Passing from these considerations not unconnected with the subject, let us inquire what saltpetre is, and how it is formed. The term Saltpetre is applied to a variety of bodies, distinguished, however, by their bases, as potash saltpetre, soda saltpetre, lime saltpetre, etc., which occur naturally. They are all compounds of nitric acid and bases, or the gases nitrogen and oxygen united to bases, and are found in all soils which have not been recently washed by rains, and which are protected from excessive moisture. The decomposition of animal and of some vegetable bodies in the soil causes the production of one constituent of saltpetre, while the earth and the animal remains supply the other. Evaporation of pure water from the surface of the earth causes the moisture which rises from below to bring to the surface the salt dissolved in it; and as this salt is not volatile, the escape of the moisture leaves it at or near the surface. Hence, under buildings, especially habitations of men and animals, the salt accumulates, and in times of scarcity it may be collected. In all cases of its extraction from the earth several kinds of saltpetre are obtained, and the usual course is to decompose these by the addition of salts of potash, so as to form from them potash saltpetre, the kind most generally consumed. 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.