state it possesses very remarkable
properties. All substances burn in it with greater brilliancy than they
do in atmospheric air, and its affinity for most of the elements is
extremely powerful. When diluted with nitrogen, it supports the
respiration of animals; but in the pure state it proves fatal after the
lapse of an hour or two. It is found in plants, in quantities varying
from 30 to 36 per cent.

It is worthy of observation, that of the four organic elements, carbon
only is fixed, and the other three are gases; and likewise, when any two
of them unite, their compound is either a gaseous or a volatile
substance. The charring of organic substances, which is one of their
most characteristic properties, and constantly made use of by chemists
as a distinctive reaction, is due to this peculiarity; for when they are
heated, a simpler arrangement of their particles takes place, the
hydrogen, nitrogen, and oxygen unite among themselves, and carry off a
small quantity of carbon, while the remainder is left behind in the form
of charcoal, and is only consumed when access of the external air is
permitted.

Now, in order that a plant may grow, its four organic constituents must
be absorbed by it, and that this absorption may take place, it is
essential that they be presented to it in suitable forms. A seed may be
planted in pure carbon, and supplied with unlimited quantities of
hydrogen, nitrogen, oxygen, and inorganic substances, and it will not
germinate; and a plant, when placed in similar circumstances, shows no
disposition to increase, but rapidly languishes and dies. The obvious
inference from these facts is, that these substances cannot be absorbed
when in the _elementary_ state, but that it is only after they have
entered into certain forms of combination that they acquire the property
of being readily taken up, and assimilated by the organs of the plant.

It was at one time believed that many different compounds of these
elements might be absorbed and elaborated, but later and more accurate
experiments have reduced the number to four--namely, carbonic acid,
water, ammonia, and nitric acid. The first supplies carbon, the second
hydrogen, the two last nitrogen, while all of them, with the exception
of ammonia, may supply the plant with oxygen as well as with that
element of which it is the particular source.

There are only two sources from which these substances can be obtained
by the plant, viz. the atmosphere and