128 MANUAL OF BACTERIOLOGY. 



or salt of alum. The carbonates in the water decompose the 

 aluminum and produce aluminum hydroxide. It precipitates 

 as a white, flocculent deposit, entangling solid particles, in- 

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

 a trace of aluminum 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.f 



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 effective if it is 

 properly constructed and if the filter-tubes are sterilized by heat 

 every few days — conditions which are seldom complied with. 

 Distillation of water and boiling are the most practical methods 

 for sterilizing drinking-water. 



Collection of Samples. — For bacteriological examination 

 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 col- 

 lected in sterilized tubes or bottles, taking care to avoid con- 

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

 for small samples. These samples should be examined as 

 quickly as possible, for the water bacteria increase rapidly in 

 number after the samples have been collected. When transpor- 

 tation to some distance is unavoidable the samples should be 

 packed in ice. 



The number of bacteria may be determined by making 

 plates of a definite quantity of the water with gelatin or agar. 



* See Fuller. Journal American Medical Association. Oct. 31, 1903. 

 t Consult Rosenau. Disinfection and Disinfectants. 1902. 



