By the power of these rays—­including the calorific, the luminous, and the actinic—­the natural affinities by which the above-mentioned substances were united are overcome, and they are formed into new combinations, in which they are united by very weak affinities.  Of course, they have a strong tendency to break away from the new unions, and fall back into the old.  But, by some mysterious and incomprehensible means, the sun has power to lock them, so to speak, in their new forms, so as to require a special condition of things for the releasing of them.  Thus they form a reserve of force, which can be held in restraint until the conditions required for their release are realized.

The process can be illustrated more particularly by a single case.  Water, one of the substances absorbed by plants, is composed of oxygen and hydrogen, which are united by an affinity of prodigious force.  It is the same with carbon and oxygen, in a compound called carbonic acid, which is also one of the principal substances absorbed by plants from the air.  Now the heat and other emanations from the sun, acting upon these substances in the leaves, forces the hydrogen and the carbon away from their strong bond of union with oxygen, and sets the oxygen free, and then combines the carbon and hydrogen into a sort of unwilling union with each other—­a union from which they are always ready and eager to break away, that they may return to their union with the object of their former and much stronger attachment—­namely, oxygen; though they are so locked, by some mysterious means, that they can not break away except when certain conditions necessary to their release are realized.

Hydrocarbons.

The substances thus formed by a weak union of carbon with hydrogen are called hydrocarbons.  They comprise nearly all the highly inflammable vegetable substances.  Their being combustible means simply that they have a great disposition to resume their union with oxygen—­combustion being nothing other than a more or less violent return of a substance to a union with oxygen or some other such substance, usually one from which it had formerly been separated by force—­giving out again by its return, in the form of heat, the force by which the original separation had been effected.

A compound formed thus of substances united by very weak affinities, so that they are always ready to separate from each other and form new unions under the influence of stronger affinities, is said to be in a state of unstable equilibrium. It is the function of vegetable life to create these unstable combinations by means of the force derived from the sun; and the combinations, when formed, of course hold the force which formed them in reserve, ready to make itself manifest whenever it is released.  Animals receive these substances into their systems in their food.  A portion of them they retain, re-arranging the components in some cases so as to form new compounds, but still unstable.  These they use in constructing the tissues of the animal system, and some they reserve for future use.  As fast as they require the heat and the force which are stored in them they expend, them, thus recovering the force which was absorbed in the formation of them, and which now, on being released, re-appears in the three forms of animal heat, muscular motion, and cerebral or nervous energy.