18 BACTERIOLOGY 



heat is applied, which rests upon the upper wall of the 

 evaporating vessel, and is connected with it by some small 

 apertures. 



Many substances, such as nutrient materials, are, on 

 account of their albuminoid constituents, unable to stand 

 the action of a temperature of l00° C. for any length of time 

 without undergoing changes; gelatine, for example, loses 

 the power of solidifying, which alone renders it applicable 

 to bacteriological purposes. Hence it is advisable to expose 

 all culture-media to a current of steam for not more than 

 a quarter of an hour daily on three successive days. The 

 heating on the first day kills all the micro-organisms 

 present and most of the spores ; but some of the latter 

 still remain and develop by next day into micro-organisms, 

 which perish on heating for the second time. Any that 

 remain are destroyed on the third day. 



Fractional sterilisation. — Certain substances, however, 

 particularly the serum of blood, undergo so many changes 

 even during a short sterilisation in the steam-current, as to 

 be no longer suitable for use as culture-media, and in such 

 cases recourse must be had to the process of discontinuous 

 or fractional sterilisation, introduced by Tyndall. This con- 

 sists of heating to a temperature of 54° to 56° C, for three 

 or four hours daily during one week, in a chest with double 

 walls between which there is a layer of water, the tem- 

 perature being kept at a constant height by means of a 

 thermo-regulator ; or, according to Heim's method, the 

 test-glasses can be placed in the warm water of a bath, the 

 temperature of which is kept continually at the height 

 mentioned above. 



Sterilisation by steam under pressure. — High-pressure 

 steam, appHed by means of autoclaves, acts with greater 

 rapidity than ordinary steam at 100° C. 



Chemical disinfectants. — Besides high temperatures. 



