ARTIFICIAL PURIFICATION OF WATER 65 



bicarbonate of lime, is converted into insoluble normal carbonate of 

 lime by the addition of a suitable quantity of limewater. Carbonates 

 of lime and magnesia are soluble in water containing free carbonic 

 acid, but when fresh lime is added to such water it combines with the 

 free C0 2 to form the insoluble carbonate, which falls as a sediment : 



CaCO 3 + CO 2 + CaH 2 O 2 (limewater) = 2 CaCO 3 + H 2 O. 



As the carbonate falls to the bottom of the tank it carries down 

 with it the organic particles. Hence sedimentation is brought about 

 by means of chemical precipitation. It is obviously a mechanical 

 process as regards its action upon bacteria. Nevertheless its action 

 is well-nigh perfect, and 400 bacteria per c.c. may be reduced to 4 

 or 5 per c.c. We shall refer to this same action when we come to 

 speak of bacterial purification of sewage. Alum has been frequently 

 used to purify water which contain much suspended matter. Five 

 or six grains of alum are added to each gallon of water, plus some 

 calcium carbonate by preference. Precipitation occurs, and with it 

 sedimentation of the bacteria, as before. But, as Babes has pointed 

 out, alum itself acts inimically on germs ; in such treatment, there- 

 fore, we get sedimentation and germicidal action combined. 



As a matter of actual practice, however, sedimentation alone is 

 rarely sufficient to purify water. It is true that the collection of 

 water in large reservoirs permits subsidence of suspended matters, 

 affords time for the action of light, and the suicidal competition 

 among the common water bacteria. But in small collections of 

 water it is otherwise. Here filtration is the most important and 

 most reliable method. 



Sand filtration, as a means of purifying water, has been practised 

 since the early part of last century. But it was not till 1885 that 

 Percy Frankland first demonstrated the great difference in bacterial 

 content between a water unfiltered and a water which had passed 

 through a sand filter (only about 3 per cent, of the bacteria originally 

 present being left in the water). Previous to this time the criterion 

 of efficiency in water purification had been a chemical one only, 

 and the presence or absence of bacteria in any appreciable quantity 

 was described not in mathematical terms, but in indefinite descriptive 

 words, such as " turbid," " cloudy," etc. It is needless to say that this 

 difference in estimation was largely due to the introduction by Koch 

 of the gelatine-plate method of examination. As a result of investi- 

 gation Percy Frankland formulated the following conclusions as 

 regards the chief factors influencing the number of microbes passing 

 through the filter. The efficiency of filtration, he held, depended 

 upon (a) the storage capacity for unfiltered water, by which it was 

 possible to obtain the preliminary advantage of subsidence ; (b) the 

 thickness of fine sand through which the filtration is carried on; 



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