628 APPLICATION OF METHODS OF BACTERIOLOGY 



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 objection 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 aerobio- 

 scope 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. 112) is about 35 cm. in its entire 

 length; it is 15 cm. long and 4.5 cm. in diameter 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 b), and also at the large end (c), 

 the apparatus is plugged with cotton. When thoroughly 

 cleaned, dried, and plugged, the apparatus is to be steril- 



