EESPIEATION 293 



tion has already been called, viz. that carbon dioxide is 

 not the only respiratory exhalation. The watery vapour 

 which accompanies 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 consideration 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. 



Respiration 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, 

 together with the return of the carbon dioxide and water 

 therefrom ; the latter is strictly comparable to the changes 



