296 DECOMPOSITION OF CARBONIC ACID. [BOOK n. 



carbonic acid and extrication of oxygen, takes place exclu- 

 sively in daylight. " Hence," he says, " are we not justified 

 in concluding that the production of oxygen, and its converse, 

 the formation of carbonic acid, are the unvarying results of 

 two different functions ; viz. this of respiration, that of diges- 

 tion; and that both are vegetative actions dependent upon 

 vitality ? To conclude : the formation of carbonic acid is 

 constant both by day and night, during the life of the vege- 

 table ; it is equally carried on whether in sickness or in 

 health ; it is essential to its existence for the sustentation of 

 its irritability; for, if deprived of oxygen, and confined in 

 carbonic acid gas, plants, like animals, quickly die. This 

 function, which is performed chiefly by the leaves and petals, 

 though also in a less degree by the stems and roots, like the 

 respiration of animals, is attended with, and marked by, the 

 conversion of oxygen into carbonic acid ; it is the respiration 

 of plants. 



" Again : vegetables, at certain times and under certain 

 circumstances, decompose carbonic acid, and renovate the 

 atmosphere by the restoration of its oxygen ; but this occa- 

 sional restoration is dependent, not upon the respiratory, but 

 the digestive system : it in part arises from the decomposi- 

 tion of water, but chiefly from the decomposition of carbonic 

 acid, absorbed either in the form of gas or in combination 

 with water, either by the roots or leaves, or both ; and here 

 again the analogy holds good between the functions of respi- 

 ration and digestion in animals and plants, for to both is 

 carbonic acid deleterious when breathed, and to both is it 

 invigorating to the digestive organs." (Journal of Royal Insti- 

 tution, new series, vol. i. p. 99.) This is not, however, con- 

 formable to the experiments of Mr. Haseldine Pepys, to be 

 mentioned in a future page. 



As the decomposition of carbonic acid gas is thus evidently 

 an important part of the act of digestion, it might be supposed 

 that to supply a plant with a greater abundance of carbonic 

 acid than the atmosphere will usually yield would be attended 

 with beneficial consequences. To ascertain this point several 

 experiments have been instituted ; the most important of 

 which are those of Saussure, who found that, in the sun, an 



