THE EARTH. 



91 



the obvious picture : the other pursues her 

 with close investigation, tortures her by ex- 

 periment to give up her Secrets, and mea- 

 sures her latent qualities with laborious pre- 

 cision. Much more, therefore, might be said 

 of the mechanical effects of air, and of the 

 conjectures that have been made respecting 

 the form of its parts ; how some have suppo- 

 sed them to resemble little hoops coiled up 

 a spring; others, like fleeces of wool; 



in 



others, that the parts are endued with a re- 

 pulsive quality, by which, when squeezed 

 together, they endeavour to fly off, and re- 

 cede from each other. We might have given 

 the disputes relative to the height to which 

 this body of air extends above us, and con- 



cerning which there is no agreement. We 

 might have inquired how much 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 con- 

 tinued down to the centre of the earth. At that 

 place we might, with the help of figures, and 

 a bold imagination, have shown 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 specu- 

 lations do not belong to natural history ; and 

 they have hitherto produced no great advan- 

 tages 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 consi- 

 dered our atmosphere as one large chymical 

 vessel, in which an infinite number of various 

 operations are constantly 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 a while 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 

 receive and return their share ; and animals, 

 that by living upon, consume this generalstore, 

 are found to give it back in greater quanti- 

 ties when they die." The air, therefore, that 

 we breathe, and upon which we subsist, 

 bears very little resemblance to that pure 

 elementary body which was described in the 

 last chapter; and which is rather a sub- 

 stance that may be conceived, than experi- 

 enced to exist. Air, such as we find it, is 

 one of the most compounded bodies in all 

 nature. Water may be reduced to a fluid 

 every way resembling air, by heat ; which, 



a Boyle, vol. ii. p. 593. 

 wo. 9 & 10. 



by cold, becomes water again. Every thing 

 we see gives off its parts to the air, and has 

 a little floating atmosphere of its own round 

 it. The rose is encompassed with a sphere 

 of its own odorous particles; while the night- 

 shade infects the air with a scent of a more 

 ungrateful nature. The perfume of musk 

 flies off in such abundance, that the quantity 

 remaining becomes sensibly lighter by the 

 loss. A thousand substances that escape all 

 our senses we know to be there ; the power- 

 ful emanations of the load-stone, the effluvia 

 of electricity, the rays of light, and the insi- 

 nuations of fire. Such are the various substan- 

 ces through which we move, and which 

 we are constantly taking in at every pore, 

 and returning again with imperceptible dis- 

 charge ! 



This great solution, or mixture of all earth- 

 ly bodies, is continually operating upon it- 

 self; which, perhaps, may be the cause of 

 its unceasing motion: but it operates still 

 more visibly upon such grosser substances 

 as are exposed to its influence; for scarcely 

 any substance is found capable of resisting the 

 corrodingqualities of the air. The air, say the 

 chymists, is a chaos furnished with all kinds 

 Y 



