METHODS OF EXAMINING WATER 



81 



two filters of fine sand separated from each other by wire-gauze 

 netting. 



2. After the tube is sterilized and plugged, it is placed in a 

 horizontal position, and the proximal end of the tube attached to an 

 air-pump (see Fig. 25, page 82), capable of drawing from 5 to 10 

 litres of air per minute, and the plug at the distal end of the tube 

 removed. 



3. The air-pump is worked until the gauge registers 100 litres. 



4. The sand filter at the distal end of the tube is removed and 

 mixed with 10 c.c. of sterilized liquefied gelatine, which is poured 

 into a sterile Petri-dish. The sand filter at the proximal end of the 



FIG. 24. Hesse's Apparatus for Examining Air. 



tube is used as a control, and must remain germ free when mixed with 

 nutrient gelatine, and poured into a Petri-dish. 



Powdered glass can be substituted for sand, and is more satis- 

 factory, as any developing colonies can be readily observed. 



CL. METHODS OF EXAMINING WATER. 



Samples of water can be procured in sterilized Erlenmeyer flasks 

 closed with cotton-wool plugs. They should be examined immedi- 

 ately, if possible, and never later than two hours after the sample is 



F 



