1*4 A HISTORY OF 



is M tietner 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 useful 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 contrac- 

 tions of all the vessels, which serve to force it still more strongly for- 

 ward, through the minutest channels of the circulation. Be this as it 

 may, it is well known, that that part of our blood which has just re- 

 ceived 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 journey 

 through the body, is of a fine florid scarlet colour ; while, on the con- 

 trary, the blood of the veins that is returning from having 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 use- 

 ful purposes. 



Besides 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 contrived that will burn un- 

 der water ; but none has yet been found, that will continue to burn 

 without air. Gunpowder, which is the most catching and powerful 

 fire we know, will 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 uncon- 

 sumed. Wood also set on fire, immediately goes out ; and its flame 

 ceases upon removing the air ; for something is then wanting to press 

 the body of 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 precautions 

 to exclude the beams of the sun from shining upon them, which effec- 

 tually puts them out. This they are apt to ascribe to a wrong cause ; 

 namely, the operation of the light ; but the real fact is, that the 

 warmth of the sun-beams lessens and dissipates the body of the air 

 that goes to feed the flame ; and the fire, of consequence, languishes 

 for want of a necessary supply. 



The air, while it thus kindles fire into flame, is, notwithstanding, 

 found to moderate the rays of light, to dissipate their violence, and to 

 spread a uniform lustre over every object. Were the beams of the 

 sun to darl directly upon us, without passing through this protecting 

 medium, they would either burn us up at once, or blind us with their 

 effulgence. But by going through the air, they are reflected, refract- 

 ed, and turned from their direct course, a thousand different ways ; 

 and thus are more evenly diffused over the face of nature. 



Among the other necessary benefits the air is of to us, one of the 



* Kcil Robinson. " Whytt upon vital and involuntary Motion*. 





