THE ATMOSPHERE. 171 



415. PRINCIPAL OFFICE OF NITROGEN.- 



What is the 



principal of- The principal office of the nitrogen of the 

 ficeofnitro- air is to dilute its oxygen. The latter, if 

 pure, would soon consume our bodies, as 

 it hastens the combustion of a taper, or other combus- 

 tible. 



416. THE ATMOSPHERE. The air we 



What is the 



composition breathe, and which, to the depth of fifty 

 of the air? miles or more; f orms t h e crystal shell, or 

 envelope of the globe we inhabit, is a mixture of nitro- 

 gen and oxygen gases, with aqueous vapor. It also 

 contains small and varying proportions of carbonic acid, 

 and ammonia. 



417. PROOF THAT AIR is A MIXTURE. 



How is it 



proved to be a That it is a mixture, and not a chemical 

 compound, is sufficiently evident from the 

 fact that it possesses no new and peculiar properties 

 different from those of its constituents. It is further 

 proved to be a mixture, from the fact that heat, which 

 is the usual attendant on chemical combination, is never 

 occasioned when air is artificially produced by the ad- 

 mixture of its constituents. 



yse 418. USE OF CARBONIC ACID AND AMMONIA 



served by IN THE AIR Carbonic acid and ammonia. 



its carbonic 



acid and aw although present in the air in extremely 

 small quantity, subserve the most impor- 

 tant purposes in administering to the growth of plants. 

 They constitute the gaseous food of all forms of vege- 

 table life, as will be more fully explained in succeeding 

 chapters of this work. 



