DESTROYING GERMS BY MOIST HEAT. 103 



77. Destroying: Germs by Moist Heat. 



The opinion expressed in a former chapter, that the seat of 

 the high powers of resistance enjoyed by bacterial spores is to 

 be sought in their membrane, is supported by their behaviour 

 towards the influence of warmth, in so far as has been ascertained 

 by numerous experiments that, under otherwise identical con- 

 ditions, moist heat, e.g. in the form of steam, exerts a more 

 violent action and kills them much sooner than dry heat at the 

 same temperature. This behaviour is explained by the unusually 

 low heat-conducting power of the unaltered spore membrane. 

 By the influence of moisture, however, the structure of this pro- 

 tective envelope is loosened and its permeability to heat rays 

 increased. 



Although the use of moist heat may thus appear preferable 

 to the method described in the preceding paragraph, it is never- 

 theless inapplicable in many special instances. For example, 

 air-filters must be sterilised by dry heat alone, but when liquids 

 have to be freed from living germs by the aid of heat, then moist 

 heat must be decided upon. This can now be employed in one of 

 two ways : either by boiling the liquid over a naked flame, or by 

 exposing it to the influence of water vapour heated to a sufficiently 

 high temperature. 



That every liquid can be sterilised by simple boiling at 100 C. 

 was shown by HUEPPE (III.) in 1882 ; the time of exposure 

 necessary in order to secure the desired result with certainty 

 being, however, very long. In this connection we may recall 

 the experience of Brefeld, mentioned in 53, according to which 

 the killing of the spores of the species of hay bacillus examined 

 by him necessitated their exposure for full three hours in boiling 

 water. However, the nutrient solutions destined for the cultiva- 

 tion of organisms, and requiring to be sterilised anterior to use, 

 must not be treated in this manner, since they would be con- 

 centrated too much by such prolonged boiling. Such solutions 

 are generally sterilised by exposure to low-pressure steam, for 

 which purpose the so-called "steam steriliser," proposed by Gaffky, 

 R. Koch, and Lb'ifier, and resembling in arrangement an ordinary 

 potato-steamer, is employed. It consists principally of a high 

 cylindrical tin pot, covered over with asbestos board or felt, and 

 fitted with two, bottoms, the upper one, which is perforated, 

 serving as the support for the vessels to be sterilised by exposure 

 to the steam evolved by the boiling water below. This process 

 is known as sterilising by direct steam; it obviates the incon- 

 venience arising from the evaporation of the nutrient media, and 

 also prevents local over-heating. The samples are surrounded on 

 all sides by steam, which drives away the protecting envelope of 

 air and raises the temperature uniformly throughout to that of 



