OXYGEN AND HYDROGEN. 169 



" From any one of these sources an ample supply 

 of oxygen can readily be obtained ; and it may be 

 considered as a proof of the vast importance of this 

 element to the maintenance of animal and vegetable 

 life, that it is everywhere placed abundantly within 

 their reach. It is from the first of these sources, 

 however, — water, — that plants derive their prin- 

 cipal supply, 



2. Hydrogen. " This elementary substance is not 

 known to exist in nature uncombined. Therefore 

 it must, like carbon, enter plants in union with 

 some other element. 



" Water consists of hydrogen in combination with 

 oxygen. In the form of vapor, this compound per- 

 vades the atmosphere, and plays among the leaves 

 of plants ; in the liquid state it is diffused through 

 the soil, and is unceasingly absorbed by the roots 

 of all living vegetation. In the cells of plants — at 

 least during their growth — water is continually 

 undergoing decomposition ; and this is unquestion- 

 ably the principal source of the hydrogen which 

 enters into the constitution of their several parts. 

 In explaining the properties of water, we have 

 already referred to the apparent facility with which 

 its elements are capable either of separating from 

 or of reuniting with each other in the vascular 

 system of animals or plants. 



" In light, carburetted hydrogen, given off during 

 the decay of vegetable matter, and said to be always 

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