240 



SCIENTIFIC AMUSEMENTS. 



whence it flows to a suitable reservoir. Most domestic waters, 

 however, contain more or less organic matter not removed by such 

 treatment, but capable to some extent of being removed by filtering 

 again on the small scale through miniature filtering beds made of 

 certain kinds of materials capable of exerting a purifying action 

 by enabling oxygen from the air to act on the organic matter in 

 the pores of the filtering material, as well as effecting mechanical 

 removal of suspended solid particles. 



The chemical purification thus effected depends on much the 

 same general principles as the use of charcoal for trapping drains 

 and the like ; obviously it is essential for the proper action to be 

 carried on that the filtering material should from time to time be 

 exposed to the air, so as to take up oxygen again, to replace that 

 exhausted whilst the purification is going on. This precaution is 

 often neglected, with the result of rendering the filter wholly 

 useless for the purpose for which it is mainly intended, viz., as a 

 means of causing the oxygen to act on the organic matter and 

 destroy it. 



Amongst the materials practically used for filtration on the 

 principle of not merely mechanically intercepting solid suspended 

 particles, but of also exerting a chemical purification by oxidation, 

 finely divided charcoal of various kinds and spongy metallic iron 

 are those most largely employed ; but fine sand, porous earthenware, 

 and similar materials possess the power of causing a small effect 

 of the kind ; in some instances this is partly 

 due to the presence of oxide of iron in the 

 material, this substance acting as a sort of 

 " carrier " pf oxygen, i.e., it absorbs oxygen 

 from the air and to some extent parts with 

 it again to the organic matters causing their 

 oxidation. Certain other metallic oxides, such 

 as oxide of manganese, have been used in 

 the same way as ingredients in filtering beds, 

 &c. Fig. 99 represents a simple form of small 

 filter consisting of a block of a kind of porous 

 coke or charcoal cemented into the bottom of 

 a conical jar which stands on the top of a 

 water bottle so that the water must pass 

 through the charcoal before dropping into 

 the bottle. 



* r S- 9 jL Domestic A natural purification of impure water as 



it flows down a river is brought about by 



the gradual absorption of air by the water, and the action of 



this dissolved oxygen on the noxious matter; so that some 



