THE EARTH. 223 



dispute is, whetlier tlie air is only useful by its weight to foico 

 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 useful offices 

 at the same time. Its weight may give the blood its progi-es- 

 sive motion, through the larger vessels of the body ; and its ad- 

 mixtm-e with it, cause those contractions of all the vessels, which 

 serve to force it still more strongly forward, through the miiiut- 

 est channels of the circulation. Be this as it may, it is well 

 k:no\vn, that that part of our blood which has just received the 

 influx of the air in our bodies, is of a very different colour from 

 that which has almost performed its circuit. It has been found, 

 that the arterial blood, which has been immediately mixed with 

 the air in the lungs, and, if I may so express it, is just beginning 

 its joiumey through the body, is of a fine florid scarlet colour ; 

 while, on the contrary, the blood of the veins, that is returning 

 from ha\'ing performed its duty, is of a blackish crimson hue. 

 Whence this difference of colour should proceed, is not well 

 understood ; we only know the fact, that this florid colour is 

 communicated by the air ; and we are well convinced, that this 

 air has been admitted into the blood for very useful purposes. 



Beside this -vital principle in animals, the air also gives life 

 and body to flame. A candle quickly goes out in an exhausted 

 receiver ; for having soon consumed the quantity of air, it then 

 expires for want of a fresh supply. There has been a flame con- 

 trived that will burn under water ; but none has yet been found 

 that will contiime to burn «-ithout air. Gunpowder, which is the 

 most catching and powerful fire we know, v/ill not go off in an 

 exhausted receiver ; nay, if a train of gunpowder be laid, so as 

 that one part may be fired in the open air, yet the other part in 

 vacuo will remain untouched, and unconsumed. Wood also set 

 on fire, immediately goes out ; and its flame ceases upon remov- 

 ing the air ; for something is then wanting to press the body ot 

 the fire against that of the fuel, and to prevent the too speedy 

 diffusion of the flame. We frequently see cooks and others, 

 whose business it is to keep up strong fires, take proper precau- 

 tions to exclude the beams of the sun from shining upon them, 

 which effectually puts them out. This they are apt to ascribe 



1 Keil. Robinsoa 2 Wli) tt upon vital and involuntary motioD& 



