uried under layers
of sediment which subsequently become stone, and thus shut the
hydrocarbonaceous compounds beneath them from all access to oxygen. The
beds of coal thus formed retain their reserved force for periods of immense
duration; and when at length the material thus protected is brought to
the surface, and made to give up its treasured power, it manifests its
efficiency in driving machinery, propelling trains, heating furnaces, or
diffusing warmth and comfort around the family fireside. In all these
cases the heat and power developed from the coal is heat and power derived
originally from the sun, and now set free, after having lain dormant
thousands and perhaps millions of years.

This simple case of the formation of hydrocarbons from the elements
furnished by carbonic acid and water is only adduced as an illustration of
the general principle. The modes by which the power of the sun actually
takes effect in the decomposition of stable compounds, and the formation
of unstable ones from the elements thus obtained, are innumerable, and
the processes as well as the combinations that result are extremely
complicated. These processes include not only the first formation of the
unstable compounds in the leaf, but also an endless series of modifications
and re-arrangements which they subsequently undergo, as well in the other
organs of the plant as in those of the animal when they are finally
introduced into an animal system. In all, however, the general result is
substantially the same--namely, the forcing of elements into unnatural
combinations, so to speak, by the power of the sun acting through the
instrumentality of vegetation, in order that they may subsequently, in the
animal system, give out that power again by the effort they make to release
themselves from the coercion imposed upon them, and to return to the
natural unions in which they can find again stability and repose.

One of the chief elements employed in the formation of these
weakly-combined substances is _nitrogen_--its compounds being designated as
nitrogenous substances, and noted, as a class, for the facility with which
they are decomposed. Nitrogen is, in fact, the great _weak-holder_ of
nature. Young students in chemistry, when they learn that nitrogen is
distinguished by the weakness of its affinities for other elements, and its
consequent great _inertness_ as a chemical agent, are often astonished
to find that its compounds--such as nitric a