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 aërial 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 A portion only of this quantity can be withdrawn in natural operations, such as the falling of rain and the deposition of dew,—the larger part always remaining behind.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. This ozonized oxygen, when brought into mixture with many nitrogenized bodies, forms with them nitrous acids, completely destroying their former condition and composition; hence, in the atmosphere, this part of the oxygen becomes a purifier of the whole mass, from which it removes putrescent exhalations, miasmatic vapors, and the effluvia from every source of sea or land. Very curious are the effects of this active oxygen, which is ever present in some portion of the atmosphere. Moved by the wind, mixed with the impure upward currents rising from cities, it seizes on and changes rapidly all foulness, and if the currents are not too voluminous, the impure air becomes changed to pure. As ozonized oxygen can be easily detected, we may pass from the city, where (overpowered by the exhalations) it does not exist, and find it in the air of the vicinity; and moving away several miles, ascertain that a normal amount there prevails, and that step by step, on our return to abodes of a dense population, the quantity diminishes and finally all disappears.

We are now prepared to answer the second part of the question which was suggested, and to find that nitrous acids formed in the atmosphere by direct oxidation of nitrogenous matter may unite with the ammonia present to produce one kind of saltpetre; and when the rains or the dews carry this to the earth, the salts of lime, potash, and soda there found will decompose this ammoniacal saltpetre, and set the ammonia free, to act over again its part. So in regard to decomposing organic matters in the soil: ozonized oxygen changes them in the same way. The earth and calcareous rocks of caves, penetrated by the air, slowly produce saltpetre, and before the theory of the action was understood, artificial imitation of natural conditions enabled us to manufacture saltpetre. Animal remains, stratified with porous earth or the sweepings of cities, and disposed in long heaps or walls, protected from rain, but exposed to the prevailing winds, soon form nitrous salts, and a large space covered with these deposits carefully tended forms a saltpetre plantation. France, Prussia, Sweden, Switzerland, and other countries, have been supplied with saltpetre from similar artificial arrangements.

But the atmosphere is washed most thoroughly by the rains falling in and near tropical countries, and the changes there are most rapid, so that the production of saltpetre, favored by moisture and hot winds, attains its highest limit in parts of India and the bordering countries.

During the prevalence of dry winds, the earth in many districts of India becomes frosted over with nitrous efflorescences, and the great quantity shipped from the commercial ports, and that consumed in China, is thus a natural production of that region. The increased amount due to tropical influences will be seen in the instances here given of the produce from the rich earths of different countries:—

Natural.