STERILISATION BY CHEMICAL AGENTS 35 



sation purposes, great risk is incurred by traces of the 

 germicide escaping removal, and thus destroying the or- 

 ganisms under examination or introducing other elements 

 of uncertainty into the work. Great care must be taken 

 when using such substances ; in fact, it is advisable only 

 to resort to their use under special circumstances. For 

 ordinary purposes it is best to rely upon the careful fulfil- 

 ment of all the details required in the sterilisation by the 

 usual methods. 



Probably the most ready means of sterilising plates, 

 tubes, instruments, etc., wherever possible, is prolonged 

 boiling in water, taking care to protect from dust when 

 cooling. 



Sterilisation by Filtration. — Air and other gases are 

 readily freed from micro - organisms by drawing them 

 through a tube containing a plug of dry sterile cotton- 

 wool, or packed with sugar or sand. The application of 

 this principle is seen in the plugging of culture tubes and 

 flasks with cottonwool to protect the contents from aerial 

 organisms. 



In order to deprive water or other liquid, which is not too 

 viscid, of bacteria, it is forced through cylinders made of un- 

 glazed porcelain (Pasteur-Chamberland filter) or baked 

 infusorial earth (Berkefeld filter). When it is necessary to 

 free water from organisms without chemical change, or for 

 the purposes of concentration, as in the testing of water for 

 the typhoid bacillus, or the separation of bacteria from the 

 products of their vital activity (in the preparation of toxines, 

 etc.), the use of these filters is invariably resorted to. 

 Not only will these filters keep back the bacillus of mouse 

 septicaemia, one of the smallest known micro-organisms, 

 but putrid blood serum can be fiiltered, and the filtrate 

 is rendered not only perfectly clear, but quite free from 

 organisms. 



