THE DESTRUCTION OF BACTERIA 81 



powers of penetration. An experiment carried out by Koch and 

 his associates illustrates this point clearly. Small packages of garden 

 soil were surrounded by varying thicknesses of linen with thermom- 

 eters so placed that the temperature under a definite number of 

 layers could be determined. Exposures to hot air and to steam 

 were then made for comparison, and the results were as tabulated : 27 



This great penetrating power of steam is due presumably to its 

 comparatively 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. 28 



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, accord- 

 ing 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 



27 Koch, Gaffky und Loeffler, loc. cif., p. 339. 



28 Gruber, Cent, f . Bakt., iii, 1888. 



