iii

in which both Oxygen and Nitrogen form the atmosphere, is by
their weight about 23 per cent, of the former, to 77 per cent, of the
latter. Each particle of Oxygen is therefore mixed with about 4
particles of Nitrogen. By this arrangement a dilution of the pure
Oxygen is effected and consequently its destructive influence upon
living and dead bodies diminished, to such an extent as is in best
accordance with the wants of nature.

Carbonic Acid is the product of the decomposition of organic
substances, arising from the combination of Carbon with atmos-
pheric oxygen. All organic substances consist chiefly of Carbon;
and therefore when oxydized, produce Carbonic Acid, which
escapes in gaseous form, and diffuses itself through the atmosphere.

The functions which this constituent of the atmosphere performs,
are of high importance in the economy of nature. It is not alone
the chief nourishment of plants, which by separating the Oxygen,
appropriate to themselves its Carbon; but in its chemical character,
which is that of a weak acid, it is a powerful agent under the in-
fluence of which, as will be seen below, the process of the degra-
dation of rocks and formation of soils is produced. The quantity
of Carbonic Acid contained in the atmosphere is not constant—it
ranges however between certain limits, and depends on many par-
ticular circumstances; e. g. on the season of the year, on the time of
the day, on the locality, on the weather, &c.

Ammonia, a substance composed of Hydrogen and Nitrogen, is
also a product of the decomposition of organic bodies, (which
contain Nitrogen,) and for this reason is likewise diffused through
the atmosphere, of which it makes a constituent; and to it we must
also ascribe important functions, though its quantity is comparatively
very small. Experiments expressly made to ascertain its import-
ance, have shown that plants can live and develop themselves
without having any other source capable of furnishing them with
Nitrogen, but the Ammonia contained in the atmosphere; and if
practical experience has shown that a rational husbandman should
not always rely on this supply of Nitrogen, the result of the exper-
iments just mentioned nevertheless teaches us a fact which throws
sufficient light on the high importance of this substance.

Water is, in the form of gas and vapor, another constituent of the
atmosphere, of which it forms about one per cent, of its volume.
Its quantity, however, varies very much on account of circumstances
which have been noticed already, when speaking of Carbonic Acid.
It originates in exhalations from the surface of our globe, and is
dissolved by the atmosphere in form of a gas, the specific gravity
of which is much lighter than that of the atmosphere itself. For
this reason, it goes up to a certain height, where it is collected and
finally condensed to water (clouds,) which when heavy, falls down
(as rain) to its original place, the surface of our globe. Rain-water
contains in solution small quantities of all the constituents of the