40 METHODS OF STERILISATION 



Filtration. (a) Cotton-wool Filter. Practically the 

 only method in use in the laboratory for the sterilisa- 

 tion of air or of a gas is by filtration through dry cotton- 

 wool or glass-wool, the fibres of which entangle the 

 micro-organisms and prevent their passage. 



Perhaps the best example of such a filter is the cotton- 

 wool plug which closes the mouth of a culture tube. 

 Not only does ordinary diffusion take place through it, 

 but if a tube plugged in the usual manner with cotton- 

 wool is removed from the hot incubator, the tempera- 

 ture of the contained air rapidly falls to that of the 



laboratory, and a partial vacuum is formed ; air passes 

 into the tube, through the cotton-wool plug, to restore 

 the equilibrium, and, so long as the plug remains dry, 

 in a germ-free condition. If, however, the plug be- 

 comes moist, either by absorption from the atmos- 

 phere, or from liquids coming into contact with it, 

 micro-organisms (especially the mould fungi) com- 

 mence to multiply, and the long thread forms rapidly 

 penetrate the substance of the plug, and gain access to 

 and contaminate the interior of the tube. 



METHOD. 



If it is desired to sterilise gases before admission to 

 a vessel containing a pure cultivation of a micro- 

 organism, as, for instance, when forcing a current of 

 oxygen over or through a broth cultivation of the 

 diphtheria bacillus, this can be readily effected as 

 follows : 



