o large an amount of water.

[What is the result if a field be deficient in nitrogen?]

The protein substances are necessary to animal and vegetable life, and
none of our cultivated plants will attain maturity (complete their
growth), unless allowed the materials required for forming this
constituent. To furnish this condition is the object of nitrogen given
to plants as manure. If no _nitrogen_ is supplied the protein
substances cannot be formed, and the plant must cease to grow.

When on the contrary _ammonia_ is given to the soil (by rains or
otherwise), it furnishes nitrogen, while the carbonic acid and water
yield the other constituents of protein, and a healthy growth continues,
provided that the soil contains the _mineral_ matters required in the
formation of the ash, in a condition to be useful.

The wisdom of this provision is evident when we recollect that the
protein substances are necessary to the formation of muscle in animals,
for if plants were allowed to complete their growth without a supply of
this ingredient, our grain and hay might not be sufficiently well
supplied with it to keep our oxen and horses in working condition, while
under the existing law plants must be of nearly a uniform quality (in
this respect), and if a field is short of nitrogen, its crop will not be
large, and of a very poor quality, but the soil will produce good plants
as long as the nitrogen lasts, and then the growth must cease.[I]

ANIMALS.

That this principle may be clearly understood, it may be well to explain
more fully the application of the proximate constituents of plants in
feeding animals.

[Of what are the bodies of animals composed?

What is the office of vegetation?

What part of the animal is formed from the first class of proximates?

From the second?

Which contains the largest portions of inorganic matter, plants or
animals?

Must animals have a variety of food, and why?]

Animals are composed (like plants) of organic and inorganic matter, and
every thing necessary to build them up exists in plants. It seems to be
the office of the vegetable world to prepare the gases in the
atmosphere, and the minerals in the earth for the uses of animal life,
and to effect this plants put these gases and minerals together in the
form of the various _proximates_ (or compound substances) which we have
just described.

In animals the compounds containing _no nitrogen_ comprise the fatty
substances, parts of t