THE KAOLIN FILTER 



123 



allows the water to trickle through sandstone, and in that of 

 David it is forced in turn through layers of wool treated 

 with iron tannate, sandstone, animal charcoal, and gravel. 



The kaolin niters on the Chamberland-Pasteur principle 

 consist of porous tubes of porcelain (the so-called * candles') 

 about 20 cm. long and 2 cm. thick, closed at one end and 

 provided at the other with enamelled points for the outflow. 



Supply-pipe connected with 

 the metal case 



Water 



Porcelain tube 



Enamelled discharge-pipe 

 FIG. 44. KAOLIN FILTER, ox THE CHAMBERLAND-PASTEUR SYSTEM. 



These are placed in the water to be filtered or fixed in metal 

 cases and screwed on to the supply-pipes ; several may 

 also be connected to form a battery (fig. 44). 



The micro-membrane filter of Breier consists of a fine 

 netting of metal covered with densely-packed asbestos, 

 which thus forms a thin filtering layer having excessively 

 fine pores. 



