BACTERIOLOGICAL AIR ANALYSIS. 509 



the mark correspondiog to the amount of air re- 

 quired.^ 



A sterilized aerobioscope is now to be fixed in the 

 upright position and its small end connected by a rubber 

 tube with a stopcock on the cylinder, or to a glass tube 

 tightly fixed in the neck of an aspirating bottle by 

 means of a perforated rubber stopper. The cotton 

 plug is then moved from the upper end of the aerobio- 

 scope, and the desired amount of air is aspirated through 

 the sugar. Dust particles and bacteria will be held 

 back by the sugar. During manipulation the cotton 

 plug is to be protected from contamination. 



When the required amount of air has been aspirated 

 through the sugar the cotton plug is replaced, and by 

 gently tapping the aerobioscope while held in an almost 

 horizontal position, the sugar, and with it the bacteria, 

 are brought into the large part (e) of the apparatus. 

 When all the sugar is thus shaken down into this part 

 of the apparatus about 20 c.c. of liquefied, sterilized 

 gelatin is poured in through the opening at the end o, 

 the sugar dissolves, and the whole is then rolled on ice, 

 just as is done in the preparation of an ordinary Esmarch 

 tube. 



The gelatin is most easily poured into the aerobio- 

 scope by the use of a small, sterilized, cylindrical funnel 

 (Fig. 105), the stem of which is bent to an angle of 

 about 110° with the long axis of the body. 



The larger part of the aerobioscope is divided into 

 squares, to facilitate the counting of the colonies. 



1 Such a cylinder and air-pump are not necessary. A pair of ordinary as- 

 pirating bottles of known capacity graduated into litres and fractions thereof 

 answer perfectly well. Or one can determine by the weight of water that 

 has flowed from the aspirator the volume of air that has passed in to take 

 its place— i, e>, the volume of air that has passed through the aerobioscope. 



