AMMONIA CONVEYED IN RAIN AND DEW. 



291 



ammonia or nitric acid is received by plants directly from 

 the air, simultaneously with carbonic acid. 



Li the elevated plateaus of Central America, where it 

 scarcely ever rains, the cultivated and wild plants receive 

 their nitrogenous food only from the dew or directly 

 from the aii' ; and we may assume, without risk of error, 

 that the plants which grow in the cultivated fields of 

 Europe have as much ammonia and nitric acid furnished 

 to them by the air and the dew, as is conveyed to them 

 in rain-water. A sandy plain, where no plants grow, 

 receives from the rain as much ammonia and nitric acid 

 as a cultivated field ; but the latter derives a greater 

 quantity through the plants, and more from the leaiy 

 plants, than fi'om those which are poor in leaves. Let 

 us assume that in the Saxon experiments the cereal 

 plants, potatoes, and clover, raised upon the unmanured 

 land, derived the whole of their nitrogen from the 

 ground, and that nitrogenous food had not been received 

 either from the air or from the dew ; then the profit and 

 loss of the field in nitrogenous nutriment (according to 

 the assumptions made p. 228, that -^ of the nitrogenous 

 constituents in clover and potatoes were carried off" in 

 the form of cattle), may be thus represented : — 



Thejield at Cunnersdorf. 



II 2 



