606 A MANUAL OF BACTERIOLOGY 



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- 



A B c 



FIG. 67. Frankland's tube for air analysis. 



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 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. 



(4) Frankland's method. The air to be examined is aspirated 

 through a tube 5 in. in length and in. in diameter (Fig. 67). One 

 end of the tube is open, the other (c) is plugged with cotton-wool. 

 At a distance of 1 in. from the open end the tube is slightly con- 

 stricted to support a plug of glass wool (A). At a distance of 2^ in. 

 from this plug the tube is again constricted to support a second 

 plug (B), consisting of glass-wool and finely powdered cane-sugar, 

 supported in front and behind by plugs of glass-wool. Several 

 such tubes having been prepared, they are placed in a tin box and 

 sterilised at 130 C. for three hours, and can then be easily trans- 

 ported without risk of contamination. When required for use, 

 a tube is quickly removed from the box, being handled by the 



