gem, decomposing light with the power of a
dewdrop, would be priceless to the jeweller.
Newton hence surmised that water contained a
combustible principle; which has since been
proved by experiment.

The proportions, in weight, of oxygen and
hydrogen required to form water, are eight-
nine parts and nine-tenths of the first, to eleven
parts and one-tenth of the latter, to make in all
one hundred parts, as may be demonstrated by
synthesis, that is, by putting the two ingredients
together. It may be effected by passing
an electric spark through a bladder or other
vessel containing the gases duly mixed. But
very considerable quantities of gas are required
to produce an appreciable quantity of water.
Cavendish was the first to reveal the real nature
of water, and to pursue the experiment with
sufficient perseverance to obtain a few spoonfuls.
Monge, Lavoisier, and Laplace, manufactured
it in larger quantities. Whether much water
is naturally thus formed now, may be doubted;
but imagination is overwhelmed on attempting
to conceive the discharges of electricity
requisite to combine the gases which furnished
the water existing on the earth as seas, rivers,
clouds, and ice.

Spring, well, rain, river, pond, and ice or
snow-water, are the ordinary condition in which
that liquid is presented to us. They are not all
potable, or at least not wholesome. Many springs
are too laden with either carbonate of sulphate
of lime; many pools with decomposing vegetable
or animal matter; many wells are impregnated
by the soil in which they are dug, the strata
through which they pass, the materials of which
they are built, or by unhealthy infiltrations
which escape from sewerage. Water, for drinking,
should contain a certain quantity of air in
dissolution. Ice and snow-water have none, and
are therefore unfit both for drinking and as a
medium for fish to live in. The air, however,
may be restored by agitation. Thus, trout are
found in streams that spring from glaciers at no
great distance from the source; because the
water has been aerated by falling and being
broken while leaping from rock to rock. It is
curious that the air contained in water should
hold more oxygen than atmospheric air; which
explains why so small a quantity should serve
for the respiration of fishes. The liquid appears
to have the power of changing the composition
of the atmosphere. The air which enters into
water at its conversion into ice and separates
by distillation, contains even a greater proportion
of oxygen.

Easy tests of good drinking water are, that it
readily dissolves soap without curdling, and that
it cooks vegetables well, especially dry
vegetables, as peas. Drinking water should be
running, limpid, scentless, insipid (not flat), giving
no sensation of weight when taken into the
stomach, yielding but a slight precipitate to the
nitrate of siver, the nitrate of barytes, and the
oxalate of ammonia. Its temperature should
not greatly differ from that of the atmosphere.
The best is water which flows over a flinty
bed, and whose source is not in calcareous
ground.

Water in casks from ponds and rivers is apt
to acquire, after a time, a putrid and offensive
smell, which renders it disgusting and even
dangerous. An efficacious remedy is to mix
with it a little coarsely-powdered, well-calcined
charcoal, or, still better, charred bones, to stir
well, and then strain or filter. When the quantity
of charcoal is sufficient, the water is
immediately disinfected. It results from Lowitz's
further experiments that sulphuric acid greatly
assists the action of the charcoal, and also allows
the dose to be diminished by nearly two-thirds,
which is a great advantage in long sea voyages.
Three pounds four ounces of putrid water require
four ounces and a half of powdered charcoal
for their complete purification, whilst by
adding thirty-four drops of sulphuric acid to the
same quantity of water, an ounce and a half of
charcoal suffices. The sulphuric acid need cause
no apprehension, because the quantity is too small
to produce any injurious effects, and it is, moreover,
absorbed by the charcoal itself. A good
precaution is to char the inside of the
water-casks before filling them.

We have seen, that within the short range of
one hundred degrees centigrade, water passes
through three different states. It is first a solid,
then a liquid, and lastly an elastic fluid. The
first is generally known as ice, the latter as
steam or vapour. Clouds, fogs, dew, rain, snow,
hail, and hoar-frost, are only varieties of those
states.

At a given temperature, and under the same
atmospherical pressure, the evaporation of water
is abundant in proportion to the surface exposed
to the air. Advantage is taken of this circumstance
to obtain solid salt from saline springs. The
water is made to fall on fagots of brushwood
disposed in strata under the shelter of sheds.
The water, as it falls on the brushwood, is
divided into very fine rain, which, by offering
numberless points of contact with the air, is in
great part evaporated. The evaporation of the
saline solution is then completed by boiling it in
caldrons.

Under whatever circumstances water is
evaporated, the resulting vapours mingle with the
atmosphere, which therefore always contains
more or less water in the state of elastic fluid.
The quantity is in proportion to the temperature,
whatever be the density of the air. Thus,
it is capable of holding much more vapour in
summer than in winter; and during hard frosts,
the transparent air is as dry as it can be without
being desiccated by artificial means. But
even then, it still contains a certain quantity of
gaseous water.

The vapours in the atmosphere remain invisible
as long as they do not exceed its capacity
of saturation. But if it cools, a portion of the
vapour becomes visible, since the capacity of a
given bulk of air to hold vapour diminishes with
the temperature. Cold squeezes the atmosphere,
just as the hand squeezes a sponge. And
according to the height where the contraction