FREEING THE AIR FROM GERMS. 97 



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 opera- 

 tion 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 cul- 

 ture 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 micro-biological analysis of air. Of 

 the numerous methods since proposed for the estimation of the 

 number of germs in the air, that given by FRANKLAND 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 charged with sterilised glass powder or 

 sterilised fine sand, the contents of the filter being then intimately 

 mixed with a gelatinised nutrient medium, and the whole poured 

 into flat glass basins. The separate germs then develop into multi- 

 cellular families (colonies). When counted, their number re- 

 ferred to unit volume gives the germ content of the air. The 

 difficulty in the way of studying the cultures, caused by the 

 presence of the powdered glass and sand, can be overcome by 

 substituting a soluble filtering medium, such as coarsely powdered 

 crystals of sodium sulphate of about 0.5 mm. in diameter. This is 

 specially recommended by MIQUEL (IH.)> to whom (be it remarked 

 en passant) we owe the most comprehensive experiments on the 

 percentage of germs in the air. Regular reports of his researches 

 appear in the Year-Book (published annually since 1879) f the 

 observatory established, under his direction, for studies of this 

 kind, in the southernmost district of Paris. Readers are hereby 

 referred to this Annuaire de I'Observatoire de Montsouris. The 



VOL. I. G 



