EXAMINATION OF AIR AND WATER. 41 



lower part of the tube. When the air is to be examined, the 

 outer india-rubber cap is removed, and air slowly drawn 

 through the tube. The germs contained in the air then 

 settle down on the gelatine, and after the aspiration is con- 

 cluded the tube is again closed and placed in the incubator, 

 where some of the germs then produce visible colonies, 

 which are easily counted. The results show that with a 

 sufficiently slow current of air the bacteria, which are often 

 floating about in the air in larger or smaller aggregations, 

 frequently clinging to dust-particles, settle sooner than the 

 mould-spores ; so that in consequence the gelatine in the front 

 part, of the tube generally contains the majority of the 

 bacteria colonies, whilst the mould-spores develop further 

 along the tube. 



Hueppe, v. Schlen, and others, employ liquid gelatine for 

 air-analyses, the air being aspirated through the gelatine, after 

 which the latter is poured on to glass-plates. 



Frankland, Miquel, and Petri, use porous solid substances 

 for the nitration of air for analytical purposes ; as, for example, 

 powdered glass, glass-wool, sand, sugar, etc. The sand-filter 

 employed by Petri is 3 cm. long and 1*8 cm. wide. It is 

 filled with sand which has been heated, the size of the grains 

 being O25 to 0*5 mm. Two such sand-filters are placed one 

 behind the other, in a glass-tube. The first filter should re- 

 tain all the dust- particles containing germs, whilst the other 

 filter should remain sterile, and thus serves as a control. The 

 sand charged with germs is distributed in shallow glass-dishes 

 and covered with liquid gelatine. The germs contained in 

 the dust-particles will then develop colonies in the gelatine. 



When samples of air are to be sent from one place to 

 another, these air-filters will answer the purpose. On receipt, 

 the sand may be washed into gelatine or, preferably, into 

 sterilised water. After vigorously agitating the water, it is 

 added in drops to flasks containing nutritive liquid, or it may 

 be used in plate-cultures. 



Against the employment of gelatine plates for these 



