AIR, SOIL, AND WATER. 185 



using chiefly the rivers and other large waters which course 

 near them for drinking purposes. A purification or filtration 

 can in a certain measure render these waters harmless. 



Filtration is often carried on on a large scale in the water- 

 works of cities and towns. 



Bacteriological examination is here of great service to deter- 

 mine if a water, which has been filtered and may have a very 

 clear appearance, and give no harmful chemical reaction, yet 

 be entirely free, or nearly so, from germs ; in other words, if 

 the filter is a germ filter or not. 



Charcoal Sponge and Asbestos, the materials formerly in use, 

 are objectionable because germs readily develop on them and 

 clog them, so that they require frequent renewal. In very 

 large filters, sand and gravel give the best results ; the number 

 of germs in a cubic centimetre is reduced to forty or fifty and 

 kept at that number. This is a very pure water for a city water, 

 though, as we stated before, not a safe one, for among those 

 forty germs very dangerous ones may be found. It is then 

 necessary for the users to refilter the water before drinking it, 

 through a material which will not allow any germs to pass. 



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, infusorial earth, 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 passage in the form of an aspirator or air-pump. 



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. 



All the cylinders or bougies are not germ proof, so that they 

 must be tested, and most of them must be cleaned every fourth 

 day, or they will allow germs to pass through. 



