observed. But additions and subtractions are being constantly made,
and the atmosphere, as distinguished from the pure air, is mixed with
exhalations from countless sources on the land and the sea. Wherever man
moves, his fire, his food, the materials of his dwellings, the soil he
disturbs, all add their volatile parts to the atmosphere. Vegetation,
death, and decay pour into it copiously substances foreign to the
composition of pure air. The combustion of one ton of coal adds at least
sixteen tons of impurity to the atmosphere; and when we estimate on
the daily consumption of coal the addition from this source alone, the
amount becomes enormous.

Experiments have been made for the purpose of estimating these
additions, and the results of those most carefully conducted show how
very slightly the combined causes affect the general composition of our
atmosphere; and although the present refined methods of chemists enable
them to detect the presence of an abnormal amount of some substances, no
research has yet been successful in determining how far this varies from
the natural quantity at all times necessarily present in the atmosphere.

It is, however, the comparatively minute portions of nitrogenous matter
in the atmosphere that we are to consider as the source of the nitrous
acids formed there, and of part of that found in the earth. From some
experiments made during the day and night it has been found, that, under
the most favorable circumstances, six millions six hundred and seventy
thousand parts of air afford one part of nitrogenous bodies, if the
whole quantity be abstracted! A portion only of this quantity can be
withdrawn in natural operations, such as the falling of rain and the
deposition of dew,--the larger part always remaining behind.

When the oxygen of our atmosphere is exposed, while in its usual