THE EARTH. 283 



more settled and durable causes to maintain 

 them." 



From the recital of this train of mischiefs pro- 

 duced by the air upon minerals, plants, animals, 

 and man himself, a gloomy mind may be apt to 

 dread this indulgent nurse of nature as a cruel 

 and an inexorable step-mother ; but it is far 

 otherwise : and although we are sometimes in- 

 jured, yet almost all the comforts and blessings 

 of life spring from its propitious influence. It 

 would be needless to observe, that it is absolutely 

 necessary for the support of our lives ; for of 

 this every moment's experience assures us. But 

 how it contributes to this support is not so rea- 

 dily comprehended. All allow it to be a friend, 

 to whose benefits we are constantly obliged ; and 

 yet, to this hour, philosophers are divided as to 

 the nature of the obligation. The dispute is, 

 whether the air is only useful by its weight to 

 force our juices into circulation ;* or whether, 

 by containing a peculiar spirit, it mixes with the 

 blood in our vessels, and acts like a spur to their 

 industry.! Perhaps it may exert both these use- 

 ful offices at the same time. Its weight may give 

 the blood its progressive motion through the larger 

 vessels of the body, and its admixture with it 

 cause those contractions of all the vessels, which 

 serve to force it still more strongly forward, 

 through the minutest channels of the circulation. 

 Be this as it may, it is well known that that part 

 of our blood which has thus received the influx of 



* Keil, Robinson. 



f Whytt upon Vital and Involuntary Motions. 



