water has approached them, and surrendered its 
carbonic acid to their leaves, it is driven away 
by the pressure of other portions, which are ap- 
proaching to perform the same duty. But in the 
case of aerial plants, the leaves unfold a broad 
surface to the atmosphere, and readily, by reason 
of the darkness of their colour, absorb the calo- 
rific rays descending upon them, and in conse- 
quence establish and maintain a warm current 
for the constant withdrawal of expended air and 
the constant approach of carbonaceous air; in 
multitudes of instances also, the sensitive and 
waving leaves stand upon slender footstalks, and 
| give way to the slightest motion in the atmo- 
sphere, and are hence brought continually into 
extensive and ever-changing volumes of carbonic 
| acid; and in all instances, winds more or less 
| prevail, to sweep away expended portions of the 
atmosphere, and bring fresh supplies of carbona- 
ceous portions, or to mix the air of the seas and 
the deserts with that of the land and the fertile 
regions, and to make the carbonaceous air of the 
middle strata of the atmosphere exchange places 
with the oxygenous air of the lower strata. An 
animal, whose chemical task is oxidizement, is 
provided with instincts and locomotive powers 
to go in search of his food; but a plant, whose 
chemical task is the more delicate one of car- 
bonizement, or of reversing the effects of the 
animal’s task, and of maintaining a fair balance 
of chemical forces in the momentous relations of 
the atmosphere to the organic world, obtains a 
| supply of all its wants by means of exquisite and 
ever-operating exterior contrivances of the all- 
wise and all-benevolent Creator. 
“But in what state,” asks Keith, “is the car- 
bonic acid actually assimilated to the plant? Is 
it assimilated in the state in which it is inhaled, 
| or is it previously decomposed? It had been 
|| observed by Ingenhoutz that the leaves of plants, 
if placed in water and exposed to the action of 
the sun’s rays, will evolve a quantity of oxygen 
gas. It was afterwards ascertained by Senebier, 
that this process takes place only when the leaves 
are fresh, and the water impregnated with car- 
bonic acid. For when the water was deprived of 
its carbonic acid by boiling, or in the course of 
experiment, there was no more oxygen evolved. 
But when the water was again impregnated with 
carbonic acid, the extrication of oxygen recom- 
menced as before. Thus, the conclusion is ob- 
|| vious,and the phenomenon satisfactorily account- 
ed for ;—the carbonic acid gas contained in the 
water is abstracted and inhaled by the leaf, and 
immediately decomposed ; the carbon being assi- 
milated to the substance of the plant, and the 
oxygen evolved. Yet in the process of the fixa- 
tion of carbon, there seems, according to Saus- 
sure, to be alsoa partial fixation of oxygen, as well 
as the disengagement of a portion of nitrogen, of 
which the origin is a matter of doubt. Has it 
entered the plant with the air of the atmosphere, 
or mingled with carbonic acid, or with ani- 
se 
CARBONIC ACID. 
697 
mal substances soluble in the moisture of the 
soil ?” 
A question, implying a doubt, has been raised, 
whether a possibility exists of all plants obtain- 
ing their carbon from the atmosphere, or, in 
other words, whether a sufficiency of carbonic 
acid exists in the atmosphere for their supply. 
But this question can easily and most satisfac- 
torily be settled by a brief calculation. A col- 
umn of air of 2,517 lbs. weight rests upon every 
square foot of the surface of the earth ; a thou- 
sandth part of this is carbonic acid ; and twenty- 
seven per cent. of carbonic acid is carbon. Now 
a calculation on these data, jointly with the known 
diameter and superficies of the world, will show 
that so vast a quantity as 3,085 billions of pounds 
of carbon is contained in the atmosphere,—a 
quantity greater than that existing in a solid 
form, whether in living plants or in all the strata 
of mineral and brown coal throughout the earth ; 
and this quantity—especially as the supply of it 
from respiration, combustion, fermentation, ere- | 
macausis, and putrefaction, is maintained undi- 
minished—cannot but be most ample for the 
supply of all vegetation. The solicitude of all 
agriculturists of the old school, to afford their | 
crops a supply of carbon in the form of humus | 
or of decayed vegetable fibre in the soil, there- 
fore, was an idle waste of thought, and occasioned 
a perfectly gratuitous expenditure of labour. 
The manuring of land, as is shown in many of 
our articles, ought always to have reference to 
the supply of elements far difrerent from carbon. 
See the articles Manures, ALKALInS, AMMONIA, 
CHARcoAL, and many others. 
Carbonic acid has been proposed as at once an 
easy, an effective, and a very economical means 
of destroying the red spider and the other in- 
sects which infest plant-houses; but to prevent 
risk to the life or health of the gardener, it 
requires to be administered and dissipated in 
some manner which will protect him from breath- 
ing it; and to prevent damage to the plants, it | 
ought to be administered under a play of clear 
sunshine, when their power of absorbing it is at 
a maximum. One proposed method of applying 
it is to generate it in a separate building, and to 
pump it, through an orifice, into the top of the 
plant-house, by means of a syringe or an engine 
hose; and another and far simpler and more 
economical method, is to spread a four-inch stra- 
tum of powdered chalk on the floor of the plant- | 
house, and, by means of a garden or vinery en- | 
gine, with its rose introduced through a pane, to | 
let down upon it a sufficient shower of diluted 
sulphuric acid, to effect its thorough efferves- 
cence, and the consequent extrication of all its | 
carbonic acid. A coincident advantage of this 
latter method is to transmute the chalk into 
gypsum, and render it fit to be used as a field 
manure.—Zwurner’s Elements of Chemistry.—Dra- 
pers Chemistry of Plants—Davy’s Agricultural 
Chemistry. —Liebig’s Chemistry of Agriculture—_ | 
| 
