FILTERING 



81 



placed in the funnel. The 

 liquid is poured from a 

 beaker, down a glass rod, 

 upon the filter. The filtrate 

 runs through the filter. 



water is more agreeable, because some dissolved solids are still present; 

 but both kinds of water are better if they are aerated; that is, if they 

 are made to dissolve air. We can add the air 

 to a water by filtering the water through 

 porous stone, or by pouring the water several 

 times from one vessel to another. 



84. Filtering. A filter is a screen 

 or sieve with openings so small that 

 only liquids and the substances dis- 

 solved in them can get through. 

 Substances suspended in the liquid, 

 which make the liquid roily, or turbid, A circular ^f; folded and 

 can not get through. In the laboratory 

 we make a filter by folding a piece of 

 porous paper, as shown in Fig. 68, and 

 placing it in a funnel. If a milky mixture of water and 

 powdered chalk be poured upon the filter, the water 



goes through, but the chalk does 

 not. 



In household filters (Fig. 69) por- 

 ous stone and charcoal are used as 

 filtering materials. These strain 

 out the suspended impurities of 

 the water, including the bacteria. 

 However, unless a filter is cared 

 for, it may become so clogged with 

 organic matter that it will serve as 

 a breeding-place for bacteria, in- 

 stead of removing them. Water filters should be cleaned 

 frequently, and exposed to direct sunlight, to keep them 

 fresh and wholesome. This is especially true of filters 



Charcoal 



FIG. 69. 

 A Household Filter. 



