170 Biological Chemistry. 



The carbon atom (which is not indicated in these diagrams) 

 occupies the centre of the tetrahedron. If these two 

 figures are regarded carefully, it will be noticed that they 

 are not super-imposable, but that the one is a mirror image 

 of the other. The reader will find it easy to convince 

 himself, if he actually handles the model of a tetrahedron, 

 and represents the atoms by coloured hollow pyramids 

 which slide over the solid angles at the corners, that 

 the formation of mirror images, as described above, is only 

 possible when the element or radicles A, B, C, D are all 

 different. If any two of them are the same, it is not 

 possible to obtain two mirror images when the tetrahedron 

 is employed. According to the Van't Hoff conception, 

 " optical isomerism " is possible when the compound con- 

 tains what is known as an " asymmetric carbon atom " 

 that is, a carbon atom united to four different atoms or 

 radicles. In a compound containing only one asymmetric 

 carbon atom, it is possible to obtain two isomerides, which 

 can be represented by means of the tetrahedron formula, 

 of which the one form is the mirror image of the other. 

 Substances containing an asymmetric grouping, such as 

 have been described above, possess the power (if they are 

 liquids, without dilution, or if they are solids, when dis- 

 solved in a suitable solvent) of causing plane polarized 

 light to be rotated when passing through them. Sub- 

 stances which differ from one another only in the arrange- 

 ment of the groups and atoms relatively to one another 

 in space, in the manner described above, are stated to 

 be " stereoisomeric," and such a description of " stereo- 

 isomerism " is only possible in substances containing asym- 

 metric groupings. The one mirror image will rotate the 

 plane of polarized light as much in one direction as the 

 other, or so-called " enantiomorph," will rotate it in the 

 other direction. 



