164 SCIENCE AND THE HUMAN MIND 



had failed to impress the minds of chemists, and had 

 to be rediscovered when time had gradually under- 

 mined the theory and prepared the way for the natural 

 reinterpretation of the phenomena. In 1669, a century 

 before Priestley's final discovery of oxygen, its exist- 

 ence in air and its significance in the phenomena of 

 respiration and combustion had been demonstrated 

 by John Mayow (1640-1679), a physician who practised 

 in Bath and London. Again oxygen was prepared 

 from heated saltpetre by Borch in 1678, and once more 

 in 1729 by Hales, who actually collected it over water. 

 The isolation of hydrogen may even be traced back to 

 Paracelsus, who described the action of iron filings 

 on vinegar. Yet all these observations were forgotten 

 and their meaning lost ; air was still believed to be 

 the only ponderable gaseous element. 



Thus once more we are impressed by the rarity of 

 finding that the accepted discoverer of a scientific 

 phenomenon, or the received originator of a scientific 

 theory, stands alone or even at the beginning of the 

 episode. A study of previous records nearly always 

 discloses others who trembled on the verge of the 

 discovery, or beheld premonitory visions of the 

 coming revelation. Leonardo da Vinci buried in his 

 note-books many of the ideas which created the 

 science of the succeeding centuries ideas which only 

 attained an accepted position in the structure of 

 knowledge when Galileo or Huygens had fitted them 

 into place. Behind Leonardo, again, stand other 

 shadowy figures of whom we catch here and there a 

 faint glimpse. It is only when the frame is ready, 

 when a place is waiting for the new conception, that 

 it can be certain of an immediate and enduring re- 



