308 CARBONIC ACID. [BOOK n. 



CHAPTER XV. 



OF RESPIRATION. 



THE general facts belonging to vegetable respiration have 

 been mentioned in the Chapter on Digestion. There are, 

 however, certain phenomena connected with this process 

 which may be most conveniently treated of separately. 



Carbonic Acid. It has been already stated that the expe- 

 riments of Burnett led him to conclude that a small quantity 

 of carbonic acid is continually evolved by plants. This seems 

 to be proved by the carbonate of lime which gradually forms 

 upon the surface of fresh leaves when plunged in lime-water ; 

 and Liebig regards it as a phenomenon necessarily attendant 

 upon the function of perspiration; for he says that in the 

 escape of water from the surface of plants, carbonic acid dis- 

 solved in it cannot do otherwise than accompany it. It is, 

 however, to be observed that the very careful and extensive 

 series of experiments carried on by Mr. Haseldine Pepys, 

 and recorded in the Philosophical Transactions for 1843, do 

 not confirm this opinion. This gentleman selected for expe- 

 riment specimens of plants which had been previously habi- 

 tuated to respire constantly under an inclosure of glass; 

 and employed the apparatus which he had formerly used in 

 experimenting on the combustion of the diamond, consisting 

 of two mercurial gasometers, with the addition of two hemi- 

 spheres of glass closely joined together, so as to form an 

 air-tight globular receptacle for the plant subjected to experi- 

 ment. The general conclusions he deduced from his nume- 

 rous experiments, conducted during several years, were, first, 

 that in leaves which are in a state of vigorous health, 

 vegetation is always operating to restore the surrounding 

 atmospheric air to its natural condition, by the absorption 

 of carbonic acid and the disengagement of oxygen : that 



