ticularly described xuider the head of digestion, consists !54,^/^e 

 decomposition of carbonic acid by the green tissues of the leaves, 

 under the stimulus of the light^^he fixation of the solid carbon, 

 and the evolution ef 2^^"'^ oxygenj\ 



282. Carboiyls one of the principal in^'edients in the vegetable stractnre. Tlia 

 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- 

 mals, from the earth, &c. 



a. ' Now if we place some fresh- leaves in an inverted bell-glass, containing air 

 charged 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/tliat a large proportion of the carbonic 

 acid \vill have disappeared, and will be replaced by pnre oxygen.' Bnt 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 whicli carbon is given off. The 

 formei^ccurs only in the liglit 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 wlien we consider 

 how large a portion of the tissues of every plant is solid cai-bon, 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 gi-eat kingdoms of nature rendered mntualiy subser- 

 vient, each to the well-being, and even the existence, of the other, f 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 stelid._/ It is impossible not to admire 

 this beautiful arrangement of Providence, by which, as in a tliousand otlier cases, 

 the means and ends are rendered recipi-ocal, affording the highest proof of wis- 

 dom and design. 



X Another view of respiration, different from the above, has heea ably maintained; viz. that 

 iKSsnota, vital actio n,\>xi\, only a necessary result of a temporary suspension «f vital action. 

 During the absence of the vivifying stimulus of the light, a part of the carbonic acid absorbed by 

 day is lost, from the want of power to retain it, and a small quantity of oxygen i.s absorbed to 

 recombine with some of the carbon recently set free. But as tliis theory does not account for the 

 loss of carbonic acid by day as well as by night, and moreover supposes imptrjiction in the origi- 

 nal design of the Creator, I have not yet seen fit to adopt it. 



