THE ATMOSPHERE. 281 



of them deriving from it a part, and some of them the whole 

 of their carbon. Extensive forests, such as those of the 

 Landes in France, which grow upon sands absolutely destitute 

 of carbonaceous matter, derive their carbon entirely from this 

 source. But the oxygen of the carbonic acid is not retained 

 by the plant, for the lignin and other constituent principles 

 of vegetables, contain, it is well known, no more oxygen than 

 is sufficient to form water with their hydrogen, and which 

 indeed has entered the plant as water. The oxygen of the 

 carbonic acid must therefore be returned in some form to the 

 atmosphere. The discharge of pure oxygen gas from the 

 leaves of plants was first observed by Priestley, and the general 

 action of plants upon the atmosphere has subsequently been 

 minutely studied by Sir H. Davy and Dr. Daubeny. It 

 appears that plants have a double action upon the atmosphere ; 

 they withdraw carbonic acid from it, appropriating the car- 

 bonaceous part of that gas to their own wants and evolving 

 its oxygen ; and they also absorb oxygen from the atmosphere 

 and return carbonic acid in its place, an action corresponding 

 with the respiration of animals. Of these actions the latter 

 predominates during the night, and the former during the day, 

 but the result of both is that plants during twenty-four hours 

 yield considerably more oxygen than they consume. That 

 they fully compensate for the loss of oxygen occasioned by 

 the respiration of animals and other natural processes is not 

 improbable. But the mass of the atmosphere is so vast that 

 any change in its composition must be very slowly effected. It 

 has indeed been estimated that the proportion of oxygen con- 

 sumed by animated beings in a century does not exceed 

 1 -7200th of the whole quantity. 



Other gases and vaporous bodies are observed to enter 

 the atmosphere, but none of them can afterwards be detected 

 in it, with the exception perhaps of hydrogen in some form, 

 probably as the light carburetted hydrogen of marshes, of 

 which Boussingault believes that he has been able to detect 

 the presence of an appreciable but exceedingly minute trace.* 

 He observed concentrated sulphuric acid to be blackened when 

 exposed in a glass capsule to the air, protected from dust, 

 and at a distance from vegetation, which he ascribes to the 

 occasional presence in the air of some volatile carbonaceous 



* An. de Ch. et Fh t. 57, p. 148. 



