BACTERIOLOGY OF WATER 145 



a temporary makeshift. There are some very good types 

 of domestic filters, but they are costly, and require very 

 intelligent and conscientious handling to give good results. 

 Many of the cheap ones are worse than useless because 

 they cannot be cleaned at all, and thus soon become veri- 

 table culture-media for bacteria. This is true of sand, 

 charcoal, and sponge filters, which, after the first few days, 

 are merely "strainers," keeping back gross impurities, 

 but allowing bacteria to grow in them and pass out into 

 the "filtered" water in greatly increased numbers. 

 Good types of domestic filters are the Pasteur and the 

 Berkefeld filters. Both consist of a cylinder of porous 

 porcelain called a "candle," fitted within a larger metal 

 cylinder. The metal cylinder is attached to the faucet, 

 the water enters it and cannot leave it except through 

 the porcelain candle, which retains all bacteria. But 

 there is danger of the bacteria "growing through" the 

 candle; hence it becomes necessary to boil and scrub the 

 candle every few days, and then dry it in the oven to 

 kill all germs. After that the filter is again efficient. 

 There must be a tight connection between the two 

 cylinders, as otherwise unfiltered water may mix with 

 the filtered. 



Purification by Chlorination. Many cities, New York, 

 for example, now disinfect their water supplies by means 

 of chlorin. At first chlorinated lime was used, but now 

 compressed chlorin gas (liquid chlorin) is preferred. This 

 is obtainable in steel cylinders similar to those used for 

 liquid carbon dioxid. In disinfecting water supplies y 5 Q 

 part of chlorin is added to 1,000,000 parts of water. 

 While the water is not made sterile, the number of bac- 

 teria is greatly reduced, especially the number of bacilli 

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