80 BIOLOGY OP PLANTS. 



power uses for the purposes of the vegetable economy ; and 

 yet, so controlled by it as to change the order of its affinities, 

 and the character of the substances, of which it forms a part, 

 in the vegetable kingdom. 



Oxygen exerts an equally important agency upon soils and 

 manures, combining with the metals, and forming oxides and 

 acids, which, by their union, compose the soil ; and effecting 

 changes in the vegetable matter of the soil, especially con- 

 verting insoluble into soluble food. This agency will be fur- 

 ther illustrated in a future section. 



II. Influence of the Nitrogen of the Air. All plants, in some 

 of their organs, contain nitrogen in combination with other 

 substances, but do not probably derive it directly from the at- 

 mosphere. Although nitrogen seems necessary to the process 

 of vegetation, we do not know what agency that which is 

 contained in the air exerts, unless it acts, simply, as a dilu- 

 ent to the oxygen. A small quantity of nitrogen is absorbed 

 by the organs of plants, and given out again in an unaltered 

 state. 



III. Ammonia of the Atmosphere. That the atmosphere 

 contained ammonia, in small quantities, every chemist well 

 knew ; but it was first proved, beyond a doubt, by Liebig, 

 who has calculated its probable amount, both in the air, and 

 in rain water. A pound of the latter contains from one quarter 

 to one grain of this gas. Hence there would fall, on the sur- 

 face of an acre, more than eighty pounds of ammonia, annually. 

 The ammonia of the atmosphere, owing to its great solubil- 

 ity, is brought to the earth by every shower of rain, and h^ce 

 must enter the organs of plants. 



Ammonia is also found in the soil, in clays, in oxide of 

 iron, and in several other bodies, which must have derived it 

 from the atmosphere. Liebig and Dr. Wilbrand found it in 

 maple sap, the juice of the birch, and of beet root. How is the 

 atmosphere supplied with this substance ? This question is 

 easily answered by reference to changes in progress on the 

 surface of the earth. 



