64 GENERAL BIOLOGY 



Combustion and Respiration. Since oxygen is 

 so significant in organic life, it is important to 

 find out what role it plays in metabolism. It was 

 among the earlier discoveries of modern chemis- 

 try that when anything is burned, the combustion 

 involves a using-up of oxygen (oxidation) and 

 will not take place in the absence of oxygen. 

 When wood is burned, the carbon in it unites 

 with the oxygen to form CO 2 , and the hydrogen 

 to form H 2 O or water. It is easy to observe 

 that in plants as well as in- oxygen-breathing 

 animals not only is O taken in, but CO 2 is 

 expelled. This interchange is known as respira- 

 tion, and (it was an obvious step to compare 

 it with ordinary combustion, particularly as the 

 production of the bodily heat of the higher ani- 

 mals is unquestionably dependent upon oxida- 

 tions. From this standpoint, the foods which 

 the organism takes into itself were supposed to 

 be oxidized with an evolution of energy, in the 

 same way that the fuel burnt under an engine 

 boiler generates steam to drive the wheels of the 

 engine. 



In the burning of fuel the oxygen supplied by the 

 draft combines directly with the fuel, but it is not 

 difficult to show that the CO 2 -production of an 

 animal or plant bears no direct relation at all to the 

 intake of oxygen. A frog in an atmosphere of hy- 

 drogen will continue to evolve CO 2 without any 

 possible supply of gaseous oxygen. The CO 2 in 

 such a case must have been evolved as a by- 



