In this decomposition of animal remains and the formation of saltpetre the air performs an important part, and the changes it effects are worthy of our attention.

Let us consider the aerial ocean surrounding our earth and resting upon it, greatly larger in mass and extent than the more familiar aqueous ocean below it, and more closely and momentarily affecting our well-being.

The pure air, consisting of 20.96 volumes of oxygen gas and 79.04 volumes of nitrogen gas, preserves, under all the variations of climate and height above the surface of the earth, a remarkable constancy of composition,—­the variation of one one-hundredth part never having been 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  A portion only of this quantity can be withdrawn in natural operations, such as the falling of rain and the deposition of dew,—­the larger part always remaining behind.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 hygrometric state, to the influence of bodies attracting a portion of it, such as decomposing substances, or when it forms the medium of electrical discharges, it suddenly assumes new powers, acquires a greatly increased activity, affects our organs of smell, dissolves in fluids, and has been mistaken for a new substance, and even named “ozone.”  Among the new characters thus conferred on it is the power of uniting with or burning many substances.