642 BACTERIOLOGY. 



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. 98) 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. 



Upon this narrow tube, 5 cm. from the lower end, a 

 mark is made with a file, and up to this mark a small 

 roll of brass-wire gauze (a) is inserted ; this serves as 

 a stop for the filtering-material which is to be placed 

 over it. Beneath the gauze (at 6), and also at the J , 

 large end (c), the apparatus is plugged with cotton. 

 When thoroughly cleaned, dried, and plugged, the 



