cid, 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. The _processes of vegetation_ are the chief means employed for
effecting the original separations, by the power of the sun, and for
forming the unstable compounds by which this power is held in reserve. The
_animal system_, on the other hand, takes in these compounds, remodels them
so far as is required to adapt them to its structure, assimilates them, and
then, as occasion requires quires, it releases the concealed force, which
then manifests itself in the forms of _