76 METHODS OF STERILISATION 



by passing the liquid or gas through a suitable filter. The sterilisation of air on 

 a large scale is effected exclusively by the latter method, the prototype of which 

 was constituted by the tubes, plugged with cotton-wool, first employed by 

 Schroder and Dusch. The air is therefore passed through a cotton-wool filter, 

 as it is termed, such a one being used, for example, to purify the air admitted 

 to the sterilised wort in an apparatus for the pure cultivation of yeast. It will 

 not be out of place to lay stress on the fact that such a filter will only work 

 efficiently provided it be thoroughly dry ; otherwise the Eumycetes spores en- 

 tangled therein will germinate and develop into long-thread cells, which will 

 penetrate right through the filter and quickly form new spores, so that the air 

 at the end of the filter nearest the wort is not only not freed from germs, but is 

 probably richer therein than before. Attention to the air filters must, conse- 

 quently, not be neglected. E. CH. HANSEN (III.) has reported on experiments 

 made by Poulsen concerning the time during which such filters continue, under 

 normal conditions of practical working, to pass the air in a germ-free state. 



The cotton-wool plugs with which, since the time of Schroder and Dusch, it 

 is customary to close test-tubes, bottles, and flasks in which cultures of organisms 

 or stores of nutrient media are kept, are simply small cotton-wool filters. They 

 are especially brought into action when currents of air pass into the vessels as a 

 consequence of the partial vacua formed within them by a lowering of tempera- 

 ture, the germs in which are retained by the plugs. The efficiency of the filter 

 depends on its being kept dry. Its reliability is not, however, permanent, since, 

 though the fission fungi are always retained, this is not the case with the spores 

 of mould fungi, which are so abundantly met with in the air. These latter are 

 very troublesome, as they often produce much mischief even when the mycologist 

 has taken the greatest care. If the room in which the cultures are kept be free 

 from moisture, then the cultures dry up very rapidly, which, in order to pre- 

 serve their vitality, necessitates their being frequently re-inoculated into fresh 

 media a tedious and unpleasant task. On the other hand, if the surrounding 

 air be too moist, then it not infrequently happens that the spores of the mould 

 fungi on the surface of the cotton-wool stopper germinate, and the resulting 

 cell threads penetrate to the other end of the plug, and there form spores, which, 

 falling into the culture, contaminate and spoil it. 



Various remedies have been proposed to overcome this evil, one of them 

 being a previously sterilised indiarubber cap, which is drawn over the mouth of 

 the vessel (test-tube, &c.) after the outer end of the stopper has been burnt away. 

 This latter operation must always be performed when one begins a re- inoculation, 

 since the germs resting on the surface of the cotton plug are thereby annihilated, 

 and consequently prevented from falling into the culture when opened. Instead 

 of the rubber cap, one can be made out of a double layer of filter-paper tied on 

 with a string ; many cultures specially requiring air are covered with a cap of 

 this kind only, the cotton-wool plug being dispensed with. 



It is not essential that the working layer of the filter should consist of 

 cotton- wool, various other stuffs being employed for special purposes. Thus, for 

 example, Pasteur, in carrying out his researches (referred to in 7) on the 

 organised bodies present in the atmosphere, passed the air through gun-cotton. 

 This was then immersed in a mixture of ether and alcohol, which dissolved out 

 the nitro-cellulose and left the entrapped organisms behind, so that they could 

 be more closely examined as to their size, form, and structure. This was the 

 first microbiological analysis of air. Of the numerous methods since proposed 

 for the estimation of the number of germs in the air, that given by FHANKLAND 

 and PETRI (I.), which is a successful modification of the Pasteur prototype, is 

 the most suitable for the purposes of the technical mycologist. These observers 

 deprive a measured quantity of air of its germs by passage through a filter 



