250 ON THE PURIFICATION OF DRINKING WATER. 
separation of the mechanical impurities of water ; that is, of 
those foreign bodies which are suspended, not dissolved, in 
water. But under certain circumstances, filtration becomes 
a means of modifying the chemical composition of water. 
Between liquids and solids there exists that kind of attractive 
force commonly called adhesion, to which are due the pheno- 
mena of capillarity or capillary attraction; and under cer- 
tain circumstances, this adhesive force is capable of over- 
coming a feeble chemical force, and thereby of effecting the 
decomposition of bodies whose constituents are held together 
by weak affinities. Practically, however, filtration is in most 
cases to be regarded as a mechanical process, by which phy- 
sical and not any important chemical changes are effected. 
The materials employed for the filtration of water are 
perforated plates of metal or stone-ware, unsized or bibulous 
paper, flannel or cloth or other tissues, sponge, porous stone 
(filtering stone,) charcoal (animal charcoal,) and beds of 
sand and gravel. 
On the present occasion we shall confine our attention 
chiefly to those which are applicable to the filtration of water 
on the large scale, namely, beds of sand and gravel; but 
we shall premise a few observations on the use of animal 
charcoal as a filtering material. 
Of all the permeable substances used for the purpose of 
filtration, animal charcoal possesses in the highest degree 
the combined mechanical and chemical influence to which 
we have already referred. In addition to its power, in com- 
mon with other filtering media, of removing suspended or 
mechanical impurities, it also abstracts from the liquor which 
permeates it, various dissolved bodies, and thus effects a 
change in the chemical composition of the fluid which traverses 
it. Thus it removes odorous and coloring matters, bitter 
principles, vegetable alkaloids, resins, tannin, and even 
metallic substances, from their solutions. By filtration 
through it the most stinking ditch water may be deprived of 
its noxious odor and flavor, and highly colored solutions, 
