ON THE PURIFICATION OF DRINKING WATER. 25 L 
such as wines and other brown saccharine liquids, are render- 
ed colorless by it. 
These important properties of animal charcoal have often- 
times led to its recommendation and use as a filtering medium 
for water ; and accordingly it is introduced into many of the 
common domestic water filters. But its deodorizing and de- 
colorizing power is soon lost, and in order to enable it to re- 
acquire its former efflcacacy, it requires to be again burned. 
Sugar refiners are obliged to renew weekly the animal charcoal 
which they employ for the decolor ization of brown syrups. 
So that when employed in water filters, animal charcoal re- 
quires renewal every week or two. 
Of all known permeable substances, the only ones which 
present all the requisites of filtering media for water on a 
large scale, are sand and gravel. These are cheap, allow the 
passage of water through them, and, when they have been 
previously well washed, communicate no impurity to the 
waters which traverse them. Their employment must have 
been suggested to man by the observation of the numerous 
limpid springs which arise in sandy and gravelly districts. 
Their action is chiefly, if not entirely, mechanical. They 
possess little or none of that power of effecting chemical 
changes on the liquids filtering through them, which, as we 
have before remarked, animal charcoal possesses in so pre- 
eminent a degree. Yet, unless several distinguished writers 
have grossly deceived themselves, sand is not entirely devoid 
of this chemical influence. 
Wagenmann, for example, found that when vinegar is 
filtered through pure quartz sand, the first portion of liquid 
that runs through is deprived of almost all its acid, and the 
vinegar does not pass through unchanged until the sand has 
become well charged with acid. The same authority also 
states that potato-brandy diluted with water and filtered 
through quartz sand, yields at first pure water, then a mixture 
of water and alcohol deprived of its fusel-oil, and, lastly, the 
original mixture unaltered. 
