LIVING SUBSTANCE 125 



expected, i.e., a contrast that rests upon a difference as regards 

 chemical elements. But a difference does exist in the kind of 

 combinations into which the elements enter. It was seen above 

 that chemical compounds are present in living substance, that 

 never occur in the inorganic world; such are proteids, carbo- 

 hydrates and fats. Of most importance is the fact that one group 

 of these chemical bodies, the proteids, belong to all organisms 

 without exception. Just as there is no single organism, whether 

 living or dead, in which proteids are wanting, so there are no 

 inorganic bodies in nature in which even an approximately similar 

 substance is present. The possession of the highly complex 

 proteid molecule is, therefore, a definite mark of distinction of 

 the organism in its relation to all inorganic bodies. 



But some have gone still further and have endeavoured to find 

 an absolute difference between the two bodies, not only in the 

 existence of certain compounds, but also in the order of the 

 chemical processes in the active organism. It is said that living 

 substance is characterised by its metabolism, in which definite 

 compounds are formed continually, are broken down, give off* 

 their decomposition-products to the outside, and are reformed 

 at the expense of the substances taken in from the outside as 

 food ; hence a continual streaming of matter through the living 

 substance takes place, being conditioned by the construction 

 and destruction of the compounds in question. Metabolism is, 

 indeed, a characteristic process of the living organism, and 

 it will be seen later that upon it the vital process rests; but 

 it is solely a process that distinguishes the living organism 

 from the dead organism and not from inorganic substance, for 

 it is not confined to organisms, but occurs also among inorganic 

 bodies. A simple example of this is found in the behaviour of 

 nitric acid in the production of concentrated sulphuric acid. If 

 nitric acid be mixed with sulphurous anhydride, which is obtained 

 in the manufacture of sulphuric acid by roasting sulphur ore, the 

 sulphurous acid withdraws oxygen from the nitric acid and passes 

 over into sulphuric acid, while the nitric acid becomes nitrous 

 acid. If the constant entrance of fresh air and water be 

 provided, the nitric acid is constantly reformed from the nitrous 

 acid and gives a part of its oxygen again to new quantities of 

 sulphurous acid, so that the molecule of nitric acid is continually 

 being alternately broken down with loss of oxygen and built up with 

 absorption of oxygen. In this manner with the same quantity of 

 nitric acid an unlimited quantity of sulphurous acid can be changed 

 into sulphuric acid. Thus here in a simple form, i.e., in a 

 simple chemical compound, is a regular metabolism, a succession 

 of destructions and constructions of a substance along with the 

 gain and loss of substances, which corresponds in principle, even 



