It is then
passed through several covered channels and mechanically or chemically
cleansed from every kind of inorganic impurity, and finally oxygenated
or aerated with air which has undergone a yet more elaborate
purification. At every stage in this process, a phial of water is
taken out and examined in a dark chamber by means of a beam of light
emanating from a powerful electric lamp and concentrated by a huge
crystal lens. If this beam detect any perceptible dust or matter
capable of scattering the light, the water is pronounced impure and
passed through further processes. Only when the contents of the bottle
remain absolutely dark, in the midst of an atmosphere whose floating
dust renders the beam visible on either side, so that the phial, while
perfectly transparent to the light, nevertheless interrupts the beam
with a block of absolute darkness, is it considered fit for human
consumption. It is then distributed through pipes of concrete, into
which no air can possibly enter, to cisterns equally, air-tight in
every house. The water in these is periodically examined by officers
from the waterworks, who ascertain that it has contracted no impurity
either in the course of its passage through hundreds of miles of
piping or in the cisterns themselves.
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