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