332 APPLIED BACTERIOLOGY 



caps, the inner one having a hole in its centre 10 mm. in 

 diameter, and at the other end a rubjjer cork fits in the 

 cyHnder; through this cork a glass tube 100 mm. in 

 diameter passes, which is plugged with cotton-wool. The 

 cylinder is sterilised by placing for one hour in the steam- 

 steriliser. The apparatus is then prepared as follows : The 

 rubber stopper is carefully removed, and 50 c.c. of sterile 

 nutrient gelatine in a iiuid condition is introduced into 

 the tube and rolled out on the sides as in the preparation of 

 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 litres, 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. 



