reservoirs is
introduced a certain quantity of the clear lime-water, and after this
about nine times the quantity of the chalk-water. The transparency
immediately disappears--the mixture of the two clear liquids becoming
thickly turbid, through the precipitation of carbonate of lime. The
precipitate is crystalline and heavy, and in about twelve hours a
layer of pure white carbonate of lime is formed at the bottom of the
reservoir, with a water of extraordinary beauty and purity overhead. A
few days ago I pitched some halfpence into a reservoir sixteen feet
deep at the Chiltern Hills. This depth hardly dimmed the coin. Had I
cast in a pin, it could have been seen at the bottom. By this process
of softening, the water is reduced from about seventeen degrees of
hardness, to three degrees of hardness. It yields a lather
immediately. Its temperature is constant throughout the year. In the
hottest summer it is cool, its temperature being twenty degrees above
the freezing point; and it does not freeze in winter if conveyed in
proper pipes. The reservoirs are covered; a leaf cannot blow into
them, and no surface contamination can reach the water. It passes
direct from the main into the house tap; no cisterns are employed, and
the supply is always fresh and pure. This is the kind of water which
is supplied to the fortunate people of Tring, Caterham, and
Canterbury.

*****

The foregoing article, as far as it relates to the theory which
ascribes epidemic disease to the development of low parasitic life
within the human life, was embodied in a discourse delivered before
the Royal Institution in January 1870. In June 1871, after a brief
reference to the polarisation of light by cloudy matter, I ventured to
recur to the subject in these terms: What is the practical use of
these curiosities? If we exclude the interest attached to the
observation of new facts, and the enhancement of that interest through
the knowledge that facts often become the exponents of laws, these
curiosities are in themselves worth little. They will not enable us
to add to our stock of food, or drink, or clothes, or jewellery. But
though thus shorn of all usefulness in themselves, they may, by
carrying thought into places which it would not otherwise have
entered, become the antecedents of practical consequences. In
looking, for example, at our illuminated dust, we may ask ourselves
what it is. How does it act, not upon a beam of light, but