itrogen, and it cannot be
doubted, from what has been already stated, that it is from nitric acid
only that it can be obtained.

It must be admitted, then, that carbonic acid, ammonia, nitric acid, and
water, are the great organic foods of plants. But while they have
afforded to them an inexhaustible supply of the last, the quantity of
the other three available for food are limited, and insufficient to
sustain their life for a prolonged period. It has been shown by
Chevandrier, that an acre of land under beech wood accumulates annually
about 1650 lb. of carbon. Now, the column of air resting upon an acre of
land contains only about 15,500 lb. of carbon, and the soil may be
estimated to contain 1 per cent., or 22,400 lb. per acre, and the whole
of this carbon would therefore be removed, both from the air and the
soil, in the course of little more than 23 years. But it is a familiar
fact, that plants continue to grow with undiminished luxuriance year
after year in the same soil, and they do so because neither their carbon
nor their nitrogen are permanently absorbed; they are there only for a
period, and when the plant has finished its functions, and dies, they
sooner or later return into their original state. Either the plant
decays, in which case its carbon and nitrogen pass more or less rapidly
into their original state, or it becomes the food of animals, and by the
processes of respiration and secretion, the same change is indirectly
effected. In this way a sort of balance is sustained; the carbon, which
at one moment is absorbed by the plant, passes in the next into the
tissues of the animal, only to be again expired in that state in which
it is fitted to commence again its round of changes.

But while there is thus a continuous circulation of these constituents
through both plants and animals, there are various changes which tend to
liberate in the free state a certain quantity both of the carbon and
nitrogen of plants, and these being thus removed from the sphere of
organic life, there would be a gradual diminution in the amount of
vegetation at the earth's surface, unless this loss were counterbalanced
by some corresponding source of gain. In regard to carbonic acid the
most important source is volcanic action, but the loss of nitrogen,
which is far more important and considerable, is restored by the direct
combination of its elements. The formation of nitric acid during thunder
storms has been long familiar; but