the lower, or at the summit of the loftiest moun- 
tains. as at the surface of well-cultivated and 
thoroughly - ventilated plains. The former of 
these seeming anomalies is accounted for by the 
fact of all vegetables assimilating the carbon of 
carbonic acid, and giving back its oxygen to. the 
atmosphere; and the latter is explained by the 
well known tendency of gases to make equal dif- 
fusions throughout one another without refer- 
ence to their respective gravity. See the arti- 
cles ArmospHERE, AGRICULTURAL CHEMISTRY, and 
VEGETABLE PHysioLoay. 
The average proportion of carbonic acid in the 
atmosphere is usually stated, in round numbers, 
at a thousandth part; but according to the most 
exact and most recent experiments, it is ‘000415th 
of the atmosphere’s own volume. “ We have rea- 
son to believe,” says Liebig, “ that this propor- 
tion was much greater in past ages; and, never- 
theless, the immense masses of carbonic acid 
| which annually flow into the atmosphere from so 
many causes, ought perceptibly to increase its 
quantity from year to year. But we find that 
all earlier observers describe its volume as from 
one-half to ten times greater than that which it 
has at the present time; so that we can hence at 
most conclude that it has diminished. It is quite 
evident that the invariable quantities of car- 
bonic acid and oxygen in the atmosphere, must 
stand in some fixed relation to one another; a 
cause must exist which prevents the increase of 
carbonic acid by removing that which is con- 
stantly forming; and there must be some means 
of replacing the oxygen removed from the air by 
the processes of combustion and putrefaction, as 
well as by the respiration of animals. Both these 
causes are united in the process of vegetable life. 
* * * The various layers of wood and mineral 
coal, as well as peat, form the remains of a prime- 
val vegetation. The carbon contained in them 
must have been originally in the atmosphere as 
carbonic acid, in which form it was assimilated 
by the plants which constitute these formations. 
It follows from this that the atmosphere must be 
richer In oxygen at the present time than in 
former periods of the earth’s history. The in- 
crease must be exactly equal in volume to the 
carbonic acid extracted in the nourishment of a 
former vegetation, and must, therefore, correspond 
to the quantity of carbon and hydrogen contained 
in the carboniferous deposit. Thus, by the de- 
position of ten cubic feet Hessian (5:51 cubic feet 
English) of Newcastle splint coal, (of the formula 
C 24 H 13 O, and specific gravity 1-228), the at- 
mosphere must have been deprived of above 
18,000 cubic feet Hessian (9,918 cubic feet Eng- 
lish) of carbonic acid, and must have been en- 
| riched with the same quantity of oxygen. A 
further quantity of oxygen, amounting to 4,480 
cubic feet Hessian (2,468 English) must have 
been furnished to the air by the decomposition 
of water, for ten cubic feet Hessian of coal con- 
tains hydrogen corresponding to this amount. 
CARBONIC ACID. 
695 
In former ages, therefore, the atmosphere must 
have contained less oxygen, but a much larger 
proportion of carbonic acid, than it does at the 
present time; a circumstance which accounts 
for the richness and luxuriance of the earlier 
vegetation. When this became entombed, higher 
forms of animal life were capable of existing. But 
a certain period must have arrived in which the 
quantity of carbonic acid contained in the air 
experienced neither increase nor diminution in 
any appreciable quantity. For if it received an 
additional quantity to its usual proportion, an 
increased vegetation would be the natural con- 
sequence, and the excess would thus be speedily 
removed. And, on the other hand, if the gas 
was less than the normal quantity, the progress 
of vegetation would be retarded, and the propor- 
tion would soon attain its proper standard. When 
man appeared on the earth, the air was rendered 
constant in its composition.” 
The fact that plants assimilate the carbon of 
carbonic acid, and return its oxygen to the at- 
mosphere, was ascertained by Priestley and Sen- 
ebier, and established by Percival and De Saus- 
sure; and it has been tested and illustrated by 
multitudes of more modern chemists and phytolo- 
gists, and may be ascertained, to his perfect satis- 
faction, by any ordinary intelligent observer. The 
leaves and other green parts of a plant absorb 
carbonic acid and emit an equal volume of oxy- 
gen, even independently of their connexion with 
the stem and root; for if they be detached from 
the rest of the plant, and placed in water which 
contains carbonic acid, and then exposed to a 
play of sunshine, they will speedily be found to 
deprive the water of all its carbonic acid; and if 
the experiment be conducted under a glass re- 
ceiver filled with water, the oxygen which they 
evolve from the absorbed and decomposed acid, 
may be collected and examined. Butif the same 
leaves, or others precisely similar, be placed, un- 
der a glass receiver, in water which does not 
contain carbonic acid, or in water which, while 
containing it, contains also such an alkali as re- 
sists its assimilation, no oxygen whatever will be 
evolved. Sir Humphrey Davy placed a piece of 
turf, four inches square, in a porcelain dish which 
floated on the surface of water impregnated with 
carbonic acid; he covered the turf with a glass 
vessel of 230 cubic inches in capacity; he occa- 
sionally supplied this vessel with water, through 
a funnel furnished with a stopcock ; he daily 
supplied the water in which the porcelain dish 
floated, with new water saturated with carbonic 
acid, so as to cause a small quantity of the acid 
to be constantly present in the receiver; and he 
found that—in consequence of the action of the 
grasses of the turf upon the carbonic acid thus 
fully and steadily supplied to them in the water 
—the air in the receiver acquired, in the course 
of eight days, an increase of thirty cubic inches 
to its volume, and of four per cent. to its propor- 
tion of oxygen. Ditches, pools, and small lakes, 
f 5 x ~ : NN 
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