152 THE PATH OF SCIENCE 



give off "dephlogisticated air" in sunlight; in darkness, on 

 the contrary, they produce the gas that we call carbon dioxide. 

 These discoveries were not fully understood at the time. We 

 now know, of course, that green plants take up carbon dioxide 

 from the air through their leaves and under the influence of 

 sunlight build the carbon into the substance of their tissues. 

 In both light and dark they use oxygen and produce carbon 

 dioxide in respiring, just as animals do, but it is only in dark- 

 ness that the carbon dioxide is passed out into the air, for it 

 cannot then be used as a source of nourishment. It was not 

 until the beginning of the nineteenth century that the Swiss 

 investigator Nicolas de Saussure put the subject of plant 

 respiration and nutrition on a firm basis by means of quanti- 

 tative studies. 



Meanwhile something was being learned about the respira- 

 tion of animals. Up to the middle of the seventeenth century 

 no one had the slightest idea why one must breathe to live; 

 respiration was not in the least understood. In 1660 Robert 

 Boyle, the famous chemist, showed that mice and sparrows 

 die in partial vacua. Eight years later a more fundamental 

 discovery was announced by John Mayow, the lawyer and 

 Oxford don (though Boyle was probably partly responsible 

 for it). It was shown that it is not air as a whole, but some- 

 thing in air, that is necessary for life. Mayow called that 

 something igneo-aerial particles; it was, of course, oxygen. 

 Nearly a century then elapsed without further discoveries 

 being made on this momentous subject. At last Joseph 

 Black, professor of chemistry at Glasgow, showed that ''fixed 

 air" (carbon dioxide) is a product both of combustion and 

 of respiration. Not long afterward a young Scottish medical 

 man Daniel Rutherford showed that "fixed air" is not the 

 only irrespirable matter in air; but he missed the actual dis- 

 covery of nitrogen. It was in 1780 that the fundamental dis- 

 covery about respiration was made by the famous French 

 scientists Lavoisier and Laplace: "Respiration is therefore 

 a combustion, slow it is true, but otherwise perfectly similar 

 to the combustion of charcoal." They had realized that 



