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 acid, nitre, which gives its explosive character to gunpowder, nitro-glycerine, gun-cotton, and various other explosive substances which it helps to form—­are among the most remarkable in nature for the violence and intensity of their action, and for the extent to which the principle of vitality avails itself of them as magazines of force, upon which to draw in the fulfillment of its various functions.

186 GENTLE MEASURES.

But this is really just what should be expected.  It is the very weakness of the hold which nitrogen maintains upon the elements combined with it that facilitates their release, and affords them the opportunity to seize with so much avidity and violence on those for which they have a strong attraction.

It is as if a huntsman should conduct a pack of ferocious dogs into a field occupied by a flock of sheep, quietly grazing, holding the dogs securely by very strong leashes.  The quiet and repose of the field might not be seriously disturbed; but if, on the other hand, a child comes in, leading the dogs by threads which they can easily sunder, a scene of the greatest violence and confusion would ensue.

In the same manner, when nitrogen, holding the particles of oxygen with which it is combined in the compounds above named by a very feeble control, brings them into the presence of other substances for which they have a very strong affinity, they release themselves at once from their weak custodian, and rush into the combinations which their nature demands with so much avidity as to produce combustions, deflagrations, and explosions of the most violent character.