AGRICULTURAL CHEMISTRY. 
73 | 
monia represents the substance assimilated by | crops of algze that grow on the bottom and shores 
the plant, and the formic acid the excrementi- | of the sea derive their carbon. 2d, The humates 
tious matter. 
By means of chemical transformations a great 
variety of products may now be obtained artifi- 
cially, which, having been found in plants and 
animals, were supposed to result from the vital 
principle. The volatile oil of valerian may be 
obtained from the oil generated during the fer- 
mentation of potatoes ; the oil of Spzrwa ulmaria 
from the crystalline matter of the bark of the 
willow. We can form malic, oxalic, and formic 
acids, urea, &c. 
Transformations of existing compounds are 
constantly taking place during the whole life of 
a plant, in consequence of which, and as the re- 
sults of these transformations, there are produced 
gaseous matters which are excreted by the leaves 
and blossoms, solid excrements deposited in the 
| bark, and’ fluid soluble substances which are 
eliminated by the roots. Substances containing 
a large proportion of carbon are excreted by the 
| roots and absorbed by the soil. Through the ex- 
pulsion of these matters unfitted for nutrition, 
the soil receives again with usury the carbon 
| which it had at first yielded to the young plants 
as food, in the form of carbonic acid. 
The soluble matter thus acquired by the soil is 
still capable of decay and putrefaction, and by 
undergoing these processes furnishes renewed 
sources of nutrition to another generation of 
plants ; it becomes humus. 
Humus does not nourish plants by being taken 
| up and assimilated in its unaltered state, but by 
presenting a slow and lasting source of carbonic 
acid, which is absorbed by the roots, and is the 
principal nutriment of young plants at a time 
when, being destitute of leaves, they are unable 
to extract food from the atmosphere. “Many 
physiologists,” says Mr. Shier, in the notes to his 
edition of Sir H. Davy’s ‘ Elements of Agricultu- 
ral Chemistry,’ “maintain that plants derive a 
great part of their carbon from humous matter 
in the soil, and that they absorb it in solution in 
the form of humates of lime and ammonia. Lie- 
big, while he admits that organic matter in the 
soil is useful in supplying carbon, denies that it 
is in the form of humates that it is taken up. 
Organic matter in soils to which air has access 
is continually undergoing decay, and carbonic 
acid thus formed constitutes the carbonaceous 
nutritive matter supplied by the soil. Liebig’s 
principal arguments against the common theory 
may be thus condensed. Ist, The humic acid of 
chemists does not occur in appreciable quantity 
in fertile soils; it is formed in the course of the 
chemical processes had recourse to to procure it. 
If humates existed in the soil and constituted 
the food of plants, they would communicate a 
brown tint, and be readily detected in the water 
of springs, brooks, and rivers, but they are not 
so; neither do they occur in sea-water, hence, it 
must be from carbonic acid that the immense 
are so little soluble, that it can be shown, that 
all the rain that falls during the growth of com- 
mon crops is incapable, even if it were all to be 
saturated with humates, and to pass through the 
plants, of affording a tithe of the carbon they 
require. 3d, There was no original humus, and 
hence it is not absolutely essential. 4th, Many 
plants have but a point of attachment in the | 
soil, and live almost entirely by absorption from | 
the air. 5th, Neither in cultivated land that is | 
regularly manured, nor in forest and meadow 
lands that are not, does the humus decrease; in 
the latter cases, indeed, it increases. Boussing- 
ault shows in the case of the five-course rotation 
already referred to, that the carbon contained by 
the crops exceeded that contained by the man- 
ure, by 4745°5 kilogrammes per hectare = 4233'9 
Ibs. per imperial acre. The carbon, therefore, 
must, to this extent at least, have been derived 
from the air; and at the close of the rotation the | 
humus had not decreased. Hence, it may be in- 
ferred, that the carbon derived from the organic 
matter in the soil was also taken up in the form 
of carbonic acid. Of recent attempts to settle 
the question by direct experiment, those of Saus- 
sure have attracted most attention. He endeav-| 
oured to show that soluble humates are taken up 
and assimilated by causing plants of the bean | 
and polygonum to grow in a decoction of mould 
in bicarbonate of potash. Liebig has criticised 
these experiments, and shown, that they are in- 
exact and inconclusive; and that the results are 
capable of a satisfactory explanation only on the 
principles they were intended to refute. On the 
whole then, it appears that there is no sufficient | 
reason for holding that soluble humates form any 
appreciable or important part of the food of 
plants, however useful humus may be, both as a | 
textural constituent of soils, and as affording car- 
bonic acid by gradual decay.” 
Assimilation of Hydrogen.—We can conceive 
of the formation of wood by the decomposition 
of water; its hydrogen uniting with the elements 
of carbonic acid, and oxygen being eliminated. 
Thus 100 parts carbonic acid unite with 8:04 
hydrogen, to form woody fibre, and separate 
72°35 oxygen, which was combined with the 
hydrogen. 
From their generating caoutchouc, wax, fats, 
and volatile oils containing hydrogen in large 
quantity, and no oxygen, we may be certain that 
plants possess the property of decomposing water, | 
because from no other body could they obtain 
the hydrogen of those matters. It has also been 
proved by the observations of Humboldt on the 
fungi, that water may be decomposed without 
the assimilation of hydrogen. Water is a re- 
markable combination of two elements, which 
have the power to separate themselves from one 
another, in innumerable processes, in a manner 
imperceptible to our senses; while carbonic acid, 
