98 BACTERIA 



entrance tube closed. The water (plus bacteria) is now 

 poured out into test-tubes of media or plated out on Petri's 

 dishes. Provided the apparatus has been absolutely steril- 

 ised, and that the water was also sterile, any colonies 

 developing upon the Petri dish are composed of micro- 

 organisms from the air examined. 



4. The Method of Hesse. This method is somewhat akin 

 to Pouchet's aeroscope, but is in addition a culture method. 

 Hesse's tube is about 2 feet long and i inches bore through- 

 out. At one end is an india-rubber stopper bored for a glass 

 tube to the aspirator. The other end is open. Before using, 

 the tube is sterilised, and 40 or 50 cc. of sterilised gelatine 

 replaced in it. The tube is now rapidly rotated in a groove 

 on a block of ice or under a cold-water tap, and by this 

 simple means the gelatine becomes fixed and forms a layer 

 inside the tube throughout. We have therefore, so to 

 speak, a tube of glass with a tube of gelatine inside it. The 

 apparatus is now ready for use. It is fixed on the tripod, 

 and fifteen litres of air are drawn through, and the tube is 

 properly plugged and incubated at room temperature. In 

 a day or two days the colonies appear upon the gelatine. 

 They are most numerous generally in the first part of the 

 tube, and might be roughly estimated as follows : 



15 litres of air, 6 colonies. 



.'.-^g- X 10,000 = 4000 aerobic bacteria in the cubic metre. 



The disadvantages of this process are that dried gelatine 

 does not catch germs like the broth cultures of Pasteur or 

 Miquel, and that many organisms are able to go straight 

 through the tube, and failing to be deposited, pass out at 

 the aspirator exit, and thus are neither caught nor counted. 

 The Hesse tube is generally used in practice with a pump 

 consisting of two flasks and a double-way india-rubber tube. 

 The flasks have a capacity for one litre of water. By a 

 simple adaptation it is possible to secure siphon action, and 



