284 HISTORY 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 begin- 

 ning its journey through the body, is of a fine 

 florid scarlet colour ; while, on the contrary, 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 useful 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 under wa- 

 ter ; but none has yet been found that will conti- 

 nue 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 vacua will remain untouched and unconsumed. 

 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 



