THE CHEMISTRY OF LIGHT PRODUCTION 91 



of seeds and showed that repeatedly respired air was 

 unfit for further breathing. About the same time R. Hooke 

 discovered the true meaning of respiratory movements 

 and by forcing a blast of air continuously through the 

 lungs with bellows, was able to keep animals alive. He 

 concludes **that as the bare Motion of the Lungs, without 

 fresh air, contributes nothing to the life of the Animal, he 

 being found to survive as well as when they were not 

 moved as when they were; so it was not the Subsiding 

 or Movelessness of the Lungs that was the immediate 

 cause of death, or the stopping of the circulation of the 

 Blood through the Lungs, but the Want of a sufficient 

 Supply of fresh Air." The cause of death on collapse 

 of the lungs could not be better stated to-day. Thus com- 

 bustion, respiration and luminescence of flesh or wood 

 were early recognized as related phenomena. 



Although the "gas sylvestre'^ (CO2) of burning char- 

 coal and fermentation of wine was known to van Helmont 

 (1577-1644) and Mayow (1646-1679) in 1674 showed that 

 ^^spiritus nitroaerens ' ' (oxygen) was responsible for the 

 life of animals and for combustion, a century elapsed 

 before the true significance of these gases became known. 

 In the meantime the phlogiston theory of combustion had 

 been developed. Black (1728-1799) in 1755 had rediscov- 

 ered carbon dioxide ("fixed air'^) in the expired air and 

 Priestley (1733-1804) and Scheele (1742-1786) had both 

 rediscovered oxygen ("dephlogisticated air") in 1774. 

 About the same time Lavoisier overthrew the phlogiston 

 doctrine and showed that in the combustion of organic sub- 

 stances water and COo are formed. 



Later it was realized that this slow combustion did 

 not take place in the lungs, or in the blood, but in the 



