50 SOURCE OF HYDROGEN AND OXYGEN. 



true in cold climates, where during winter fires are so 

 numerous and constant. 



3. In some districts large quantities of carbonic acid 

 pass off into the air from fissures in the earth's surface : 

 this is no doubt produced by volcanic action at a great 

 depth. 



4. Another source is natural decay and decomposi- 

 tion. It is a curious fact, that if you leave a piece of 

 wood to decay, the ultimate results will be the same 

 as if it had been burned in the commencement. The 

 action is slower, requiring often years to complete it; 

 but the products are the same, that is, carbonic acid 

 and water. Decay has, for this reason, been called a 

 slow combustion. 



We see therefore that the constant tendency in every 

 species of destruction, decomposition, or decay in 

 animal and vegetable bodies, is to the production and 

 liberation of carbonic acid. The sources already 

 indicated are quite sufficient to supply the quantities 

 annually withdrawn from the atmosphere by vegeta- 

 tion. 



The hydrogen required by plants is readily obtained. 

 Water consists of hydrogen and oxygen : in the form 

 of a liquid, it is drawn up by the roots; as a vapor, it 

 is absorbed by the leaves from the atmosphere. This 

 may be seen in the great effect of a trifling shower 

 during dry weather. Even if there is only enough 

 rain to barely moisten the surface of the parched earth, 

 the leaves before drooping are revived, and the whole 

 plant assumes a flourishing appearance : no water has 

 reached its roots, but it has absorbed a portion of the 

 shower through its leaves. This one source of supply 

 affords ample store of hydrogen. 



Oxygen is also to be obtained by the plant from 

 water. Carbonic acid too, it will be remembered, is 

 partly composed of this gas. There can thus be no 

 difficulty as to the plants obtaining oxygen, and no 

 fear of exhausting it from the atmosphere. 



