tric acid in
air, but its quantity is undoubtedly excessively minute, and materially
smaller than that of ammonia. At least this conclusion seems to be a
fair inference from Way's researches, as well as the recent experiments
of Boussingault on the proportion of nitric acid contained in rain, dew,
and fog, made in a manner exactly similar to those on the ammonia,
already quoted. According to his experiments an imperial gallon
contains--

Grs.
Rain. {Paris 0.0708
{Liebfrauenberg 0.0140

Dew. {Maximum 0.0785
{Minimum 0.0030

Fog. {Paris 0.7092
{Liebfrauenberg 0.0718

Although it thus appears that Barral's results have been only partially
confirmed, enough has been ascertained to show that the quantity of
ammonia and nitric acid in the air is sufficient to produce a material
influence in the growth of plants. The large amount of these substances
contained in the dew is also particularly worthy of notice, and may
serve to some extent to explain its remarkably invigorating effect on
vegetation.

_Carburetted Hydrogen._--Gay-Lussac, Humboldt, and Boussingault have
shown, that when the whole of the moisture and carbonic acid have been
removed from the air, it still contains a small quantity of carbon and
hydrogen; and Saussure has rendered it probable that they exist in a
state of combination as carburetted hydrogen gas. No definite proof of
this position has, however, as yet been adduced, and the function of the
compound is entirely unknown. It is possible that the presence of carbon
and hydrogen may be due to a small quantity of organic matter; but,
whatever be its source, its amount is certainly extremely small.

_Sulphuretted Hydrogen and Phosphuretted Hydrogen._--The proportion of
these substances is almost infinitesimal; but they are pretty general
constituents of the atmosphere, and are apparently derived from the
decomposition of animal and vegetable matters.

The preceding statements lead to the important conclusion, that the
atmosphere is capable of affording an abundant supply of all the organic
elements of plants, because it not only contains nitrogen and oxygen in
the free state, but also in those forms of combination in which they are
most readily absorbed, as well as a large quantity of carbonic acid,
from which their carbon may be derived. At first sight it may indeed
appear that the quantity of the latter