There are other modes besides the processes of animal life by which the reserved force laid up by the vegetable process in these unstable compounds may be released.  In many cases it releases itself under ordinary exposures to the oxygen of the atmosphere.  A log of wood—­which is composed chiefly of carbon and hydrogen in an unstable union—­lying upon the ground will gradually decay, as we term it—­that is, its elements will separate from each other, and form new unions with the elements of the surrounding air, thus returning to their normal condition.  They give out, in so doing, a low degree of heat, which, being protracted through a course of years, makes up, in the end, the precise equivalent of that expended by the sun in forming the wood—­that is, the power expended in the formation of the wood is all released in the dissolution of it.

This process may be greatly accelerated by heat.  If a portion of the wood is raised in temperature to a certain point, the elements begin to combine with the oxygen near, with so much violence as to release the reserved power with great rapidity.  And as this force re-appears in the form of heat, the next portions of the wood are at once raised to the right temperature to allow the process of reoxidation to go on rapidly with them.  This is the process of combustion.  Observations and experiments on decaying wood have been made, showing that the amount of heat developed by the combustion of a mass of wood, though much more intense for a time, is the same in amount as that which is set free by the slower process of re-oxidation by gradual decay; both being the equivalent of the amount absorbed by the leaves from the sun, in the process of deoxidizing the carbon and hydrogen when the wood was formed.

The force imprisoned in these unstable compounds may be held in reserve for an unlimited period, so long as all opportunity is denied them of returning the elements that compose them to their original combinations.  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.