io6 



MOLECULAR STRUCTURE 



and R 4 are alike. Not only the occurrence and non- 

 occurrence of isomerism agree with this fundamental 

 assumption, but also the character of the isomerism. In 

 ordinary isomerism, as that of C 6 H 4 C1 2 (1.2) and (i . 3), there 

 is a difference in every respect, in melting point, boiling 

 point, solubility, density, and also in chemical behaviour. 

 The two isomeric lactic acids, on the other hand, agree in 

 almost all points, as in those just mentioned, in accordance 

 with the complete equality of their internal dimensions. 

 Differences occur only in respect to qualities that refer 

 to the difference between two unsymmetrical reflected 

 images, such as Figs. 16 and 17 are. 



The most noticeable difference is in optical activity in 

 the dissolved or the liquid condition ; this shows itself in 

 an opposite (and equal) rotation of polarized light, and 

 every substance containing a carbon atom united to four 

 different atoms or groups a so-called asymmetric carbon 

 atom occurs in these two oppositely active forms, the 

 optical antipodes. The simplest body yet known that 

 illustrates this is chlorobromofluoracetic acid l , 

 CClBrF(COOH), 



which has been obtained in two antipodes, and from which, 

 presumably, the isomeric chlorobromofluormethanes, 



CC1BFH, 



should be got by splitting off carbon dioxide. 



A second difference shown by the isomers in question is 

 in their crystalline form. This 

 expresses itself in the formation 

 of two so-called enantiomorphic 

 forms due to hemihedry such as 

 Figs. 1 8 and 19 show for laevo- 

 and dextro - ammonium bimalate. 

 Fig. 18. Fig. 19. The two forms, like the diagrams 

 Figs. 1 6 and 17, representing the 



atomic arrangement, constitute a pair of unsymmetrical 



1 Swartz, Bull, de I'Acad. de Bdg. (3) 31. 28. 



