506 BACTERIOLOGY. 



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," 

 Sedg wick-Tucker. ) 



The former methods have given place almost entirely 

 to the latter for reasons of greater exactness possessed 

 by the latter. 



In some of the methods 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 the water analysis. This is the simplest pro- 

 cedure. An objection raised against it is that organisms 



FIG. 103. 



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. 



