290 Purification of Water. 
getting attacked, and particularly so if the water was 
strongly impregnated with organic matter. I had paid a — 
good deal of attention to the subject at the time, and, 
from my practical experience, | knew well that the Doctor 
was right in his surmise. To more than half of about two 
hundred water tanks, which I inspected, none or hardly any 
attention had been paid to the cleansing and vegetation, and 
animalculze were flourishing in abundance. 
The lead which lined the inside had in many instances 
the appearance of corrosion, and when the test was applied, 
it proved the presence of lead in the water. — 
There has been a great controversy as to how ordinary 
water may attack lead. The theory in which I believe is 
this, that in the first instance 1t emanates from the dissolving 
action of soft water, which rapidly accelerates when in 
motion, and the pressure of organic matter when in a state 
of repose. Lankester and others assert that experiments 
have proved it beyond doubt that distilled water will dissolve 
lead to a limited extent. 
To be wholesome, the water used for drinking and ordinary 
purposes must be free from injurious matter. I don’t mean 
that it should be pwre in the literal sense of the word, in- 
deed there are bodies, such as iron, carbonic acid gas, ete., 
the limited presence of which might be considered rather co- 
inducive to health than otherwise, and I have not un- 
frequently employed such media as will cause an impurity in 
this direction, when I had soft and flat water to deal with. 
Contrivances to purify water for domestic purposes have 
been known since time immemoriable. The old inhabitants 
of Keypt, the Greeks, and the Romans had them. Sponges 
were used to free the water of the Niger from its accidental 
contaminations. The Japanese use a porous stone, hollowed 
in the form of an egg, and set in a frame over a vessel into 
which the water drops as it percolates through the stone. 
The Eeyptians have the same for the filtration of the water 
of the Nile. be 
A favourite medium in France is a porous limestone, found, 
I believe, in Brittany, and a similar one used to be imported 
into England from Teneriffe, but is not so now, because 
equally good filterstones are found in Derby and Northamp- 
tonshire, besides, other media have supplanted them in 
modern times. 
In the latter part of the last century the filteration of 
water seems to have attracted public attention in England, 
