730 A MANUAL OF BACTERIOLOGY 



obtained by this method are roughly comparative, but no estimate 

 can be formed from it of the number of organisms contained in a 

 given volume of the air. 



(2) Hesse's method. This is a quantitative method for estimat- 

 ing the n.umber of organisms contained in a given volume of air. 

 The apparatus consists of a glass tube 30 in. long by 1^ to 2 in. 

 in diameter. One end of this tube is plugged with a rubber cork 

 through which a glass tube plugged with cotton-wool passes, 

 the other end is covered with a piece of sheet rubber perforated 

 with a hole to in. in diameter ; over this is placed another 

 sheet of rubber, unperforated. The whole is sterilised for an 

 hour in the steam steriliser. Just before use 40 to 50 c.c. of 

 melted sterile nutrient gelatin are poured into the tube, and 

 it's walls coated with the medium. The tube is then strapped 

 horizontally on to a tripod stand, and the small tube connected 

 by means of a piece of rubber tubing to an aspirator consisting 

 of two flasks arranged so as to form a reversible syphon. A 

 litre of water is poured into the flask connected with the tube, 

 and the outer sheet of rubber having been removed from the end 

 of the tube, the water is syphoned over to the second flask, placed 

 at a lower level, and an equal, volume of air is thus aspirated 

 through the tube. The second flask is then connected with the 

 tube, and the position of the flasks being reversed the water is 

 again syphoned over, and a second litre of air passes through the 

 tube, and this process is repeated until 5, 10, 15, or 20 litres of 

 air have been drawn through the tube. The rate of flow is 

 controlled by a screw-clamp on the rubber connecting-tube ; 

 it should not exceed half a litre per minute. With this rate of 

 flow all the organisms are deposited on the gelatin-coated tube. 

 The aspiration being completed the rubber tube is disconnected, 

 and the sheet of rubber replaced over the end of the tube, which 

 is then incubated, and the colonies are counted when they have 

 developed. 



(3) Petri's method. Petri aspirates the air through a glass tube 

 containing sterilised sand, kept in place by fine wire-gauze wads. 

 When the sample has been N taken the sand is distributed in Petri 

 dishes, and melted sterile gelatin is poured over it and allowed to 

 solidify, plate cultures being thus prepared. The objection to 

 this method is the presence of the opaque particles of sand in the 

 culture medium. 



