AIR. 

 CHAPTEE VI. 



THE ATMOSPHERE. 



Meteorology is a science that at one time in- 

 cluded astronomy, but now it is restricted to 

 the weather, seasons, and all phenomena that 

 are manifested in the atmosphere in its rela- 

 tion to heat, electricity, and moisture, as well 

 as the laws that govern the ever-varying con- 

 ditions of the circumambient air of our globe. 

 The air is made up chiefly of oxygen and 

 nitrogen, in the proportions of about twenty- 

 one parts of oxygen and seventy-nine parts 

 nitrogen by volume, and by weight about 

 twenty-three parts oxygen and seventy-seven 

 of nitrogen. These gases exist in the air as 

 free gases and not chemically combined. The 

 air is simply a mixture of these two gases. 



There is a difference between a mixture and 

 a compound. In a mixture there is no chem- 

 ical change in the molecules of the substances 

 mixed. In a compound there has been a re- 



