134 SEVENTEENTH CENTURY. PT. in. 



other was still breathing. This proved that the air which 

 had lost \\sfire-air particles was lightest and 'rose to the top, 

 so that the top mouse could no longer breathe. By these 

 and a great many other experiments Mayow proved that air 

 is made up of two portions one heavy, which supports flame 

 and life ; the other light, and which is useless for burning or 

 breathing, and this last was the largest portion. I want you to 

 notice this particularly, because you will see by-and-by that 

 Mayow had really discovered and described two gases. The 

 one which he called fire-air was oxygen, which was not known 

 to other chemists for more than one hundred years later, and 

 the other and lighter one is now called nitrogen. 



Having now proved that an animal in breathing uses up 

 the same part of the air which a candle does in burning, 

 Mayow wanted next to know what this fire-air did inside the 

 animal. Harvey, as you remember, had proved that the 

 blood passes through the lungs and there meets the air which 

 we draw in at each breath. Here then, said Mayow, the 

 fire-air particles must come in contact with the blood, and, 

 joining with it in the same way as they do with the fat of 

 a candle, must cause the heat of the blood. If anyone wants 

 to prove this let him run fast. He will find that he is obliged 

 to breathe more quickly and draw more air into his lungs, 

 which will soon make his blood hotter and move more 

 quickly, till his whole body glows with warmth. But if this 

 mixture of the air with the blood does really take place, the 

 arteries into which blood has just flowed from the lungs and 

 heart ought to be full of air ; and this is easily proved to 

 be the case by putting warm arterial blood under an air- 

 pump, where, as soon as the pressure of the outside air is 

 taken off, innumerable bubbles rise out of the blood as fast 

 as they can come. 



