ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 63 



a surface of ,o'oo of an acre, the entire raiu-fall (dews, etc., 

 included) for those years. Prof. Way, at that time chem- 

 ist to the Royal Ag. Soc. of England, analyzed the waters, 

 and found that the total amount of ammonia contained in 

 them was equal to 7 lbs. in 1855, and 9^ lbs. in 1856, for 

 an acre of surface. These amounts were yielded by 

 663,000 and 616,000 gallons of rain-water respectively. 



In the waters gathered at Insterburg during the twelve- 

 month ending March, 1865, Pincus and Rollig obtained 

 6.38 lbs. of ammonia per acre. 



Bretschneider found in the waters collected at Ida-Ma- 

 rienhiitte from April, 1865, to April, 1866, 12 lbs. of am- 

 monia per acre of surface. 



The significance of these quantities may be most appro- 

 priately discussed after we have noticed the nitric acid of 

 the atmosphere, a substance whose functions towards vege- 

 tation are closely related to those of ammonia. 



§7. 



When lightning strikes the earth or an object near 

 its surface, a person in the vicinity at once perceives a 

 peculiar, ; >called " sulphureous " odor, which must belono- 

 to something developed in the atmosphere by electricity. 

 The same smell may be noticed in a room in which an 

 electrical machine has been for some time in vigorous 

 action. 



The substance which is thus pioduced is termed ozone^ 

 from a Greek word signifying to smell. It is a colorless 

 gas, possessing most remarkable properties, and is of the 

 highest importance in agricultural science, although our 

 knowledge of it is still exceedingly imperfect. 



Ozone is not known in a pure state free from other 

 bodies ; but hitherto has only been obtained mixed with 



