The alchemists dreamed and talked of that universal solvent which they so
long and vainly endeavored to discover; still, for all this, not only the
alchemist of old, but his more immediate successor, the chemist of to-day,
has found no solvent so universal as water. No liquid has nearly so wide a
range of dissolving powers, and, taking things all round, no liquid
exercises so slight an action upon the bodies dissolved--evaporate the
water away, and the dissolved substance is obtained in an unchanged
condition; at any rate, this is the general rule.

The function of water in nature is essentially that of a solvent or a
medium of circulation; it is not, in any sense, a food, yet without it no
food can be assimilated by an animal. Without water the solid materials of
the globe would be unable to come together so closely as to interchange
their elements; and unless the temperatures were sufficiently high to
establish an igneous fluidity, such as undoubtedly exists in the sun, there
would be no circulation of matter to speak of, and the earth would be, as
it were, locked up or dead.

When we look upon water as the nearest approach to a universal solvent that
even the astute scientist of to-day has been able to discover, who can
wonder that it is never found absolutely pure in nature? For wherever it
accumulates it dissolves something from its surroundings. Still, in a
rain-drop just formed we have very nearly pure water; but even this
contains dissolved air to the extent of about one-fiftieth of its volume,
and as the drop falls downward it takes up such impurities as may be
floating in the atmosphere; so that if our rain-drop is falling
immediately after a long drought, it becomes charged with nitrate or
nitrite of ammonia and various organic matters--perhaps also the spores or
germs of disease. Thus it will be seen that rain tends to wonderfully clear