BACTERWLOOIOAL AIR ANALYSIS. 607 



The methods of filtration through porous substances 

 appear, on the whole, to give the best results. Petri 

 recommends the aspiration of a measured volume of air 

 through glass tubes into which sterilized sand is packed. 

 (Fig. 103.) When the aspiration is finished the sand 

 is mixed with liquefied gelatin, plates are made, and 

 the number of developing colonies counted, the results 

 giving the number of organisms contained in the volume 

 of air aspirated through the sand. 



The main objection to this method is the possibility 

 of mistaking a sand granule for a colony. This objec- 

 tion has been overcome by Sedgwick and Tucker, who 

 employ granulated sugar instead of the sand; this, when 

 brought into the liquefied gelatin, dissolves, and no such 

 error as that possible in the Petri method can be made. 



Sedswick-Tucker Method. — On the whole, the 

 method proposed by Sedgwick and Tucker gives such 

 uniform results that it is to be recommended above the 

 others. It is as follows : 



The apparatus employed by them consists essentially 

 of three parts : 



1. A glass tube of a special form, to which the name 

 aerobiosGope has been given. 



2. A. stout copper cylinder of about sixteen litres 

 capacity, provided with a vacuum-gauge. 



3. An air-pump. 



The aerobiosoope (Fig. 104) is about 35 cm. in its 

 entire length; it is 15 cm. long and -4.5 cm. in diam- 

 eter at its expanded part; one end of the expanded 

 part is narrowed down to a neck 2.5 cm. in diameter 

 and 2.5 cm. long. To the other end is fused a glass 

 tube 15 cm. long and 0.5 cm. inside diameter, in which 

 is to be placed the filtering material. 



