380 
SOUTHERN CULTIVATOR. 
any carbonous body with which the plant comes in con- 
tact. 
Moreovet', we must observe that carbonic acid is found 
in water, in which it is also soluble; both of which facts 
speak much for the above-mentioned supposition. For 
carbonic acid is found everywhere in the atmosphere, 
which is constantly supplied anew: and all water, wher- 
ever it is collected, contains carbonic acid. No one has, 
as yet, attempted to prove that a plant did ever grow under 
a perfect exclusion of carbonic acid. But the most effectual 
proofof the plant’s subsistance by means of carbonic acid. 
I am inclined to see in the circumstance, that while the 
plant is absorbing carbonic acid, it gives off oxygen ; and 
thus a chemical activity is evidently called forth, to which 
we must pay strict attention. Chemistry, the science 
which is alone competent to give judgment in this expul- 
sion of oxygen, knows of no other explanation than that 
the decomposition of water and consequently expulsion of 
oxygen is effected only by means of the carbonic acid, 
which is absorbed and made use of for the formation of 
the carbonous constituents. 
I will finally introduce a relative experiment, in order 
more fully to support the proposition, that carbonic acid 
yields the carbon of the plant. “In spite of the innumer- 
able quantity of works on the nourishment of plants,” says 
a German professor of Botany, “nothing has, thus far, 
been more uncertain than the views of the requisite sub- 
stances for vegetable nourishment; and this from the simple 
fact, that the matter v/as commenced at the wrong end, ex- 
periments and researches being lavished on fully develop- 
ed plants, instead of first seeking the laws for the simplest 
cases. But the simplest and most natural object for such 
inquiries is Protocacus viridis or some other conferva 
which consists of one or a few cells, contains all the usual 
substances requisite for cellular life live freely swimming 
in the water. These, in order to grow, need nothing but 
pure water(l) that may absorb carbonic acid and ammonia 
from the atmosphere, in which both of these are always 
found, and perhaps a very small quantity of non-organic 
salts, the necessity of which in the life of these simple 
plants, is in conformity with the higher order of plants, 
pre-supposed only, but is not proved. They do, however, 
thrive more luxuriantly in water impregnated with a 
larger and larger quantity of carbonic acid, than in water 
mixed with a solution of mud, consequently with a solu- 
tion of humus and humusates. In the latter they do not 
even thrive as well as in pure water — sufficient proof of 
how non-essential these constituents are to the life of the 
cell.” 
Considering what has thus far been said of the subject 
under consideration, we might almost here pass a final 
iudgment ; but, as it is my intention to be wholly impartial, 
we shall still have to inquire into what may be said against 
the supposition, that the plants are nourished by means of 
carbonic acid. 
Not long after the supposition had been advaitced, that 
the expulsion of oxygen of the living plants by the opera- 
tion of light is very closely related to the absorption of 
carbonic acid, it was found that the same plant expels car- 
bonic acid at night, but absorbs oxygen, and now certain 
people did not hesitate at once to place these day and- 
night-occurrences in vegetable life side by side For, 
without inquiring into the relation of the quantity of the 
expelled and absorbed oxygen and carbonic acid, it was 
asserted that the oxygen, which is absorbed at night, 
should be the same that is again expelled by day, and 
that, vice versa, the carbonic acid which, is expelled at 
night, could be regarded only as that absorbed by day. 
It is easily perceived, that, if the case did not really 
stand thus, then the idea of the carbonic acid supplying 
the plant with carbon, would be wholly out of the ques- 
tion ; for equal quantities of expelled carbonic acid drawn 
from equal quantities of absorbed carbonic acid leave no 
carbonic acid which might be used for the nourishment of 
plants. But the case does not stand thus. Indeed, we 
are not to doubt the above-mentioned effect, that oxygen 
is absorbed and carbonic acid expelled at night; but the 
experiment has proved that the oxygen, which is expelled 
by day when a plentiful supply of carbonic acid is pre- 
sent, amounts to more; so that it is a necessary conse- 
quence of vegetable life, that there is formed an air more 
rich in oxygen. One of the relative experiments is the 
following: — A grass -turf, four inches square, was laid in 
a porcelain bowl which swam on the water of a large 
basin and covered with a large glass bell (230 cubic 
inches),’ so that a certain quantity of air became excluded 
from the air without, at the same time with the grass. The 
grass was now and then moistened and carbonic acid added 
to the water on which the bow! swam. After the lapse of 
eight days, during which time the growing grass had, of 
course, been exposed to the interchanging influences of 
day- light and night, the capacity of the air had increased 
thirty cubic inches, and analysis taught that the confined 
air contained four per cent, of oxygen more than the air 
without. It is evident that this result could not have been 
obtained if at night there had been absorbed as much oxy- 
gen as was expelled by day. The same is the case with 
carbonic acid. 
It was, moreover, a great error to attempt to correct 
these nightly occurrences with those that happen in the 
day-time. The plant does, at all times, both day and 
night, perform alternate operations with its associations 
and their constituents, and as a consequence of this alter- 
nate operation, a continual absorption and expulsion of one 
or other among these constituents takes place. Thus, in 
order to give an example, there happens both night and 
day a quantity of oxidations of vegetable substances, con- 
sequently occurrences, whereby oxygen is taken from the 
association; but the decrease thus occasioned in the at- 
mospheric oxygen cannot be observed by day, because the 
oxygen that is given off from the living plant under the 
effect of light amounts to a far greater quantity. Accord- 
ing to the above-mentioned experiment, the air does, con- 
sequently, ill spite of this continual oxidation, receive 
more oxygen back again than is taken from it. As re- 
gards the carbonic acid, this, too, is absorbed both day 
and night by the living plant; but it is only in the day- 
time that this is used for the nourishment of the plant : at 
night, on the central y, it is not used by the plant, but re- 
turns into the air together with the water, that is constant- 
ly evaporating from the surface of the leaves, consequent- 
ly pursuant to a purely mechanical law, and there is no 
reason to suppose that it is the same carbonic acid which 
is absorbed by day. 
I have, at last, got so far as to be able to pass a decisivo 
judgment upon ihe question propounded at tJie beginning 
of this essay, namely, whether it is the humus of the earth, 
as such, or the carbonic acid of the air, that yields the 
carbon used by the plant I But it seems to me I may be 
spared the trouble, inasmuch as the answer — that it can 
be nothing but the carbonic acid — is too evident that I 
should have need to make use of logic. For wm have seen 
that plants may grow without humus; that humus, pro- 
vided it is really found in the soil, is most often present in 
so small a quantity that its contents cf carbon is far from 
sufficient fer coveting the carbonous constituents of the 
vegetation in this soil; and, finally, that, even if it were 
found plentifully in the soil, it would be wanting sufficient 
water for its solution. Hence, we conclude that it is not 
humus from which the vegetable carbon originates. On 
the other hand, we recognised in car(>onic acid the 
body which provides the plant with the requisite car- 
bon, because it is constantly present in ^he associations- 
of the plant, and is alwmys replenished ; and, finally , be- 
cause oxygen is expeiled while it is absorbed by tJie plant, 
wdiich must be regarded as a proof of the fact, that car- 
