DESTROYING GERMS BY MOIST HEAT 81 



been ascertained by numerous experiments that, under otherwise identical 

 conditions, 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 struc- 

 ture of this protective 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 nevertheless 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 cultivation of organisms, and requiring to be sterilised anterior 

 to use, must not be treated in this manner, since they would be concentrated 

 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 Gatfky, R. Koch, and Loftier, and resembling in arrangement an 

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

 drical 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 

 inconvenience arising from the evaporation of the nutrient media, and also 

 prevents local overheating. 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 the boiling water. This is, of course, dependent 

 on the prevailing atmospheric pressure, and generally ranges between 96 and 

 1 00 0. A reduction of the time of exposure is not to be thought of, since here, 

 as before, we have to do with a temperature of only about 100 ; this must be 

 particularly emphasised, since the Koch school at one time fell into error on this 

 point, by promulgating the maxim that " the spores of bacilli cannot withstand 

 the temperature of boiling water for more than a few minutes." We have 

 already recalled a fact controverting this, and will now cite a second example, 

 given by GLOBIG (II.), viz., that the endospores of a species of bacterium, dis- 

 covered by this observer on potatoes, originating therefore in cultivated soil, and 

 named the " potato bacillus," resisted the influence of a current of steam at 

 100 C. for as much as six hours. This is the most powerfully resistant of all 

 organisms hitherto observed. 



Numerous modifications have been made in Koch's steriliser, in accordance 

 with the special purposes for which it is intended. Thus, for example, the 

 water-chamber has been separated from the steam-chamber, and the steam 

 introduced into the latter from above. This pattern is specially preferred in 

 the case of the large apparatus employed for disinfecting invalids' linen, 

 hospital bedding, and the like. Readers desirous of obtaining full information 

 on this point are referred to a treatise by DUNCKER (I.), who subjected a 



