a 
oxygen is evolved, but at night this action ceases, 
and the acid is emitted while oxygen is absorbed. 
The emission of the acid is a simple mechanical 
process, the absorption of oxygen a chemical pro- 
cess, due to its action on the various organic sub- 
stances in the flowers, fruits, &c., but neither 
have any thing to do with the process of assimi- 
lation. Thus volatile oils, tannin, &c., have a 
tendency to oxidize, and hence plants containing 
them absorb more oxygen than others; the taste- 
less leaves of the Agave Americana absorb 0°3 of 
their volume of oxygen in the dark during 24 
hours, the leaves of the Pinus abies absorb 10 
times, and those of the Quercus robur 14 times 
their volume. 
These and other facts prove that the quantity 
of oxygen thrown into the atmosphere by plants, 
is greater than that which they receive from it. 
To conclude, the carbon contained in plants is 
derived from the carbonic acid of the atmosphere, 
through the leaves or from that contained in the 
soil through the roots. 
Origin and action of Humus—All plants and 
vegetable structures undergo two processes of de- 
composition after death. One of these is named 
fermentation; the other, putrefaction, decay, or 
eremacausis. 
The decay of woody fibre—the principal con- 
stituents of all plants—is accompanied by a phe- 
nomenon of a peculiar kind. This substance, in 
contact with air or oxygen gas, converts the latter 
into an equal volume of carbonic acid, and its 
decay ceases upon the disappearance of the oxy- 
gen. If the carbonic acid is removed, and oxy- 
gen replaced, its decay recommences, that is, it 
again converts oxygen into carbonic acid. 
Woody fibre in a state of decay is the substance 
called humus. Its property of converting sur- 
rounding oxygen gas into carbonic acid dimin- 
ishes in proportion as its decay advances, and at 
last a certain quantity of a brown coaly-looking 
substance remains, in which this property is en- 
tirely wanting. This substance is called mould ; 
it is the product of the complete decay of woody 
fibre. 
Humus is a continued source of carbonic acid, 
which it emits very slowly. An atmosphere of 
carbonic acid is therefore contained in every fer- 
tile soil, and is the first and most important food 
for the young plants which grow in it. 
The roots perform the functions of the leaves 
from the first moment of their formation ; they 
extract from the soil their proper nutriment, 
namely, the carbonic acid generated by the 
humus. 
By loosening the soil which surrounds young 
plants, we favour the access of air, and the for- 
mation of carbonic acid, which is absorbed, and 
is replaced by atmospheric air, by which process 
the decay is renewed, and a fresh portion of car- 
bonic acid formed. A plant at this time receives 
its food both by the roots and by the organs above 
ground, and advances rapidly to maturity. 
2 AGRICULTURAL CHEMISTRY. 
When a plant has quite risen above ground, 
and when the organs by which it obtains focd 
from the atmosphere are formed, the carbonic 
acid of the soil is no further required. 
The power which roots possess of taking up 
nourishment does not cease as long as nutriment 
is present. When the food of a plant is in greater 
quantity than its organs require for their own 
perfect development, the superfluous nutriment 
is not returned to the soil, but is employed in the 
formation of new organs. At the side of a cell, 
already formed, another cell arises: at the side 
of a twig and leaf, a new twig and a new leaf are 
developed. 
The functions of the leaves and other green 
parts of plants, to absorb carbonic acid, and with 
the aid of light and moisture, to appropriate its | 
carbon, are continually in operation. But the 
new products arising from this continued assimi- 
lation are no longer employed by the perfect | 
leaves in their own increase ; they serve for the 
formation of woody fibre, and all the solid mat- 
ters of similar composition. The leaves now pro- | 
duce sugar, amylin or starch, and acids, which | 
were previously formed by the roots, when they | 
were necessary for the development of the stem, 
buds, leaves, and branches of the plant. 
The organs of assimilation, at this period of | 
their life, receive more nourishment from the at- 
mosphere than they employ in their own sus- 
tenance ; and when the formation of the woody | 
substance has advanced to a certain extent, the | 
expenditure of the nutriment, the supply of which 
still remains the same, takes a new direction, 
and blossoms are produced. The functions of the 
leaves of most plants cease upon the ripening of | 
their fruit, because the products of their action | 
are no longer needed. They now yield to the | 
chemical influence of the oxygen of the air, gen- 
erally suffer a change in colour, and fall off. 
A peculiar transformation of the matters con- 
tained in all plants takes place in the period be- 
tween blossoming and the ripening of the fruit ; | 
new compounds are produced, which furnish con- | 
stituents of the blossoms, fruit, and seed. An 
organic chemical transformation is the separa- | 
tion of the elements of one or several combina- | 
tions, and. their reunion into two or several | 
others, which contain the same number of ele- | 
ments, either grouped in another manner, or in | 
different proportions. Of two compounds formed 
in consequence of such a change, one remains as 
a component part of the blossom or fruit, while 
the other is separated by the roots in the form of 
excrementitious matter. 
Hydrocyanic acid and water are decomposed 
by contact with muriatic acid into formic acid 
and ammonia; the muriatic seeking to be satu- 
rated by a base, selects the elements nitrogen and 
hydrogen to form ammonia, with which it unites, 
and its power of producing farther change is lost. 
By the separation of ammonia, the remaining ele- 
ments unite to produce formic acid. The am- 
— aes — 
