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.
The force which the elements display in these reunions is always--and this is one aspect of the great discovery of modern times in respect to the _persistence_ or _constancy_ of force which has already been referred to--precisely the same in amount as that which was required for dissevering them from their original combinations with such substances at some previous time.