SEDGWICK'S METHOD. 187 



the number of developing colonies counted, the results 

 giving the number of organisms contained in the volume 

 of air aspirated through the sand. 



FIG. 30. 



The tube packed with sand is seen at the point a. 



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, who employs 

 granuated 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. 



SEDGWICK'S METHOD. On the whole, the method 

 proposed by Sedgwick gives such uniform results that 

 it is to be recommended above the others. It is as fol- 

 lows : 



The apparatus employed consists essentially of three 

 parts : 



(1) A glass tube of a special form to which the name 

 aerobioscope has been given. 



