100 BACTERIOLOGY 



may be able to flow into it ; and in this way a volume of 

 air, corresponding to the quantity of water used, is aspirated 

 into the receiving bottle. For the purpose of regulating 

 the flow, two little glass tubes, drawn out to fine points, 

 are fixed in the india-rubber tube which connects the two 

 aspirating bottles. Cultivation is effected by conveying 1 

 c.cm. of the fluid in the receiving flask (after it has been 

 thoroughly mixed) by means of a sterile pipette into 10 

 c.cm. gelatine, and pouring this out on plates. 



Hesse's method. — In this method the air is caused to 

 pass by means of a small slowly-acting aspirator through a 

 disinfected tube, the walls of which are coated with gelatine 

 after the manner of Esmarch's roll cultures. This tube is 

 70 cm. long, and has a diameter of 3 to 4 cm. ; it is placed 

 horizontally, and covered at one end with a tightly- stretched 

 rubber cap having a round piece cut out of the centre, and 

 over which a second cap, not perforated, can be drawn ; 

 while the other end is closed with a caoutchouc cork, bored, 

 and fitted with a small glass tube about 1 cm. wide and 

 10 cm. long, connected with the aspirator. While the air 

 is being aspirated, the unperforated cap must be removed. 

 Two bottles are used by way of aspirator, as in Welz's 

 method, one filled with water, the other empty (fig. 31). 

 The bacteria develop chiefly in the fore part of the tube, 

 while the spores of moulds, being isolated and therefore 

 lighter, are carried further and develop further on in the in- 

 terior. When air is examined which presumably contains 

 but few germs — for example, air out of doors during a calm— 

 10 to 20 litres are drawn through, but if it is probable that 

 large numbers are present only 1 to 5 litres are aspirated. 

 The process is concluded by replacing the unperforated 

 rubber cap. In a few days the gelatine is seen to be 

 covered with colonies which can be distinguished from one 

 another by their form, their colour, and their action on the 



