CARBON. 104 
ticularly described under the head of digestion, consists in the 
decomposition of carbonic acid by the green tissues of the leaves, 
under the stumulus of the light, the fixation of the solid carbon, 
and the evolution of pure oxygen. 
282. Carbon is one of the principal ingredients in the vegetable structure. The 
chief source from which plants obtain it is the atmosphere, which always contains 
it in the form of carbonic acid, evolved by combustion, by the respiration of ani- 
oe from the earth, &c. 
‘Now if we place some fresh leaves in an inverted bell-glass, cosiiinaal air 
dace with 7 or 8 per cent. of carbonic acid, and expose them to the direct light 
of the sun for a few hours, it will be found that a large proportion of the carbonic 
acid will have disappeared, and will be replaced by pure oxygen.’ But this 
change will not be effected in the dark, or by any degree of artificial light. Ac- 
cordingly we find that plants which grow in the dark become blanched from the 
want of the proper supply of carbon, on which their green color depends. 
283. We have before stated that this fixation of carbon in the substance of the 
plant, contravenes the process of respiration, in which carbon is given off. ‘The 
former occurs only in the light of day, the latter by night as well as by day. But 
as to the relative amount of carbon thus absorbed by the former process, and 
evolved by the latter, there can be no reasonable doubt; for when we consider 
how large a portion of the tissues of every plant is solid carbon, and that too, 
derived chiefly from the atmosphere, it is evident that much more carbonic acid 
is, on the whole, consumed by vegetation than is evolved. In accordance with 
this are the results of the experiments of Dr. Daubeny, who has recently shown, 
. that ‘in fine weather, a plant, consisting chiefly of leaves and stems, if confined 
in a capacious vessel, and duly supplied with carbonic acid during sunshine, as 
fast as it removes it, will go on adding to the proportion of oxygen present, as long 
as it continues healthy.’ 
284. Thus are the two great kingdoms of nature rendered mutually subser- 
vient, each to the well-being, and even the existence, of the other. Animals 
require an atmosphere comparatively pure, although, by their respiration and 
decay, they are continually adding to the proportion of its deleterious gases. 
Plants, on the other hand, thrive by the decomposition of these gases and the res- 
toration of pure oxygen to the air in their stead. It is impossible not to admire 
this beautiful arrangement of Providence, by which, as in a thousand other cases, 
the means and ends are rendered reciprocal, affording the highest proof of wis- 
dom and design. 
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¢ Another view of respiration, different from the above, has been ably maintained; viz. that 
it is not a vital action, but only a necessary result of a temporary suspension of vital action. 
During the absence of the vivifying stimulus of the light, a part of the carbonic acid absorbed by 
day is Jost, from the want of power to retain it, and a small quantity of oxygen is absorbed to 
recombine with some of the carbon recently set free. But as this theory does not account for the 
loss of carbonic acid by day as well as by night, and moreover supposes imperfection in the origi- 
nal design of the Creator, I have not yet seen fit to adopt it. 
¥ 
