a rearrangement of the atoms, new molecules
are formed, and a new substance is the result.

About 99-1/2 per cent. of air is oxygen and nitrogen and one-half per
cent. is chiefly carbon dioxide. Carbon dioxide is a product of
combustion, decay, and animal exhalation. It is poison to the animal,
but food for the vegetable. However, the proportion in the air is so
small that its baneful influence upon animal life is reduced to a
minimum. The nitrogen is an inert, odorless gas, and its use in the air
seems to be to dilute it, so that man and animals can breathe it. If all
the nitrogen were extracted from the air and only the oxygen left to
breathe, all animal life would be stimulated to death in a short time.
The presence of the nitrogen prevents too much oxygen from being taken
into the system at once. I suppose men and animals might have been so
organized that they could breathe pure oxygen without being hurt, but
they were not, for some reason, made that way.

Air contains more or less moisture in the form of vapor; this subject,
however, will be discussed more fully under the head of evaporation. The
air at sea-level weighs fifteen pounds to the square inch, and if the
whole envelope of air were homogeneous--the same in character--it would
reach only about five miles high. But as it becomes gradually rarefied
as we ascend, it probably extends in a very thin state to a height of
eighty or ninety miles; at least, at that height we should find a more
perfect vacuum than can be produced by artificial means. The weight of
all the air on the globe would be 11-2/3 trillion pounds if no deduction
had to be made for space filled by mountains and land above sea-level.
As it is, the whole bulk weighs something less than the above figures.

As we have said, the air envelopes the globe to a height at sea-level of
eighty or ninety miles, gradually thinning out into the ether that fills
all interstellar space. We live and move on the bottom of a great ocean
of air. The birds fly in it just as the fish swim in the ocean of water.
Both are transparent and both have weight. Water in the condensed state
is heavier than the air and will seek the lowest places, but when
vaporized, as in the process of evaporation, it is lighter than air and
floats upward. In the vapor state it is transparent like steam. If you
study a steam jet you will notice that for a short distance after it
issues from the boiler it is transparent, but soon it conden