vidence that there is no atmosphere.

The atmosphere of the earth is composed mainly of very volatile
elements, known as nitrogen and argon. This is commingled with oxygen,
also a volatile element. Into this mass a number of other substances
enter in varying but always relatively very small proportions. Of
these the most considerable are watery vapour and carbon dioxide; the
former of these rarely amounts to one per cent of the weight of the
air, considering the atmosphere as a whole, and the latter is never
more than a small fraction of one per cent in amount. As a whole, the
air envelope of the earth should be regarded as a mass of nitrogen and
argon, which only rarely, under the influence of conditions which
exist in the soil, enters into combinations with other elements by
which it assumes a solid form. The oxygen, though a permanent element
in the atmosphere, tends constantly to enter into combinations which
fix it temporarily or permanently in the earth, in which it forms,
indeed, in its combined state about one half the weight of all the
mineral substances we know. The carbon dioxide, or carbonic-acid gas,
as it is commonly termed, is a most important substance, as it affords
plants all that part of their bodies which disappear on burning. It is
constantly returned to the atmosphere by the decay of organic matter,
as well as by volcanic action.

In addition to the above-noted materials composing the air, all of
which are imperatively necessary to the wonderful work accomplished by
that envelope, we find a host of other substances which are
accidentally, variably, and always in small quantities contained in
this realm. Thus near the seashores, and indeed for a considerable
distance into the continent, we find the air contains a certain amount
of salt so finely divided that it floats in the atmosphere. So, too,
we find the air, even on the mountain tops amid eternal snows, charged
with small particles of dust, which, though not evident to the
unassisted eye, become at once visible when we permit a slender ray of
light to enter a dark chamber.

It is commonly asserted that the atmosphere does not effectively
extend above the height of forty-five miles; we know that it is
densest on the surface of the earth, the most so in those depressions
which lie below the level of the sea. This is proved to us by the
weight which the air imposes upon the mercury at the open end of a
barometric tube. If we could deepen these c