the same atom of nitrogen perform many different offices?]

After ammonia has entered the plant it may be decomposed, its hydrogen
sent off, and its nitrogen retained to answer the purposes of growth.
The changes which nitrogen undergoes, from plants to animals, or, by
decomposition, to the form of ammonia in the atmosphere, are as varied
as those of carbon and the constituents of water. The same little atom
of nitrogen may one year form a part of a plant, and the next become a
constituent of an animal, or, with the decomposed dead animal, may form
a part of the soil. If the animal should fall into the sea he may become
food for fishes, and our atom of nitrogen may form a part of a fish.
That fish may be eaten by a larger one, or at death may become food for
the whale, through the marine insect, on which it feeds. After the
abstraction of the oil from the whale, the nitrogen may, by the
putrefaction of his remains, be united to hydrogen, form ammonia, and
escape into the atmosphere. From here it may be brought to the soil by
rains, and enter into the composition of a plant, from which, could its
parts speak as it lies on our table, it could tell us a wonderful tale
of travels, and assure us that, after wandering about in all sorts of
places, it had returned to us the same little atom of nitrogen which we
had owned twenty years before, and which for thousands of years had been
continually going through its changes.

[Is the same true of the other constituents of plants?

Is any atom of matter ever lost?]

The same is true of any of the organic or inorganic constituents of
plants. They are performing their natural offices, or are lying in the
earth, or floating in the atmosphere, ready to be lent to _any_ of their
legitimate uses, sure again to be returned to their starting point.

Thus no atom of matter is ever lost. It may change its place, but it
remains for ever as a part of the capital of nature.

FOOTNOTES:

[A] By _saturated_, we mean that it contains all that it is capable of
holding.

CHAPTER IV.

INORGANIC MATTER.

[What are ashes called?

How many kinds of matter are there in the ashes of plants?

Into what three classes may they be divided?

What takes place when alkalies and acids are brought together?]

We will now examine the ashes left after burning vegetable substances.
This we have called inorganic matter, and it is obtained from the soil.
Organic matter, although forming so lar