THE EARTH. 265 



of the air we breathe is elementary, and not 

 reducible to any other substance ; and of what 

 density it would become, if it were supposed to 

 be continued down to the centre of the earth. 

 At that place, we might, with the help of figures 

 and a bold imagination, have shewn it twenty 

 thousand times heavier than its bulk of gold. 

 We might also prove it millions of times purer 

 than upon earth, when raised to the surface of 

 the atmosphere. But these speculations do not 

 belong to natural history ; and they have hither- 

 to produced no great advantages in that branch 

 of science to which they more properly appertain, 



CHAPTER XIX. 



AN ESSAY TOWARDS A NATURAL HISTORY OF 

 THE AIR. 



A LATE eminent philosopher has considered our 

 atmosphere as one large chemical vessel, in which 

 an infinite number of various operations are con- 

 stantly performing. In it, all the bodies of the 

 earth are continually sending up a part of their 

 substance by evaporation, to mix in this great 

 alembic, and to float awhile in common. Here 

 minerals, from their lowest depths, ascend in 

 noxious, or in warm vapours, to make a part of 

 the general mass ; seas, rivers, and subterranean 

 springs, furnish their copious supplies ; plants re- 



