give the names and chemical formulae of those which have
been analysed and described,--

Ulmic acid from long Frisian turf C_{40} H_{18} O_{16}
Humic acid from hard turf C_{40} H_{15} O_{15}
Humic acid from arable soil C_{40} H_{16} O_{16}
Humic acid from a pasture field C_{40} H_{14} O_{14}
Geic acid C_{40} H_{15} O_{17}
Apocrenic acid C_{48} H_{12} O_{24}
Crenic acid C_{24} H_{12} O_{16}

It is only necessary to observe further, that these formulae indicate a
close connection with woody fibre, and the continuous diminution of the
hydrogen and increase of oxygen shows that they must have been produced
by a gradually advancing decay.

The earlier chemists and vegetable physiologists attributed to the humus
of the soil a much more important function than it is now believed to
possess.

It was formerly considered to be the exclusive, or at least the chief
source of the organic constituents of plants, and by absorption through
the roots to yield to them the greater part of their nutriment. But
though this view has still some supporters, among whom Mulder is the
most distinguished, it is now generally admitted that humus is not a
_direct_ source of the organic constituents of plants, and is not
absorbed as such by their roots, although it is so _indirectly_, in as
far as the decomposition which it is constantly undergoing in the soil
yields carbonic acid, which can be absorbed. The older opinion is
refuted by many well-ascertained facts. As regards the exclusive origin
of the carbon of plants from humus, it is easy to see that this at least
cannot be true, for humus, as already stated, is itself derived solely
from the decomposition of vegetable and animal matters; and if the
plants on the earth's surface were to be supported by it alone, the
whole of their substance would have to return to the soil in the same
form, in order to supply the generation which succeeds them. But this is
very far from being the case, for the respiration of animals, the
combustion of fuel, and many other processes, are annually converting a
large quantity of these matters into carbonic acid; and if there were no
other source of carbon but the humus of the soil, the amount of
vegetable life would gradually diminish, and at length become entirely
extinct. Schleiden, who has discussed this subject very fully, has made
an approxima