174 Biological Chemistry. 



The abbreviations d and I are used to represent dextro- 

 and laevo-rotatory forms. The " externally compensated " 

 racemic form is, according to this system of nomenclature, 

 designated cZ-Z-tartaric acid that is, a mixture of the d- 

 and Z-forms.* 



Before concluding this section, the properties of some 

 of the stereoisomerides of tartaric acid may be considered. 



The ordinary tartaric acid obtained from argol is, as 

 already mentioned, dextro-rotatory, and melts with de- 

 composition at 167. 



The meso-acid, melting point 140, is obtained to- 

 gether with the d-l- (racemic) acid, melting point 204, 

 by synthetical processes (see p. 119). Both these acids 

 differ from the natural (d-) and the -acid in their melt- 

 ing points, solubilities, and other physical properties, and 

 they also differ from one another. There are also charac- 

 teristic differences in their various salts. There is, how- 

 ever, very little difference between the d- and the Z-acids ; 

 the main distinction is that their solutions rotate polarized 

 light (to the same degree) in opposite directions. Both 

 acids, on heating with water to 165 and by other methods 



* The above description will be easier to understand if the student pre- 

 pares (or obtains from an apparatus manufacturer) tetrahedra with hollow 

 coloured pyramids which fit over the cornel's. Various other forms of models 

 are made, such as those with black balls to represent carbon atoms, into holes 

 in which rods can be inserted, which carry at the other end coloured balls to 

 represent the various elements of radicles to which the carbon atom is united. 

 No difficulty will be found in grasping, with the aid of such models, the funda- 

 mental conceptions of stereoisoraerism. 



