BACTERIOLOGICAL AIR ANALYSIS. 459 



tube, and the number of colonies counted, just as was 

 done in the water analysis. This is the simplest proced- 

 ure. An objection raised against it is that organisms 

 may be lost, and not come into the calculation, by pass- 

 ing through the medium in the centre of an air-bubble 

 without being arrested by the fluid an objection that 

 appears to have more of speculative than of real value. 



FIG. 95. 



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

 packed with sand is seen at the point a. 



The methods of filtration through porous substances 

 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. 95.) 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. 



The main objection to this method is the possibility of 



