346 CHEMISTRY. 



The materials of growth, then, are produced by decay, 

 which is really not destruction, but a set of chemical 

 changes for the purpose of a recombination of the elements 

 in new forms of life and beauty. It is thus that life con- 

 tinually springs out of what we call death. 



479. Nitrogen from Nitric Acid. Considerable nitrogen 

 is furnished to plants from the nitric acid, which we have 

 stated is formed in the air and brought down in the rain. 

 As long ago as 1785, Cavendish, an English chemist, dis- 

 covered that by passing a succession of electric sparks 

 through a mixture of nitrogen and oxygen in presence of 

 aqueous vapor in a glass tube, a little nitric acid is formed. 

 This is a small representation of what takes place on a 

 large scale in the atmosphere, for traces of nitric acid have 

 been found in samples of rain collected during and after 

 thunder-storms. As one of the elements of nitric acid is 

 nitrogen, its decomposition furnishes this element to plants 

 to be used in their growth. 



480. Green Manuring. Land which has been impover- 

 ished is often rendered fertile by raising some crop upon 

 it, as buckwheat, barley, rye, etc., and plowing it in while 

 green. The manner in which this process, called green 

 manuring, enriches the soil will be clear to you by referring 

 to what we have said of the sources of the materials for 

 growth. In the first place, all the ammonia and nitric acid 

 which are washed down by the rain are used by the plants, 

 and as these are plowed in there is really a store of nitro- 

 gen laid up in the ground for the next crop. Then, again, 

 every leaf of the plants is gathering in, by its multitude 

 of open mouths, carbon from the air ; and this carbon is 

 plowed in, therefore, with the nitrogen. But, besides all 

 this, the roots as they are pushed down by the living pow- 

 er of the plant break up the mass, and then thoroughly 

 mix with it in their decay. We have, therefore, a loosen- 



