154 THE PRESIDENTIAL ADDRESS.
in Switzerland shows that a stream infected by typhoid germs will pass
unpurified through a very great distance in the coarse gravel of an
old moraine, and as the purifying organisms, whose work in shallow
sand-filters I have mentioned, require air, little or no purification
appears to take place in water streams below the surface.
The most perfect natural filter is chalk. The water from deep
chalk-wells is far freer from organic impurity and organisms than
any other natural water. In common with all other water from
deep sources it contains as much of the constituents of the rock
filter as the water can dissolve, but water containing dissolved
carbonate of lime, though inconveniently hard, is not unwholesome,
and can be easily purified either by boiling or by chemical means.
The remaining supply of pure water is afforded by the self-
purification of streams of which I have spoken. It is evident that
too much care cannot be taken to avoid initial contamination, and
zealously to guard against any interference with the stream for a
long distance above the intake. If the water from a river thus
guarded be efficiently filtered, it gives a water supply which experience
shows to be quite as wholesome as any other.
The supply obtainable from the Lea Valley naturally falls
under the two last classes, and in many ways presents exceptional
advantages.
The great Chalk basin overlaid in the lower portion by the
London Clay gives the double supply that forms the springs in the
upper valley of the Lea (which yield water of infinitely greater purity
than that of the springs and drainage that fill most rivers), or water
from artesian wells, affording an inexhaustible supply near or at the
surface.
It is hardly too much to say that the visible stream is but a
portion of the great river that flows in more or less defined channels
in the solid Chalk. In the upper valley, the water level is almost
exactly that of the stream in the valley, through however great a mass
of chalk the well may be sunk ; while in boring artesian wells in the
lower valley the depth at which water is reached, and the supply
obtained, entirely depends on the magnitude of the stream which is
struck in the chalk. The Woolwich Beds above the Chalk often
yield a considerable supply, but one not to be compared in quantity
or purity to that from the solid Chalk below, if the boring is carried
down till a good stream is struck. The level to which the water
rises in the well does not differ greatly in different bores, thus