BOYS DEPARTMENT. 
227 
Bogs’ department. 
AGRICULTURAL CHEMISTRY—No. 4. 
Boys, as the elements of the air, including those 
of water, have now been described, our next busi¬ 
ness will be to consider the manner in which their 
influence is exerted in promoting vegetable growth. 
But first permit me to refresh your memories on 
some of the leading points of my previous letters. 
It is important that you should remember, 1st, that 
plants cannot contain a particle of matter which has 
not previously existed either in the atmosphere or 
the soil; 2d, that the essential constituents of the 
air are oxygen, nitrogen, carbonic acid, ammonia, 
and watery vapor; 3d, that carbonic acid and am¬ 
monia are compound gases, the former being com¬ 
posed of carbon and oxygen, the latter of hydrogen 
and nitrogen; 4th, that water is a compound of oxy¬ 
gen and hydrogen, and that it dissolves and holds 
in solution carbonic acid, ammonia, and many sub¬ 
stances 'in, the soil, which are essential to the 
growth of vegetables. I trust that you are now 
able to understand (as far as science can explain), 
the mode by which the elements I have described 
are absorbed by plants and made to enter into new 
combinations, such as contribute to the formation 
of woody fibre, leaves, fruit, seeds, &c. 
I would next direct your attention to carbonic 
acid, and in order to show in what manner this gas 
can be of service to vegetation. All plants contain 
carbon, which, in their dried state, constitutes about 
one half of their weight—vegetable fibre being com¬ 
posed of 53 parts in 100 of carbon, 42 of oxygen, 
and 5 of hydrogen. The reason why charcoal is 
not always found on our hearths, after the w*ood is 
consumed, is, because it has nearly all united with 
the oxygen contained in the wood and air around 
it, and passed off in the form of carbonic acid. It 
is, in fact, this union of carbon and. oxygen that pro¬ 
duces combustion; for, when these two gases unite 
with rapidity, light and heat are always emitted ; 
but when wood, or any species of vegetable fibre, 
is exposed to a high temperature without the pre¬ 
sence of air or oxygen, the charcoal (carbon), which 
it contains remains, while the'"other elements are 
separated. ‘r 
What has now been said is sumtignt to convince 
you of the vast importance of carboVtq vegetable 
existence, and it only remains to be sffawn how, 
and from whence it is obtained by plants. You 
may already have been informed that it is derived 
mainly from the atmosphere, and that the large 
amount of carbon, which plants require, could not 
be furnished entirely by the soil. Leaves of trees 
and plants continually absorb carbonic acid from 
the atmosphere, and transmit it to their branches, 
stems, and roots, where it is decomposed as it is 
needed, its carbon forming new connexions with 
certain other elements belonging to vegetables, and 
its oxygen mostly returned to the leaves and is 
restored again to the air. But plants do not appro¬ 
priate all the carbon which is thus absorbed ; for, 
the absorption of carbonic acid is going on both 
night and day, yet it is only during the day, or un¬ 
der the influence of light, that it is decomposed and 
its carbon appropriated—that which is absorbed 
during the night passing off with the water that 
is constantly evaporating from the leaves. 
The atmosphere, then, may be regarded as the 
principal source for the supply of carbon, and the 
leaves of vegetables as the principal agents for ob¬ 
taining it; yet a considerable portion of this impor¬ 
tant ingredient is undoubtedly derived from the 
soil, being absorbed and transmitted by the roots. 
As rain water always contains carbonic acid, and 
as the roots of plants are always imbibing moisture 
from the soil, they must necessarily supply more 
or less carbon. Other sources from which it may 
be obtained are vegetable and animal manures; still 
there are many reasons for believing that the whole 
amount of carbon derived from the soil is compara¬ 
tively small. This subject will be resumed when 
treating of humus , or vegetable mould. 
Your curiosity maybe excited to know from what 
source the atmosphere receives sufficient carbonic 
acid to supply this immense demand. Unless there 
were a provision in nature for returning to the air 
an equivalent for the large quantity of this gas , 
which is thus drawn from it, there would indeed 
be a constant decrease, and finally a total exhaus¬ 
tion of carbonic acid. But Supreme Wisdom never 
committed such an error in designing and arranging 
the economy of nature. A wise provision is found 
for supplying, by animal respiration, as well as by 
the combustion and decay of animal and vegetable 
matter, the whole amount of carbonic acid consum¬ 
ed by plants, and thus a proper equilibrium is al¬ 
ways maintained. Every breathing creature assists 
in this operation. The oxygen contained in the 
air we inhale unites with a portion of the carbon 
contained in the blood that passes through the 
lungs, thus forming carbonic acid, which is expired 
and given to the air, The amount of this gas, thus 
formed by an average-sized person, in twenty-four 
hours, is about five pounds, and the whole quantity 
given to the atmosphere by combustion and respir¬ 
ation, in the city of London and its vicinity, has 
been estimated at 10,000,000 tons annually. When 
life is destroyed' by confinement in a close apart¬ 
ment, it is on account of the production of car¬ 
bonic acid and consequent loss of oxygen by res¬ 
piration . 
From the facts above stated, we learn, that, if 
animals did not assist to supply the atmosphere 
with carbonic acid, trees and plants would at length 
become extinct, and if vegetables did not extract it 
from the air, it would, in time, accumulate to such 
an extent as to destroy animal existence. What an 
admirable illustration of the power and wisdom of 
the Deity! Even if no practical advantage were 
to be derived from scientific investigations, there 
would be the satisfaction of understanding the glo¬ 
rious operations of nature, and the elevation of 
mind and refinement of thought, which this depart¬ 
ment of knowledge can afford, yields an ample re¬ 
turn for all the time and labor thus expended. 
J. McKinstry. 
Greenport , Columbia County , N. Y. ) 
June lsf, 1848. ) 
Experiment for the Boys. —How much water 
will a common dry brick absorb, if applied gradu¬ 
ally, drop by drop, or by a very small stream 'l 
