ON THE AIR AND WATER OF TOWNS. 73 



the sand, it is completely done by simply allowing it to stand in contact with 

 the sand. This would seem to indicate that the action was in a great mea- 

 sure, if not entirely mechanical ; at the same time it is not an action which 

 takes place in a moment ; some time is required for it. 



If the water is driven too rapidly on it will not be filtered, and wells in 

 wet weather often indicate this by becoming turbid. It is true that beco- 

 ming turbid is different fi'om discoloration, but the particles which are carried 

 forward by the water are frequently those which have organic matter in them, 

 as may be shown by the deposit on standing. Time, however, is wanted ; 

 as a continued standing with the sand shows, and as the slow percolation 

 through soil also shows, the purification is most complete when the passage 

 of the water is continued for some time. A case was related to me by a 

 gentleman who had attempted to filter beer through a barrel of sand and 

 charcoal ; the filter was too strong for it, and it came through pure water. 

 I may add, from my own experiments, that a bottle of porter after standing 

 some time was made into a bottle of something like ale, the dark colouring 

 matter quite removed, and the taste nearly destroyed. 



The material of the filter is also of inferior importance; one of the best 

 filters which I have made, as far as clearing of the water is concerned, was 

 of steel filings ; the water was of course rather a chalybeate. Oxide of iron 

 and oxide of manganese made very good filters ; pounded bricks also, as far 

 as clearness is concerned, and mere removal of colour from the water. 



Filtration, therefore, has another object in view besides the mere straining 

 through a substance, the pores of which are not sufficiently large to allow the 

 larger pieces to pass. The removal of organic matter in solution is one im- 

 portant object, as well as the formation of nitrates in some conditions. The 

 passage thi'ough the soil is very slow, and the complete act of filtration has 

 time to be elaborated. We sometimes see water flowing from land having 

 only a slight moisture visible in it, and we see from whole acres a mere 

 dropping from the drains. We know, too, that water is kept close in contact 

 with surfaces, so much so as to give us the idea of very great force. It will 

 not pass through a porous substance unless its place is supplied by other 

 water; it will not keep its level in such situations as it does in a free state. 

 We may imagine, then, the great amount of surface with which it must 

 come in contact, moving slowly through acres and sometimes miles of 

 soil. 



Mere freedom from matter in suspension is got very readily by the use 

 of fine porous substances ; the upper soil itself will take away matter in sus- 

 pension ; and peat will do so readily, sometimes without giving any colour 

 to the water. Mere fineness of pores is not therefore what is wanted, as the 

 finest part of the soil is uppermost. 



Rivers have often a brown colour, arising from clayey particles in union 

 or in conjunction with organic matter; that is, the water in these cases, if 

 allowed to stand, does not deposit mere mud, but also organized bodies, the 

 same circumstances which bring one, bringing also the other. Motion 

 through soil with fine particles in it requires to be slow, otherwise there is 

 turbid water formed. It is remarkable how bright most of the chalk waters 

 are ; although hard, the chalk removes the fine substances that are so often 

 found suspended in water like small hairs, or in an invisible form, causing only 

 a dullness of appearance. This mechanical action is not difficult to procure, 

 as even in a laboratory the most brilliant solutions are obtained by the use of 

 paper only. Some substances are readily filtered out ; others are very difla- 

 cult to filter, and some cannot be filtered ; some will not filter in water, but will 

 readily in acid. There does appear to be an action by which a substance 



