STERILISATION. 15 



There are, however, spores which withstand this treatment ; 

 thus, Sames and Christen have shown that the spores of certain 

 species of potato bacilli which frequently occur in soil will 

 withstand 10 to 16 hours' boiling. If, therefore, we have to 

 do with material which has been in contact with the earth, 

 higher temperatures must be employed, for it has been shown 

 that the disinfecting power of stearn rapidly increases when 

 its temperature rises above 100 C. Steam under pressure is, 

 therefore, used in a Papin's digester or autoclave, constructed 

 to stand a pressure of several atmospheres. This apparatus 

 is specially useful for the sterilisation of several liquids used 

 in the laboratory. If small quantities of liquids are to be 

 sterilised, a pressure of one atmosphere, corresponding with 

 a temperature of 120 C., is sufficient, if applied for half an 

 hour. During the cooling of any variety of sterilising appar- 

 atus, care must be taken that the incoming air is sterile, and 

 this is secured by passing it through sterilised cotton wool. 



(l>) Sterilisation of Liquids and Solid Nutritive Substrata. 

 All germs can be removed from nutritive liquids by filtration, 

 but this method of treatment, which is more troublesome than 

 heating, is only used for liquids when their composition is 

 affected by heat. Even in this respect it must be noted that 

 filtration is not without eft'ect, for the investigations of Fliigge, 

 Arloing, and others have shown that a filter retains or reacts 

 upon certain of the soluble constituents, for instance, upon 

 certain enzymes. As the filtering medium, either burnt clay, 

 plastic charcoal, gypsum, asbestos, or kieselguhr may be used. 

 The pores of these substances are very fine, and a thick layer 

 must be used to ensure that even the smallest bacteria are 

 retained. The pores are soon stopped up, and the filtration 

 must then be hastened by pressure or suction. In labora- 

 tories, and for the filtration of small quantities of water, the 

 Pasteur-Chamberland filter, consisting of burnt porcelain clay, 

 and Nordtmeyer's filter made of compressed diatomaceous 

 earth are frequently employed. These filters take the form 

 of a hollow candle, closed at one end ; the liquid flows into the 

 hollow and out by a tube fixed at the other end. To test 

 such a filter, it is immersed in water, and air is blown into the 

 cavity. If bubbles of air rise through the water, the filter is 



