BACTERIA IN THE AIR 



97 



2. Koch adopted the simplest of all the culture methods, 

 viz., exposing a plate of gelatine or agar for a longer or 

 shorter time to the air of which examination is desired. By- 

 gravity the suspended bacteria fall on the plate and start 

 growth. As a matter of quantitative exactitude, this method 

 is not to be recommended, but it frequently proves an 

 excellent method for qualitative estimation. 



3. The Method of Miguel. Pasteur was the first to analyse 

 air by the culture method, and he adopted a plan which in 

 principle is washing the air in some fluid culture medium 

 which will retain all the particulate matter, which may then 

 be cultured directly or sub-cultured into any favourable 

 medium. 



Miquel has contrived a simple piece of apparatus for the 

 carrying out of this principle. It consists of a flask with a 

 central tube through its own neck 

 for the entrance of the air. On 

 one side of the flask is a tube to 

 be connected with the aspirator, 

 on the other side of the flask a 

 tube through which to pour off 

 the contained fluid at the end of 

 the process. In the flask are 

 placed 30 cc. of sterilised water 

 (or, indeed, if it be preferred, 

 sterilised broth). The entrance 

 tube is now unplugged, and the 

 aspirator draws through a fair 

 sample of the air in the room (say 

 ten litres). This air perforce 

 passes through the water and by 

 the exit tube to the aspirator, and 

 is thereby washed, leaving behind 

 in the water all its bacteria. The 

 aspiration is then stopped, and the 



MiQUEL*s Flask 



