58 ANNUAL OF SCIENTIFIC DISCOVERY. 



render it extremely difficult to obtain the medium in the requisite 

 form for our purpose, and there is nothing yet discovered which 

 will perfectly meet all the requirements of the case. Those who 

 assert that it is possible to construct an apparatus to act as a uni- 

 versal filter for purifying any kind of water effectively, whatever 

 may be the impurities, remind me of the vendors of certain patent 

 medicines, who vaunt their nostrums as capable of curing every 

 disease. Their claims are about equally trustworthy. 



" I should classify the art of filtration into three systems, viz : 

 1st, where the action takes place simply on the surface of the fil- 

 tering medium ; 2d, where the whole bulk of the filtering medium 

 is calculated to operate on the water, and the detergent effect in 

 its most delicate form may be produced; and, 3d, where both of 

 these systems are conjointly employed. 



" The first system requires a filtering medium of such a fine 

 porosity that its pores must be smaller than the minute particles 

 composing the impurities suspended in the water. Such an agent 

 of course must sooner become clogged than a filtering medium of 

 coarser porosity, and which is meant to act with its whole bulk on the 

 water. But both systems employed together may prove to be use- 

 ful in several instances, as in the case of domestic filters. The 

 greatest failing of these is, that they must become clogged, and the 

 more they are liable to this, the more effectively they act. We 

 often hear of self-cleansing domestic filters, but the fact is that no 

 invention of the kind has been made yet, without involving com- 

 plications too great for the purposes of ordinary domestic use. 

 However, it is .not difficult to make a filter for general domestic 

 purposes, although the effective self-cleansing of such an appara- 

 tus is still a problem to be solved. 



"If the filtering medium employed in this case be solid, and of a 

 fine porosity in its upper part, the clogging impurities will not 

 only be retained on the surface, but may be easily removed by 

 scraping; and then, if the lower part of the filtering medium be 

 prepared of a material capable of producing a detergent effect, it 

 will act the more readily through not being interfered with by the 

 rougher and clogging impurities. It should be remembered, too, 

 that in most cases we have here only to deal with some rougher 

 impurities which have found their way into the water on its passage 

 from water-works, or other source, to the tap of the consumer. 



" The difficulty, or I may say the impossibility, of keeping water 

 which is stored in cisterns entirely free from accidental contamina- 

 tion, should lead us to provide a domestic filter capable of remov- 

 ing chemical impurities, as, for example, any lead which may be 

 held in solution ; in fact, the practice of filtering water preserved 

 in cisterns and intended for domestic use cannot be too warmly 

 recommended. 



" To remove lead from water, Professor Faraday recommends 

 the practice of stirring up animal charcoal with the water so con- 

 taminated, the same being then allowed to settle. 



"It is easy enough to purify small quantities of water, but the 

 greater the quantity the greater are the difficulties of purification 

 especially when a certain chemical effect has to be produced. 



