MICROORGANISMS IN WATER. 



207 



cipitated in large flocculent masses, entangling all particles of soil or 

 organic matter; and these, being deposited on the surface of the sand, 

 form the filtering layer. Such filters are very efficient; they remove from 

 97 to 99.8 per cent of the bacteria from the water. 



POROUS FILTERS. (Fig. 59.) These filters are either made from 

 unglazed porcelain or baked diatomaceous earth; the former are known 

 as Chamberland, and the latter as Berkefeld filters. These filters 

 are usually candle-shaped, require considerable pressure to force water 



FIG. 59. Unglazed porcelain niters. Chamberland system; A, without pressure; 

 B, fitted to main water supply; C, section of a porous porcelain filter. 



through them, and can be used only when a small supply of water is 

 needed. Water which is forced through these filters is at first sterile; but 

 with repeated use they allow bacteria to pass through the pores and thus 

 the filtering efficiency is impaired and will remain so, until the filters 

 are cleaned and baked to red heat in a muffle-furnace. Unless this is done 

 regularly, no dependence should be placed on these filters, as they only 

 put those who use them off their guard against the danger to which they 

 are exposed. 



PURIFICATION BY OZONE. The antiseptic properties of ozone are 



