INFLUENCE OF VEGETATION ON THE ATMOSPHERE. 203 



monia, and of a large proportion of carbonic acid, with the restora- 

 tion of oxygen. The latter is a constant effect of vegetation and the 

 measure of its amount. As respects the tissue of the plant, which 

 makes up almost the whole bulk of a tree or other vegetable fabric, 

 the sole consequences of its formation upon the air are the with- 

 drawal of a small quantity of water, and of a large amount of car- 

 bonic acid gas, and the restoration of the oxygen of the latter. In 

 the formation of the azotized products, a portion of ammonia or of 

 some equivalent compound of nitrogen is also withdrawn. It is 

 true, indeed, that leaves decompose carbonic acid only in daylight; 

 and that they sometimes impart a quantity of carbonic acid to the 

 air in the night, especially when vegetation languishes, or even take 

 from it a little oxygen. But this does not affect the general result, 

 nor require any qualification of the general statement. The work 

 simply ceases when light is withdrawn. The plant is then merely 

 in a passive state. Yet, whenever exhalation from the leaves slowly 

 continues in darkness, the carbonic acid which the water holds ne- 

 cessarily flies off with it, during the interruption to vegetation, into 

 the atmosphere from which the plant took it. So much of the crude 

 sap, or raw material, merely runs to waste. Furthermore, it must 

 be remembered, that the decomposition of carbonic acid in vegeta- 

 tion is in direct opposition to ordinary chemical affinity; or, in other 

 words, that all organized matter is in a state corresponding to that 

 of unstable equilibrium. Consequently, when light is withdrawn, 

 ordinary chemical forces may perhaps to some extent resume their 

 sway, the oxygen of the air combine with some of the newly de- 

 posited carbon to reproduce a little carbonic acid, and thus demol- 

 ish a portion of the rising vegetable structure which the setting sun 

 left, as it were, in an unfinished or unstable state. This is what 

 actually takes place in a dead plant at all times, and" whenever an 

 herb is kept in prolonged darkness ; chemical forces, exerting their 

 power uncontrolled, demolish the whole vegetable fabric, beginning 

 with the chlorophyll (as we observe in blanching Celery), and at 

 length resolve it into the carbonic acid and water from which it 

 was formed. But this must all be placed to the account of decom- 

 posing, not of growing vegetation ; and even if it were a universal 

 phenomenon, which is by no means the case,* would not affect the 



* In repeating the old experiments upon this subject with due precautions, 

 and with improved means of research, it is found that many ordinary plants, 



