EXAMINATION OF AlR. 455 



A third method of procedure consists in aspirating" the 

 air through sterile water or gelatin. The bubbles of air are 

 thus washed and the germs present are retained by the 

 liquid. A very useful and cheap apparatus for this purpose 

 is that employed by Straus and Wurtz, and is shown in Pig 1 . 

 60 b. It consists of a glass cylinder a in the neck of which 

 is fitted a ground pipette or tube &, the upper end of which 

 is plugged with cotton. 



The side-arm c is provided with two cotton plugs one 

 above and one below the middle constriction. The appar- 

 atus is sterilized in the dry-heat oven; 10 c.c. of gelatin are 

 then introduced and a drop of sterile oil is added to pre- 

 vent subsequent foaming. The plug is removed from the 

 end of the pipette & while the side-tube c is connected with 

 an aspirator. The air is drawn rapidly through the gela- 

 tin, and, as a result, it is deprived of most of the suspended 

 organisms. The remainder are held back by the cotton 

 plug on the inside of the constriction in the side-tube c. At 

 the close of the operation this plug is pushed down into the 

 cylinder and thoroughly agitated with the gelatin in order 

 to bring the adhering organisms into suspension. The 

 gelatin can be solidified on the inside of the cylinder, thus 

 forming an Esmarch roll-tube; or, definite portions of the 

 gelatin (0.5 and 0.1 c.c.) can be transferred to gelatin tubes 

 and Petri plates can then be made as in water analysis. 



Laboratory work. The student will examine two samples of 

 water, one drawn from the tap and the other from a well. The num- 

 ber and kind of colonies are to be reported. A specimen of milk will 

 also be plated and examined in the same manner as in water analysis. 



Esmarch roll-tubes will be made with three samples of soil. 



