MICROORGANISMS IN WATER 



325 



sand, form the filtering layer. Such filters are very efficient; they re- 

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



POROUS FILTERS. (Fig. 124.) These filters are made either 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 

 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 



FIG. 124. Unglazed porcelain niters. Chamberland system; A, without pressure; 

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



thus the filtering efficiency is impaired and will remain so, until the fil- 

 ters 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 

 well known. It is used in the purification of the water supply of some 

 towns Nice, Chartres, etc. Ozone used for this purpose is usually 

 obtained by means of the electric current; and a flowing film of water is 



