AMMONIA CONVEYED IN KAIN AND DEW. 275 



For an English acre this makes 21-9 Zollv. Ibs. (= 24: 

 Ibs. Eng.), and for a Saxon acre 30 Zollv. Ibs. These 

 numbers nearly coincide with the observations of Bous- 

 singault and Knop. 



The yearly average quantity of rain falling in vari- 

 ous districts, according to the position and elevation of 

 the localities, is very unequal ; and investigations have 

 shown that the amount of ammonia and nitric acid con- 

 tained in rain-water bears an inverse proportion to the 

 quantity of rain. In districts where the rain falls more 

 seldom or less in quantity, the water is richer in these 

 constituents than in more rainy districts. According 

 to Boussingault, dew is richest in ammonia ; according 

 to Knop, not richer than rain-water. (See his valuable 

 memoir in the 8 hefte der * Landw. Versuchstat. in 

 Sachsen.') But plants receive ammonia and nitric acid 

 not merely by means of rain-water derived from the 

 ground and in dew, but also directly from the atmos- 

 phere. The experiments of Boussingault (' Annal. de 

 Chem. et de Phys.,' 3 ser. t. liii.) leave no doubt what- 

 ever with regard to the constant presence of ammonia 

 in the air. In a kilogramme of the following sub- 

 stances heated to redness, he found these quantities of 

 ammonia, after three days' exposure to the air upon 

 porcelain plates : 



In 1 kilo, quartz-sand 0'60 milligr. ammonia 



" 1 " bone-ash 0'4Y " 



" 1 " charcoal 2'9 " 



Although we can estimate with tolerable certainty 

 the quantity of ammonia and nitric acid which a field 

 annually receives in rain-water, yet the determination 

 of the same in the dew which moistens plants is not 

 practicable. Just as little can we discover how much 

 ammonia or nitric acid is received by plants directly 

 from the air, simultaneously with carbonic acid. 



In 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 air ; and we may assume, without 



