40 



NOURISHMENT OF PLANTS. 



gen, combines with oxygen ; and in this way nitric 

 acid salts (nitrates) are formed, from which plants 

 have the power of abstracting nitrogen. In this 

 manner nitrate of lime is often generated from the 

 plaster of the walls of stables and the action of nitro- 

 gen of urinous secretions, upon heaps of rubbish, 

 and in other places. 



We find nitrogen — for the most part, indeed, in 

 small quantities — in soils and in water. Here it is 

 met with at one time in the humus, at another in the 

 form of ammoniacal or nitric acid salts ; but it is in- 

 variably derived from animal and vegetable substan- 

 ces which have putrefied or decayed in the earth. 

 The more, therefore, such decaying matter is intro- 

 duced into the ground, the richer will it become in 

 nitrogen ; and the water absorbed by plants will 

 contain this element in a degree proportioned to 

 the length of time during which it has percolated 

 through such kinds of soil as are rich in nitrogen, or 

 to the quantity of these which have been conveyed 

 into it during the process of percolation. Dew, rain- 

 water (especially that first descending from the 

 clouds), and snow always contain ammonia, because 

 by their means the ammonia, which has become vol- 

 atile during the processes of putrefaction and decay, 

 is again condensed and brought down upon the 

 earth. In the fertilizing effects of transient showers, 

 the greater quantity of ammonia which they con- 

 tain undoubtedly contributes an essential share. 



Finally, it is more than probable that the organic 



