nt acts freely for itself and according to its own laws,
important speculations (conclusions, indeed) have arisen, both as
regards temperature and climatic differences. It should be observed,
that volume, as we have used the word, is the apparent space occupied,
and differs from mass, which is the _effective_ space occupied, or the
real bulk of matter, while density is the relation of mass to volume,
or the quotient resulting from the division of the one by the other.
Those empty spaces which render the volume larger than the mass are
technically called its pores.

Has the composition of the atmosphere changed in the lapse of years?
On this point both French and German philosophers have largely
speculated. It is computed that it contains about two millions of
cubic geographical miles of oxygen, and that 12,500 cubic geographical
miles of carbonic acid have been breathed out into the air or
otherwise given out in the course of five thousand years. The
inference, then, should be, that the latter exists in the air in the
proportion of 1 to 160, whereas we find but 4 parts in 10,000. Dumas
and Bossingault decided that no change had taken place, verifying
their conclusion by experiments founded on observations for more than
thirty-five years. No _chemical_ combination of oxygen and nitrogen
has ever been detected in the atmosphere, and it is presumed none will
be.

* * * * *

The atmosphere possesses, as may be readily imagined, many important
characteristics. One of these is Weight.

This is demonstrated by simple, yet decisive experiments. The
discovery of the _fact_ is attributed to the illustrious Galileo, but
to modern science we owe all the certainty, variety, and elegance of
the demonstration. A vessel containing a quantity of air is weighed;
the air is exhausted from it and it is weighed again. An accurate
scale will then detect the difference of weight. A cubic foot of air
weighs 1.2 oz. Hence a column of air of one inch in diameter and a
mile in height weighs 44 oz.

The atmosphere is supposed to have an elevation of from 45 to 50
miles, but its weight diminishes in proportion to its height. The
whole pressure at the surface of the earth is estimated to be 15 lbs.
to the square inch; a person of ordinary size is consequently pressed
upon by a weight of from 13 to 14 tons. Happily for us, the pressure
from without is counteracted by the pressure from within.

The weight of the