ii

local causes, vary; but they are never entirely absent, and they
participate in the affairs of nature with an influence equal to that
of the former. There are a few other substances, viz: Sulphu-
reted, Carbureted, Phosphoreted Hydrogen, Miasma, &c, also
diffused through the atmosphere in minute quantities, but they do
not seem to exercise a particular influence on vegetable life, and
may therefore be here disregarded.

Oxygen, indisputably the most important constituent of the
atmosphere, is characterized by its affinity for combustible sub-
stances, in consequence of which it produces those phenomena
which in common language are termed combustion, eremacausis,
fermentation, &c. The metals e. g. are combustible substances,
and it is owing to this circumstance, that we so seldom find them
on the surface of our globe in the metalic state. We find them
already burnt or oxydized, viz: in their combination with oxygen,
as earthy substances, called Oxyds. Substances of organic nature
and origin also become exposed to the influence of the atmospheric
oxygen, as soon as the vital powers have ceased to exist in them.
Under this influence they are decomposed and converted into new
substances, which contain Oxygen as a constituent part.

In all these cases, Oxygen is consumed, and as it is furnished by
the atmosphere, the quantity contained in it should become less and
less; a conclusion, however, not confirmed by practical experience.
Philosophers have by direct experiment, succeeded in discovering
the source from which Oxygen is replaced in the atmosphere, in
'the same proportion as it is furnished by it to support the processes
just mentioned. Their observations have shown, that whilst dead
bodies of organic origin absorb Oxygen from the atmosphere, and
consequently decay; living plants, on the contrary, separate Oxygen
from its combinations, retain and assimilate the substance which
was united with Oxygen, and consequently grow and develop them-
selves. For Carbonic Acid, a product of the combustion of organic
matter, consisting of Carbon and Oxygen, becomes decomposed when
brought in contact with green leaves in such a manner that its
carbon is assimilated by them as food, whilst its Oxygen liberated
assumes the gaseous form and is diffused through the atmosphere.

Nitrogen is also a fundamental constituent of the atmosphere,
and must likewise be considered as a highly important substance,
though it takes no active part in the processes of life. Nitrogen
is, as to its properties, the opposite of Oxygen Whilst the latter
has a strong affinity to combine with other substances producing
phenomena which have been mentioned before, the former manifests
in this respect, such a degree of indifference, that only in rare and
local cases, e. g. by the action of electricity, is a direct union of
Nitrogen with other substances effected. But to this inactivity
which characterizes Nitrogen, its high importance as a constituent
of the atmosphere must be attributed. The quantitative proportion