OF THE COMPOSITION OF THE ATMOSPHERE. 27 



hydrogen into water. These may be regarded as 

 compounds of carbon with the elements of water, 

 and an excess of hydrogen. Such are the volatile 

 and fixed oils, wax, and the resins. Many of them 

 have acid characters. 



The juices of all vegetables contain organic acids, 

 generally combined with the inorganic bases, or me- 

 tallic oxides i for these metallic oxides exist in 

 every plant, and may be detected in its ashes after 

 incineration. 



Nitrogen is an element of vegetable albumen and 

 gluten ; it is a constituent of the acid, and of what 

 are termed the " indifferent substances " of plants, 

 as well as of those peculiar vegetable compounds 

 which possess all the properties of metallic oxides, 

 and are known as " organic bases." 



Estimated by its proportional weight, nitrogen 

 forms only a very small part of plants ; but it is 

 never entirely absent from any part of them. Even 

 when it does not absolutely enter into the composi- 

 tion of a particular part or organ, it is always to be 

 found in the fluids which pervade it. 



It follows from the facts thus far detailed, that 

 the development of a plant requires the presence, 

 first, of substances containing carbon and nitrogen, 

 and capable of yielding these elements to the grow- 

 ing organism ; secondly, of water and its elements ; 

 and lastly, of a soil to furnish the inorganic matters 

 which are likewise essential to vegetable life. 



OF THE COMPOSITION OF THE ATMOSPHERE. 



In the normal state of growth, plants can only 

 derive their nourishment from the atmosphere and 

 the soil. Hence it is of importance to be acquainted 

 with the composition of these, in order that we may 

 be enabled to judge from which of their constituents 

 the nourishment is afforded. 



The composition of the atmosphere has been exam- 



