438 BACTERIOLOGY. 



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. 91.) 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. 



Fig. 91. 



Petri's apparatus for bacteriological analysis of air. The tube 

 packed with sand is seen at the point n. 



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 

 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. 



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 

 follows : 



