BACTERIOLOOICAL ANALYSIS OF AIR. 605 



of allowing air to pass over solid nutrient media (Koch, 

 Hesse) and observing the colonies which develop upon the 

 media, or of filtering the bacteria from the air by means 

 of porous and liquid substances, and studying the organ- 

 isms thus obtained. (Miguel, Petri, Strauss, Wiirz, 

 Sedgwick-Tucker.) Because of their greater exactness, 

 the latter have supplanted the former methods. 



In some of the metliods which provide for the filtra- 

 tion of bacteria from the air by means of liquid sub- 

 stances 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 was 

 done in water analysis. This is the simplest procedure. 

 An objection sometimes raised against it is that organisms 



Fig. 108. 



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

 packed with sand is seen at the point a. 



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. 



