606 BACTERIOLOGY. 



Filtration through porous substances appears, on the 

 whole, to give the best results. Petri recommends as- 

 piration of a measured volume of air through glass 

 tubes into which sterilized sand is packed. (Fig. 108.) 

 When 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 sand ; this, when 

 brought into the liquefied gelatin, dissolves, and no such 

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



Sedgwick-Tucker Method. — On the whole, the 

 method proposed by Sedgwick and Tucker gives such 

 uniform results that it is to be preferred to others. It 

 is as follows : 



The apparatus employed by them consists essentially 

 of three parts : 



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

 aerobioscope has been given. 



2. A stout copper cylinder of about sixteen litres 

 capacity, provided with a vacuum-gauge. 



3. An air-pump. 



The aerobioscope (Fig. 109) 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 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. 



