OF AGRICULTURE. 15 



air, a portion of the two gases unite together chem- 

 ically; so that every spark that passes, a small portion 

 of nitric acid is formed. This effect can be produced 

 by an electrical machine. A flash of lightning is 

 nothing more than a large electric spark ; and hence, 

 every flash that crosses the air, produces along its 

 path nitric acid. When thunder-storms are frequent, 

 much nitric acid and some ammonia is formed in this 

 way. They are washed down by rains, in which 

 they have been frequently detected, and thus reach 

 the soil, when the acid combines with potash, soda, 

 lime, etc., and form nitrates. 



The soil and the air, then, are the great fountains 

 of nourishment for the vegetable world. The soil 

 provides for the mineral matter carbonic acid, humus 

 or mold, water, ammonia and nitric acid. The air. 

 too, provides all these, except the mineral matter, 

 and humus or mold. 



CHAP TEE III. 



HOW PLANTS VEGETATE AND GROW. 



Plants and animals constitute the two great depart- 

 ments of organic nature. They consist of those 

 organs necessary to sustain life, to promote growth, 

 and to reproduce their own species. Plants, as well 

 as animals, are endowed with vitality ; but they differ 

 from animals in not possessing sensation. Skillful 

 cultivation always increases the productiveness of 



