ous quality of a mass of
water so immense, and constantly kept in motion by the action of the
tides.

Some traces of animal matter may, however, be detected in the water of
the Thames; for if nitrate of lead be dropped into it,[14] "you will
find that it becomes milky, and that a white powder falls to the bottom,
which dissolves without effervescence in nitric acid. It is, therefore,
(says Dr. Thomson) a combination of oxide of lead with some animal
matter."

SUBSTANCES USUALLY CONTAINED IN COMMON WATER, AND TESTS BY WHICH THEY
ARE DETECTED.

To acquire a knowledge of the general nature of common water, it is only
necessary to add to it a few chemical tests, which will quickly indicate
the presence or absence of the substances that may be expected.

Almost the only salts contained in common waters are the carbonates,
sulphates, and muriates of soda, lime, and magnesia; and sometimes a
very minute portion of iron may also be detected in them.

EXPERIMENT.

Fill a wine-glass with distilled water, and add to it a few drops of a
solution of soap in alcohol, the water will remain transparent.

This test is employed for ascertaining the presence of earthy salts in
waters. Hence it produces no change when mingled with distilled or
perfectly pure water; but when added to water containing earthy salts, a
white flocculent matter becomes separated, which speedily collects on
the surface of the fluid. Now, from the quantity of flocculent matter
produced, in equal quantities of water submitted to the test, a
tolerable notion may be formed of the degrees of hardness of different
kinds of water, at least so far as regards the fitness of the water for
the ordinary purposes of domestic economy. This may be rendered obvious
in the following manner.

EXPERIMENT.

Fill a number of wine-glasses with different kinds of pump or well
water, and let fall into each glass a few drops of the solution of soap
in alcohol. A turbidness will instantly ensue, and a flocculent matter
collect on the surface of the fluid, if the mixture be left undisturbed.
The quantity of flocculent matter will be in the ratio of the quantity
of earthy salts contained in the water.

It is obvious that the action of this test is not discriminative, with
regard to the chemical nature of the earthy salt present in the water.
It serves only to indicate the _presence_ or _absence_ of those kinds of
substances which occasion that quality in water which is us