BACTERIOLOGICAL ANALYSIS OF AIR 



627 



In some of the methods which provide for the filtration 

 of bacteria from the air by means of Hquid substances a 

 measured volume of air is aspirated through liquefied 

 gelatin; this is then rolled into an Esmarch tube and the 

 number of colonies counted, just as is done in water analysis. 

 This is the simplest procedure. An objection sometimes 

 raised against it is that organisms may be lost, and not 

 come into the calculation, by passing through the medium 



Fig. Ill 



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

 sand is seen at the point a. 



in the centre of an air-bubble without being arrested by 

 the fluid — an objection that appears to have more of specu- 

 lative than of real value. Filtration through porous sub- 

 stances appears, on the whole, to give the best results. 

 Petri recommends aspiration of a measured volume of air 

 through glass tubes into which sterilized sand is packed. 

 (Fig. 111.) When aspiration is finished the sand is mixed 

 with liquefied gelatin, plates are made, and the number of 



