When a vapor passes into the 

 liquid state it gives out a definite amount of heat, which in the case of 

 water vapor, at 100° C, amounts to about 537 calories. This brings 

 about a rapid heating of the object in question. Following this 

 process the further heating takes place by conduction, and it is, of 

 course, well known that steam is a much better heat conductor than air.^ 



Moist heat may be applied as boiling water, in which, of course, 

 the temperature varies little from 100° C, or as steam. Steam may be 

 used as live, flowing steam, without pressure, the temperature of which is 

 more or less constant at 100° C, or still higher efficiency may be attained 

 by the use of steam under pressure, in which, of course, temperatures 

 far exceeding 100° C. may be produced, according to the amount of 

 pressure which is used. 



The spores of certain bacteria of the soil which can not be killed in 

 live steam in less than several hours may be destroyed in a few minutes, 

 or even instantaneously, in compressed steam at temperatures ranging 

 from 120° to 140° C 



In all methods of steam sterilization, it is of great practical impor- 



1 Koch, Gaffky und Loeffler, loc. cit., p. 339. 



2 Grvher, Cent. f. Bakt., iii, 1888. 



s Christen, Ref. Cent. f. Bakt., V. xiii, 1893. 



