28 NOURISHMENT OF PLANTS. 



This is readily shown by subjecting them to heat, 

 which drives off in the form of vapor their oxygen 

 and hydrogen, and leaves a charred or carbonaceous 

 mass behind. Upon continuing the application of 

 heat with free access of air, not only the color, but 

 the solid form, is altered ; for then the carbon com- 

 bines with the oxygen of the atmosphere to form a 

 kind of air or gas, which has received the name of 

 carbonic acid gas, and is colorless like the ordinary 

 air. The same thing happens in the putrescence 

 and decay of animal and vegetable matter. 



4. Nitrogen or azote constitutes the bulk, that is 

 to say, four fifths of our ordinary atmosphere, and 

 when uncombined, as is evident from the fact just 

 mentioned, is of an aerial nature, and invisible. Ex- 

 cept in the air, it is but scantily diffused in nature. 

 In the mineral kingdom it is entirely wanting, and in 

 soils we meet with it only in such as contain decay- 

 ing or putrid vegetable and animal matter. In the 

 organic kingdom, we find it in far larger quantity in 

 the bodies of animals than in those of plants. Of 

 the vegetable organs, the seeds are particularly rich 

 in nitrogen. It unites with oxygen to form an acid, 

 which has received the name of nitric acid, and 

 forms with basic or alkaline bodies (for example, 

 potash, soda, lime, &c.) the so-called nitrates or 

 nitric acid salts, which are more especially produced 

 in decaying vegetable or animal substances. When 

 united with hydrogen it generates a kind of air or 

 gas, which is called ammoniacal gas, and possesses 



