few inches in thickness; while the top layer of
all, being uncompressed, would be exceedingly
light, and still ten feet thick: so that a person
ascending from the bottom of the pit would find the
layers of equal weight thicker and thicker until
he reached the top.

Although one of their elements (oxygen) is
the same, air and water have but few qualities
in common; they are both transparent and both
fluid; that is to say, mobile, not solid, and that
is all. Yet so great is the affinity existing
between these two very dissimilar bodies, whose
common function is to sustain life, that neither
water completely deprived of air, nor air
completely devoid of water, is to be had except by
chemical means. Thus the atmosphere is a grand
reservoir; and the supply is afforded with such
regularity, that the southern hemisphere has been
likened to the boiler, and the northern to the
condenser, of a still.

Hence, although the quantity of water
contained in oceans, seas, and fresh-water lakes,
strikes us at once as enormous, still there is
more water in the world than meets the eye, or
than is even suspected by mankind in general.
Forests contain, in their tissues, millions upon
millions of tons. The water and the air which
are contained in living beings form so large a
portion of their mass, that, if thoroughly dried
and exhausted, they would become mere
shrivelled scraps of skin and bone. Birds and insects
are greatly indebted to air for their making up,
and for their general portliness. The quantity
of water which is visibly and invisibly held in
the atmosphere, is incalculable.

Following out the discrepancies of the two,
we find that water, so long as it continues
water, shrinks and swells but slightly with
change of temperature, and yields scarcely at all
under outward force. Air, on the contrary,
contracts and expands remarkably under the
influences of cold and heat. It is elastic. There
is no known limit to the degree of its condensation
under pressure, nor to its rarefaction when
pressure is removed. It adapts itself to, and
fills, whatever extent of space it is allowed to
occupy. The air-gun is a beautiful exemplification
of this property. Condensed air,
suddenly let loose, rushes out to acquire the state
of ordinary air, with such force as to drive a
bullet. Instead of the three forms so readily
assumed by water, air undergoes no change in
its constitution (except the different degrees of
density and tenuity) through any of the ordinary
influences by which it is affected; nor is any
influence known capable of effecting such a
change. The air's elastic force is like that of
every other gas composed of molecules, which
would run away from, and mutually shun, each
other, if they were not prevented by the vessel
which holds them, or the weight which
compresses them. There is a great difference
between the elasticity of gases and of solid bodies.
When a gas ceases to be compressed, it is not
only expanded, but it continues to occupy a
greater space, and appears in a larger volume
than before; whereas, solid elastic bodies,
relieved from compression, simply reassume the
form and the size which they had before.

Water and air are both fluids; their particles
move freely amongst each other in all directions.
But one, as we have seen, is an elastic fluid, or
gas; the other, a non-elastic fluid, or liquid.
Gases mix readily; many liquids will not mix.
Water mixes neither with mercury nor with oil.
Water itself is not a mixture, but a chemical
combination and union. It is not oxygen and
hydrogen, but oxygen with hydrogen; whereas
air is a mixture and not a chemical
combination of gases. Consequently, air's
pretensions to be called an element are much
better founded than water's; for the great bulk
of its mass is made up of uncombined elements
which cannot yet be reduced to any simpler
form.

Out of one hundred parts of atmospheric air
ninety-nine and a half consist of oxygen and
nitrogen or azote, mixed in the proportion of
twenty-one of oxygen to seventy of azote, by
volume, and of twenty-three to seventy-seven,
by weight. It thus appears that the specific
gravity of the two gases does not greatly differ.
Oxygen is a trifle heavier, azote a trifle lighter,
than air. In round numbers, it may be stated
that air is mainly composed of one-fifth oxygen,
and four-fifths azote.

Oxygen is the vital gas which serves for
combustion, and is indispensable to animal life.
If coals burn in a grate, it is the oxygen which
keeps them burning. If you place a lighted
candle under a bell-glass, it goes out as soon as
it has consumed all the oxygen. If you put
an animal under the same, as soon as it has
breathed all the oxygen, it dies. It is the act
of breathing atmospheric air which generates
the heat that warms our bodies. Fire is
produced by the oxygen contained in the
atmosphere combining with other bodies, as wood
and coal. Now hydrogen and carbon are also
two grand combustibles. Either of them,
combined with oxygen, becomes at once the parent
of heat. When the oxygen gathered by the
blood during its passage through the lungs
reaches with it the other organs, it finds there
hydrogen and carbon. It combines with them
and produces warmth. Our breathing, therefore,
is the same thing as lighting a gentle fire
inside us. Instead of blowing the kitchen fire
with a bellows, we draw the air into our lungs
with the bellows of our chest. The whole
secret of respiration or breathing is, that the
oxygen of the air combines with the hydrogen
and carbon of our bodies, and so gives rise to
warmth. Cold-blooded animals are chilly, solely
on account of the small quantity of oxygen
required to keep them alive.

The remaining half-part of the hundred
consists of carbonic acid and watery vapour, whose
quantity varies according to season and
locality. To verify the presence of carbonic acid,
you have only to expose lime-water to the air;
you will soon see a white film form on the
surface, which is nothing else but carbonate of
lime, that is, chalk. This small proportion of