376 THE CHEMISTRY OF CREATION. 



stream is borne on its wings to the higher 

 regions ; there, out of all danger of contamina- 

 tion, it is wafted by means of the " upper 

 current," to the north ; and descending, to form 

 the ' ' lower current," on its circuit back to the 

 equator, carries with it health, life, and vigour, 

 over every region through which it sweeps, 

 until, robbed of some of its valuable properties, 

 and becoming laden with the impurities of colder 

 climes, it reaches the tropics, where its oxygen 

 is restored, and becomes again involved in the 

 great system of circulation. By this vast circu- 

 latory scheme, which we might almost venture 

 to designate the " respiration of the atmo- 

 sphere," the carbonic acid, produced in such 

 enormous volumes in northerly regions from 

 the innumerable sources already mentioned, is 

 conveyed by the returning current to the equa- 

 tor ; there, after feeding the luxuriant vege- 

 tation of the tropics, it becomes decomposed 

 into its constituent elements, its oxygen min- 

 gles with the ascending current, and, in pro- 

 cess of time, returns by the course just indi- 

 cated to enrich the air of temperate climates. 

 The interchange thus effected is perpetual ; it 

 rests not day nor night. Hence, while this 

 great current fills the canvas of the merchant's 

 vessel, and was long considered as the exclusive 

 agent of intercourse between the Kew and Old 



