Biological Relations of Optically Isomeric Substances.

Every pure substance has a definite composition and structure. And while a composition as complex as C6H12O6admits of considerable variation in the manner in which the component atoms may be linked together, each variation in structure is associated with a variation in the identity of the substance for which it is characteristic. When two asymmetrical substances differ only as does the right hand from the left in the correspondence of their structures, they are called optical isomers. They were at first thought to be identical in every chemical and physical property excepting in the direction which they rotate the plane of a beam of incident, polarized light. But optical isomers behave as identical compounds only in their reactions with substances of symmetrical structure. Protoplasm, predominatingly asymmetrical in its make-up, reacts in a quantitatively different fashion with one of a pair of optical isomers than