DISTRIBUTION OF BACTERIA. 139 



kind of filtration has come largely into use since the cholera 

 epidemic of 1892-93, and it appears to be very effective. 



Mechanical Filtration. This method of filtration is also 

 called the American system. It is more rapid than the pre- 

 ceding method and does not require a large area for filter 

 beds. Although sand is required also, filtration is accom- 

 plished by a jelly-like layer of aluminum hydroxide. This 

 product is formed by adding to the water a small quantity 

 of alum. The carbonates in the water decompose the alum 

 and produce aluminum hydroxide. It precipitates as a 

 white, flocculent deposit, entangling solid particles, includ- 

 ing bacteria, as coffee is cleared with white of egg. Only a 

 trace of alum should appear in the water. This method of 

 filtration has not been tested so extensively as slow sand 

 filtration, but seems likely to prove efficient. With water 

 poor in carbonates, these may have to be added. 



Various methods for the purification of water by means 

 of chemicals have been proposed. The use of ozone for this 

 purpose has met with considerable favor. 1 



The filtration of water on a small scale, as is ordinarily 

 done for domestic purposes, is generally entirely useless. 

 The so-called Pasteur filter of unglazed porcelain is effec- 

 tive if it is properly constructed and if the filter-tubes are 

 sterilized by heat frequently (every few days) conditions 

 which are seldom complied with. Distillation of water, or 

 thorough boiling will usually be the most practical method 

 for sterilizing drinking-water. 



Collection of Samples. Samples from the water-supply 

 of a city may be drawn from the faucet, but the water 

 should first be allowed to run for half an hour or longer. 

 From other sources the supply should be collected in ster- 

 ilized tubes or bottles, taking care to avoid contamination. 

 Sternberg bulbs (see Fig. 38) will be found useful for 



1 Consult " Disinfection and Disinfectants/' Rosenau, 1902. 



