PHYSICS, BIOLOGY, AND PSYCHOLOGY. 139 



which were formerly given of them. Fifty years ago 

 many physiological processes which, from a physical 

 and chemical standpoint, are now seen to be extremely 

 complex and obscure, were regarded as quite simple. 

 I need only refer to such activities as the oxidative pro- 

 cesses in living tissues, the processes of secretion and 

 absorption, or reflex action. There is a prevalent idea 

 that the progress of chemistry, and particularly of 

 . physical chemistry, has furnished explanations of these 

 processes. This is most certainly not the case. What 

 physical chemistry has helped us to do is to obtain 

 measures of the processes in the living body ; but the 

 results of the measurements have been to show with 

 ever-increasing clearness that the processes in the living 

 body do not correspond with our conceptions of those 

 in non-living structures, and that we are not remotely 

 in sight of mechanical explanations of the former. 



As an example I need only take the case of the ex- 

 quisitely thin and delicate living membrane which 

 separates the blood in the lung capillaries from the air 

 in the alveoli or air-cells of the lungs. A short time ago 

 it was assumed that this membrane plays only the 

 passive part which we regard a non-living membrane as 

 playing, and allows oxygen to diffuse through just as 

 a non-living membrane would. On applying accurate 

 methods of measurement, we found that whenever there 

 is need for an extra supply of oxygen, as, for instance, 

 during muscular exertion, the membrane assumes an 

 active role, and pushes oxygen inwards, without regard 

 to the mechanical laws of diffusion. In this respect the 

 alveolar epithelium acts just like epithelium of the swim- 

 bladder, or that of the kidney or any other gland, or the 



