218 



ESSENTIALS OF BACTERIOLOGY 



The amount entering is estimated by the capacity of the 

 wash-bottle, the rate at which it enters depending upon the 

 rate of the flow of water, which can be regulated. 



Fig. 132. Petri's sand-filter for air- 

 examination (McFarland). 



Fig. 133. Sedgwick's expanded tube 

 for air-examination (McFarland). 



Hesse advises for rooms and closed spaces i to 5 liters, at the 

 rate of two minutes a liter, and for open spaces, 10 to 20 liters, 

 at four minutes a liter. Plate cultures can be made from the 

 colonies, which develop in eight to ten days in the cylinder. 



Petri's Method. The air is pumped or sucked through 

 sand-filters, and the sand then mixed with gelatin. 



