148 THE ENGINES OF THE HUMAN BODY 



life. Almost at the same time Malpighi, an Italian 

 anatomist, was examining the lung of a live frog ; he 

 then discovered capillaries — the channels which connect 

 arteries to veins. He saw, too, that respiratory chambers 

 are surrounded by capillary fields of blood. Then the 

 next important discovery, which happened some five 

 years later, was made by Dr Richard Lower, a native of 

 Cornwall. It was a simple observation made on live 

 animals, namely, that blood changed from a dark lake to 

 a bright crimson colour as it passed through the lungs. 

 The meaning of that change he could not know. Three 

 years later, in 1668 — you observe discoveries are following 

 fast on each other, — Dr John Mayow, also of Cornish 

 descent, when about to complete his studies at Oxford 

 University, made experiments somewhat like those Boyle 

 had carried out. He placed a mouse under one bell-jar 

 and a lighted candle under another, but did not exhaust 

 the air as Boyle had done. The mouse died, the candle 

 went out, and he observed that they were not choked 

 or extinguished by fumes, but died because they had 

 exhausted some essential part of the air ; of course we 

 know now what that exhausted element was. 



For many years no one saw that these discoveries threw 

 any light on the uses of the lungs ; hence men went on 

 believing that they were for cooling the heart. And so 

 we pass over nearly a whole century, until 1754, when 

 Joseph Black, a young graduate of Glasgow University, 

 had to prepare a thesis in order to obtain the degree of 

 Doctor of Medicine. He made four discoveries in quick 

 succession : (1) That when limestone was burned a gas 

 was given off which he called " fixed air," but which 

 modern chemists name carbon dioxide (C0 2 ) ; (2) he 

 found out that he could always recognise this gas by 

 passing it through a solution of quicklime, the solution 

 becoming white and chalk being precipitated in it ; (3) that 

 when air in which a candle had been burned was passed 

 through lime-water it gave the same characteristic re- 

 action ; therefore during combustion fixed air was pro- 

 duced ; (4) air which had been breathed, like air in which 



