EXAMINATION OF AIR, SOIL, AND WATER 237 



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

 number of bacteria in a cubic centimeter 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, or, in the presence of an epidemic, to boil all 

 water used for drinking purposes. 



Fig. 117. Flask fitted with porcelain bougie for filtering large quantities 



,,f ii,,;<i 



of fluid. 



Pasteur-Chamberland Filter. This very perfect filter 

 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 the faucet of the water-pipe. The water courses 

 through the porcelain very slowly and comes out nearly free 

 from germs; pipe-clay, bisque, infusorial earth, and kaolin are 

 also good filters. The only disadvantage is the long time it 

 takes for the water to pass through. Pressure in the form 



