BESPIKATION 299 



In making the estimation of the respiratory interchanges 

 we are apt to lose sight of a fact to which attention has 

 already been called, viz. that carbon dioxide is not the only 

 respiratory exhalation. The watery vapour which accom- 

 panies it must also be accounted for. On the hypothesis 

 of the direct oxidation of carbon and hydrogen, if the volume 

 of carbon dioxide is equivalent to that of the oxygen, there 

 cannot have been the absorption of sufficient of the latter to 

 unite with hydrogen to form the water. Even when the 

 respiratory quotient is less than unity, the same considera- 

 tion has a certain value. The idea of such direct oxidation 

 cannot, therefore, be accepted. 



It is evident from the foregoing considerations that the 

 vital activity of the protoplasts is somehow associated 

 with the two factors in the gaseous interchange. In the 

 absence of oxygen this vital activity gradually ceases, the 

 living substance being in fact slowly stifled or asphyxiated. 

 During its life one of the manifestations of its vitality is 

 the formation and exhalation of two fairly simple com- 

 pounds, carbon dioxide and water. To ascertain what is 

 the true relation of the two processes, it is necessary to 

 look closely at the nature of the chemical changes going 

 on in the protoplasm itself, or what is usually spoken of as 

 its metabolism. 



Eespiration in the strict sense is therefore a process 

 going on in the living substance itself. The gaseous inter- 

 change observed is the expression of the beginning and the 

 end of a series of complex changes in which the molecules 

 of the living substance are involved. The details of the 

 absorption of the oxygen of the plant from its environment, 

 and its presentation to the protoplasm, together with those 

 of the ultimate exhalation of the carbon dioxide and water 

 from the plant-body, should be regarded rather as belonging 

 to the mechanism of respiration than to respiration itself, 

 which is a function of the living substance only. The former 

 corresponds to the entry of the oxygen into the lungs of 

 an air-breathing mammal, and its transport to the tissues, 



