11

which, by uniting with the former, produce salts;* thus sulphur
rind oxygen in certain proportions form sulphuric acid or oil of
vitriol. Calcium and oxygen in certain proportions form lime, a
base which, when united to the sulphuric acid, produces a salt
(sulphate of lime or gypsum.) The phenomena attending oxyda-
tion are very variable. Ordinary combustion is only oxydation
taking place in a rapid manner; then again it takes place slowly,
without any manifest development of effects, as when iron, zinc,
or copper rusts. The burning of wood and the rusting of iron are
both examples of oxydation; in the one case occurring more rapidly
than in the other; and in each oxygen is obtained from the air.
Another instance of slow oxydation is the gradual decay, or slow
combustion, (Eremacausis, as it has been termed by Baron Liebig,)
of organic matter when exposed to the air. By it all organized
bodies are resolved into forms capable of being used as food for
plants It is this gas which when breathed supports animal life,
and almost all changes in animate or inanimate nature take place
in consequence of its increase or diminution.

Hydrogen.—This is also one of the four organic elements.
Like oxygen, it is very extensively diffused throughout all nature;
like it, it is a gas invisible, tasteless and inodorous. It is the light-
est body in nature, and hence used for filling balloons. It is never
found isolated, but always in union with some other body. With
oxygen it forms water, and with carbon the gas so generally used
for light. It is very combustible, and when burned by means of
the hydro-oxygen blowpipe, (an invention of the celebrated Dr.
Hare, of Philadelphia,) produces the most intense heat yet known.

Nitrogen—Like hydrogen and oxygen, when combined, al-
ways exists in the gaseous state, and like them is tasteless, color-
less and without smell. It forms about four-fifths of the atmos-
phere, a small proportion of vegetable, and a large share of ani-
mal matter. It has in its pure state no very sensible and direct
properties, and is better known by those which are negative. It
does not support combustion, but extinguishes all burning bodies
that are surrounded by it alone, and no animal can live in it when
pure, though it has no deleterious properties; combustible matter
refuses to burn, and the animal dies because deprived of oxygen,
or vital air. It serves as a diluent to moderate the action of the
oxygen in the air which we breathe, for no animal can live but a
short time in this latter gas; and if nitrogen was not present in
the air, in a short time not only would trees and forests, but also
iron and all other metals, be burned in one general conflagration.
When combined with hydrogen, this substance forms ammonia, so
necessary to the formation of vegetable and animal life, and that
which constitutes the chief valuable constituent in Peruvian guano.

* Hydrogen is also essential to give sulphur acid properties