AIR AND WATER. 63 



more completely developed by Hesse. A glass tube, about 

 1 meter long and 4 to 5 cm. wide, is closed at one end with 

 a perforated india-rubber membrane, over which another 

 non-perforated cap is bound. A little liquefied nutrient 

 gelatine is then poured into the tube, after which the other 

 end is closed with an india-rubber stopper, through which 

 passes a glass tube plugged with cotton- wool. The whole 

 apparatus is then heated sufficiently to render it sterile, after 

 which the tube is placed in a horizontal position, so that 

 the gelatine sets in a layer in its lower part. 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 settle down on the gelatine, and after the aspiration 

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

 where some of the germs 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, small fibres, or splinters, settle sooner 

 than the mould-spores ; so that the gelatine in the fore part 

 of the tube generally showed a preponderance of the bacteria 

 colonies, whilst the mould-spores developed further on. 



Miquel's method is to allow the air to pass through a hollow 

 cylinder of solidified gelatine, in which the germs are retained. 



Hueppe, v. Schlen, and others use liquid gelatine for air 

 analyses, the air being aspirated through the gelatine, after 

 which the latter is poured on to glass plates. 



Frankland, Miquel, Petri, and Ficker use porous solid 

 substances for the filtration 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 packed with sand, previously ignited, the size of 

 the grains being from 0-25 to 0-5 mm. Two such sand filters 

 are placed one behind the other in a glass tube. In the first 

 filter all the dust-particles containing germs should be retained, 

 whilst the second filter serves as a control. The sand charged 

 with germs is distributed in shallow glass dishes and covered 

 with liquid gelatine. The germs accompanying the dust- 

 particles will then form colonies in the gelatine. 



