STEREO-CHEMISTRY AND ITS APPLICA- 

 TION TO PHYSIOLOGY 



By a. W. STEWART, D.Sc. 

 Lecturer in Stereo-chemistry in University College, London. 



When two substances contain equal numbers of similar atoms 

 and yet show a difference in properties, they are said to be 

 isomeric. In the great majority of cases isomerism is due to 

 a structural difference between the compounds in question — 

 i.e. though they are made up of the same atoms, these atoms 

 are differently linked together in the several substances. For 

 example, ethyl alcohol and dimethyl ether are isomeric sub- 

 stances, both having the formula CoHcO ; in the alcohol the 

 oxygen atom links together a carbon and a hydrogen atom, 

 while in the other case it unites two carbon atoms : 

 H H H H 



II II 



H-C-C-0-H H-C-0-C-H 



II II 



H H H H 



Ethyl alcohol. Dimethyl ether. 



This view sufficed for a time to explain nearly every case of 

 isomerism known ; but as more and more facts were accumulated, 

 it became evident that some examples of isomerism could not be 

 classed under this head ; for compounds were discovered whose 

 structures were identical, while very considerable differences 

 were found in their properties. Organic chemists were thus 

 forced to seek some new explanation for these phenomena. 



It had been observed that several of these isomeric bodies 

 had a peculiar influence on polarised light. It is well known 

 that many crystalline compounds in the solid state will rotate 

 the plane of polarisation of a polarised ray passed through the 

 crystal ; but since quartz possesses this property only when 

 crystalline and not when in a state of amorphous silica, it is 

 evident that we are dealing in its case merely with the influence 

 of crystalline structure. But if we take a crystal of natural 

 tartaric acid and dissolve it in water, so that it loses its crystal- 

 line form, we find that in solution it still has an action on the 

 plane of polarisation. Similarly, camphor when fused does not 



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