LIBERATION OF OXYGEN. 43 



It has been already mentioned likewise, that car- 

 bon and the elements of water form the principal 

 constituents of vegetables; the quantity of the sub- 

 stances which do not possess this composition being 

 in a very small proportion. Now, the relative quan- 

 tity of oxygen in the whole mass is less than in car- 

 bonic acid ; for the latter contains tw^o equivalents 

 of oxygen, whilst one only is required to unite with 

 hydrogen in the proportion to form water. The veg- 

 etable products which contain oxygen in larger pro- 

 portion than this, are, comparatively, few in number; 

 indeed in many the hydrogen is in great excess. It 

 is obvious, that when the hydrogen of water is as- 

 similated by a plant, the oxygen in combination with 

 it must be liberated, and will afford a quantity of 

 this element sufficient for the wants of the plants. 

 If this be the case, the oxygen contained in the car- 

 bonic acid is quite unnecessary in the process of 

 vegetable nutrition, and it will consequently escape 

 into the atmosphere in a gaseous form. It is there- 

 fore certain, that plants must possess the power of 

 decomposing carbonic acid, since they appropriate 

 its carbon for their own use. The formation of their 

 principal component substances must necessarily be 

 attended with the separation of the carbon of the 

 carbonic acid from the oxygen, which must be re- 

 turned to the atmosphere, whilst the carbon enters 

 into combination with water or its elements. The 

 atmosphere must thus receive a volume of oxygen 

 for every volume of carbonic acid which has been 

 decomposed. 



This remarkable property of plants has been de- 

 monstrated in the most certain manner, and it is in 

 the power of every person to convince himself of its 

 existence. The leaves and other green parts of a 

 plant absorb carbonic acid, and emit an equal volume 

 of oxygen. They possess this property quite inde- 

 pendently of the plant ; for if, after being separated 

 from the stem, they are placed in water containing 

 carbonic acid, and exposed in that condition to the 



