THE EXAMINATION OF AIR 433 



an Esmarch's tube, leaving a somewhat thicker coating 

 along the under side of the cylinder. The cylinder and its 

 fittings are mounted on a tripod stand, and the glass tube 

 which passes through the rubber stopper is connected by 

 means of a rubber tube with an aspirator, the cotton having 

 first been removed from its outer end. The aspirator most 

 suitable for the purpose is the double wash-bottle arrange- 

 ment, which is conveniently attached to the stand by means 

 of hooks. 



The outer rubber cap is then removed, and the aspirator 

 started. Air is drawn through the tube by suction, the 

 micro-organisms contained therein falling on the gelatine. 

 The amount of air entering is estimated by the capacity of 

 the flasks forming the aspirator; the rate at which it enters 

 is controlled by the flow of the water, which can be 

 regulated by a pinch-cock. Hesse advises that the amount 

 of the rate of flow for rooms and closed spaces should be 

 about 1 to 5 Htres, passed at the rate of half a litre a 

 minute; for open spaces 10 to 20 litres is passed at 

 about four minutes per litre. The tube is then capped and 

 the colonies allowed to develop, after which they can be 

 further examined by subcultures. 



BACTERIOLOGICAL EXAMINATION OF SOIL. 



Surface-soil, particularly that which is rich in organic 

 matter, is very rich in micro-organisms. That it is only the 

 ■surface-soil that contains any quantity of micro-organisms 

 is shown by the fact that at as short a distance as about two 

 metres in depth the soil contains but few organisms. This 

 was shown by Koch in 1881, who showed that in soil which 

 had not been disturbed, even at a depth of only one metre 

 but few bacteria are to be found. This fact has since been 

 confirmed by the extended researches of Frankel and others. 



28 



