Such a case occurs when large beds of vegetable substances are buried 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.