RESPIRATION 393 



pressures of a mass of gaseous nitrogen or of a mixture of nitrogen 

 with another gas. But if we raise the temperature sufficiently, and 

 hydrogen or oxygen is present, the nitrogen combines with it, form- 

 ing ammonia or oxides of nitrogen. The properties of nitrogen 

 have thus shown themselves to be more complex than the simple 

 kinetic theory of gases assumed. But from the atomic theory as ap- 

 plied in chemistry, and the theory of valencies, we can still predict 

 more or less successfully the composition of the compounds formed 

 by the nitrogen. Most of their special properties have to be ascer- 

 tained by experiment; but once ascertained they can be used for 

 the purpose of predicting how these compounds will behave un- 

 der quite new conditions. It is exactly the same when we come to 

 the complex proteins and other organic nitrogenous compounds 

 which can be separated from the bodies of organisms. So long as 

 they are separated from living organisms we can investigate 

 them just as we investigate other chemical compounds, and they 

 present no real obstacle to such investigation. 



The obstacle appears whenever the assumed chemical mole- 

 cules are participating in the life of an organism. Their proper- 

 ties seem then to become fluid and dependent from moment to 

 moment on the position of each molecule relatively to multitudes 

 of other molecules of the most diverse kinds. We consequently 

 cannot trace the individual molecules, and cannot tell whether or 

 how they are in combination with other molecules. They seem to 

 develop a quite indefinite potentiality of exhibiting unsuspected 

 properties. 



Now this fact shows us clearly that the simple atoms and mole- 

 cules of physics and chemistry are only a sensuous illusion : for, 

 behind the supposed simplicity, indefinite potentialities are hid- 

 den and actually show themselves in connection with the phenom- 

 ena of life. The properties and activities of what we call atoms 

 or molecules are in reality a function of their relations to 

 other atoms and molecules ; and this fact, which is not at once evi- 

 dent in what we call the inorganic world, becomes perfectly evi- 

 dent in biological phenomena. Organic individuality is something 

 very evident to our perception, and has thus the same claim to 

 reality as inorganic structure; but, from a purely physical and 

 chemical point of view, living structure and activity constitute not 

 merely a molecular flux like that of a river or a flame, but an 

 altogether undefinable flux undefinable because we cannot define 

 the molecular changes. It is biological and not physical or chemi- 

 cal structure and activity which biological investigation enables 



