AIR, SOIL, AND WATER. 151 



Pasteur-Chamberland Filter. This very perfect filter, which 

 is now in almost universal use, consists of a piece of polished 

 porcelain in the form of a cylinder closed at one end and pointed 

 at the other. It is placed in another cylinder of glass or rubber 

 and the pointed portion connected with a bottle containing the 

 water, or directly with faucet of the water-pipe. The water 

 courses through the porcelain very slowly and comes out entirely 

 free from germs ; pipe-clay, bisque, and kaolin are also perfect 

 filters. The only disadvantage is the long time it takes for the 

 water to pass through. Pressure is used to accelerate the pas- 

 sage in the form of an aspirator or air-pump. (See Fig. 37.) 



The force of the hydrant water is also sufficient to produce a 

 steady, small stream. 



These porcelain cylinders can easily be sterilized and the 

 pores washed out. 



Boiling as a means of purifying. When such a filter cannot 

 be obtained, the only alternative is to boil all the water to be 

 used for drinking ; and this should especially be done in times 

 of typhoid and cholera epidemics. 



Methods of Examination. Since the germs rapidly multiply 

 in stagnant water, an examination must not be delayed longer 

 than an hour after the water has been collected. Every pre- 

 caution must be taken in the way of cleanliness to prevent con- 

 tamination ; sterilized flasks, pipettes, and plugs should, or rather 

 must, be at hand, and the gelatine tubes best inoculated on the 

 spot. If this cannot be done, the sample should be packed in 

 ice until it arrives at the laboratory, which, as before stated, 

 should not be later than an hour after collection. The sample 

 is placed in a sterilized glass flask, and the flask then closed with 

 a sterile cotton plug. A sterilized pipette is then dipped into 

 the flask and 1 c.c. of the water withdrawn in it and added to 

 a tube of gelatine, the gelatine being in a fluid condition. To a 

 second tube, c.c. is added. The tubes are then shaken so as 

 to thoroughly mix the water with the gelatine, and then poured 

 upon wide glass plates one plate for each tube ; the plates are 

 then placed in the moist chamber, and in two to three days 

 examined. If the germs are equally divided, there should be 



