252 ON THE PURIFICATION OF DRINKING tVATER. 
Berzelius filtered a saline solution through a long tube 
rilled with sand, and found that it ran out more or less com- 
pletely deprived of salt. 
Matteucci repeated Berzelius's experiment and confirmed 
his statement. He filled a tube about twenty-six feet long 
with sand, and filtered a saline solution through it, and he 
found that the density of the liquid introduced by the upper 
aperture of the tube was to that of the liquid escaping from 
the lower end as 1.00 to 0.91. But he observed that this 
difference of density was not always maintained ; for after a 
certain time the saline solution becomes as dense at its exit 
from, as its entrance into, the tube ; proving that the decom- 
position of the saline solution takes place in the first action 
of contact between it and the particles of sand. 
But a still more remarkable result, the inverse of the one 
just stated, was obtained by the last mentioned author with 
a solution of carbonate of soda. He filled a tube, nearly ten 
feet long, with sand, and filtered a solution of carbonate of 
soda through it; and he found that the density of the liquid 
at its entrance was to that at its exit as 1.000 to 1.005. In 
this case then the sand had deprived the solution of part of 
its water, and had thereby increased the gravity of the liquid 
which filtered through. 
Assuming, however, the accuracy of all these reported 
observations, it cannot be doubted that, in a practical point 
of view, the efficacy of sand, as a filtering medium, depends 
on its mechanical, not on its chemical influence. 
Domestic water filters are usually made of stone-ware, 
and usually contain a combination of filtering materials — 
such, for example, as sponge, sand, and charcoal. 
The filter beds used by several of the metropolitan water 
companies consist of a number of layers of sand and gravel, 
resting on perforated drains or tunnels. They will be more 
fully noticed hereafter. 
3. Ebullition. — Boiling effects the expulsion of air and 
carbonic acid from water. It decomposes the bi-carbonate 
