Purification of Water. 291 
and we cand that in 1790 and 1791 patents were taken out, 
one by a Chelsea potter, for a new filtering medium, which 
he made of pipeclay and coarse sand ; the other for a some- 
what complicated apparatus, the patentee using sand, char- 
coal and other loose materials. 
Various contrivances have been introduced since then, 
more or less similar to each other, some acting by ascension, 
some by descension, and in fact the numbers of patents taken 
out to protect what has been known for centuries is in- 
numerable, and persons have been always ready who, though 
perfectly unacquainted with the real requirements of filtra- 
tion, would insist upon introducing their wares as capable 
of doing impossible things, just as quacks will recommend 
their patent compounds as a certain remedy for diseases of 
which they know nothing themselves. 
Of course, since the water becomes contaminated from 
various influences, the filter ought to be made accordingly ; 
a medium which is recommendable for hard water will not do 
for soft water, and again, flatness may have to be contended 
with, which in itself requires a particular treatment. 
Thus the public could not fail, in course of time, to perceive 
that they were imposed upon, and the notion that filter 
‘makers were more or less mere pretenders, lias become 
general. 
Competent men however have taken the matter in hand, 
amongst whom is Dr. Clarke, who introduced the system of 
purifying water by adding quick-lime to it. 
Pure water will absorb two grains of carbonate of lime per 
gallon, and if the water takes up carbonic acid, this quantity 
may be increased to about twenty grains. The causticlime when 
added, will seize upon the carbonic acid present and render 
the carbonate of lime in solution insoluble, which, falling to 
the bottom, carries some of the organic impurities along with 
it. It is a beautiful manipulation, and answers well on a 
large scale Gf the water be hardened from a superfluity of 
lime). Dr. Clarke having also introduced.a test by which 
the degree of hardness of the water may be ascertained, to 
determine what quantity of lime should be added. Excess 
is easily tested with nitrate of silver. But unfortunately 
this process does not answer for domestic purposes, requiring 
as 1t does removal for each supply, besides it would not affect 
soft water and lead. : 
A similar method is that of adding alum, which will de- 
compose in the water, and fall to the bottom in insoluble 
