And out the stars came sparkling, as the moon
Launched from the cloud-bank; Cadmus swiftly then
Strode back to Thebes, and after him like slaves
Came the survivors of those dragon men.

AIR.

IN TWO CHAPTERS. CHAPTER I.

IT is solely through the impressions conveyed
to us by our senses that we know what we do
know. Passions and propensities may be innate
and instinctive, but all knowledge of facts and
things is acquired.

Much of what our senses acquaint us with,
are not material substances. Thus, light,
electricity, heat,* sound, are not things, but
motions. Formerly, with the exception of
sound, they were called imponderable fluids.
Imponderable, unweighable, in truth they are;
but instead of being fluids, they are forces.
They are the life of the universe, its manifestation
of vitality. And—as to weighing them—
one force, attraction or gravitation, is the source
and origin of all weight.

*See Fire, All the Year Round, vol. vi. p. 393.

Other objects whose existence is disclosed to
us by our senses—the earth, the waters, the
woods, and the winds—are tangible, material,
ponderable. They are things which have
weight and substance, being composed of what
men have agreed to call matter, in distinction
to spirit.

There are three forms of matter known to us;
the solid, the liquid, and the gaseous. A fourth
form is suspected, and more than suspected;
namely, the ethereal, which is supposed to be
to the lightest gas, what the lightest gas is to
the densest liquid. It has been thought that
all substances, all material things, may be
capable of assuming each of these three forms,
if placed under the required conditions. We
see many objects in daily use pass through
them all without difficulty. A lump of sulphur
readily melts, and as readily passes off in fumes.
Ice is easily converted into water, and water as
easily into steam. Camphor speedily takes its
departure, leaving no residue behind. In a
closed glass jar, it is volatilised, and then re-
solidified upon the sides of the jar. What is
still more curious, one substance, usually seen
in the form of gas, carbonic acid, may be
reduced to a fluid state, and then, by artificial
freezing, to a solid. Therefore, although we are
ignorant of the conditions required to transform
hydrogen gas, for instance, to a liquid or a
solid, we have no right to assume that such a
transformation is impossible. All we can say
is, that hydrogen is known to us only in the
gaseous state.

The air we breathe is a permanent gas, and
has never been made liquid, still less solid. It
does not change its state, to whatever
circumstances of temperature and pressure it is
subjected. It has neither taste nor smell of its
own, only what it borrows from foreign bodies
with which it is laden.

Air is said to be invisible and colourless;
which is correct only when it is presented to
the eye in small or limited quantity, of equal
temperature throughout its mass, and laden
with a proper proportion of watery vapour
varying with that temperature. Otherwise, air
is perfectly visible. We see its colour in the
azure sky; we see its substance in the purple
veil which hangs between us and the distant
mountain. We see it when certain conditions
of dryness (as during some easterly winds)
diminish its transparency. We see it when,
in the shape of a mirage, it assumes the
semblance of a sheet of water. Whenever two
strata of air of considerably different temperatures
come in contact (as when a cool breeze
blows over heated sands, or on the top of a burning
brick-kiln, or by the side of a furnace chimney),
we see the two airs trying to mingle, as clearly
as we see white syrup, still undissolved, curling
and circling in a glass of water. We see what
air holds in clouds, fogs, mists, and hazes. We
see the air in the glowing tints which it refracts
in the west and reflects in the east after the
sun is below the horizon. When the sky is
clear, we behold the air in sunsets and
sunrises, as distinctly as we behold a diamond in
its sparkling.

"As light as air" is a proverbial expression.
Air, nevertheless, is heavier than is generally
imagined. It presses on us with a load of
fifteen pounds on every square inch of surface
of our bodies, although we do not so much as
suspect its weight. The reason why we do not
feel the pressure is, because the air penetrates
everywhere; it presses in all directions, both
within and without our organs. The result of
the equilibrium of the two pressures is as if
they did not exist at all.

With the barometer at thirty inches and the
thermometer at freezing-point, a cubic foot of
dry air weighs more than an ounce and a quarter.
The weight, therefore, of the air contained in any
apartment of respectable dimensions amounts
to something considerable. The total weight of
the entire atmosphere is equal to that of a solid
globe of lead sixty miles in diameter.

That air has weight, is proved by a very
simple experiment. A bottle, from which the
air is exhausted, is weighed. Air is allowed to
fill it; it is then weighed again, and an
augmentation of weight is perceptible. The proof
strikes one as being of the easiest; but to carry
it out, two instruments were necessary which
science did not possess in the olden time, namely,
an air-pump, and an exact and delicate balance.
In handling a bladder full of air, we do not feel
the weight of the contents. We only feel the
weight of the bladder, because we are handling
it in air. A similar effect would be produced
on handling a bladder full of water in water;
the weight of the water would be imperceptible.

Air also strikes us as being lighter than it
really is, in consequence of its elasticity. There
is a springiness in its contact which
immediately conveys the idea of great legerity.
When we see the boundings of a ball filled