DIVISION OF THE KACEMIC COMPOUNDS. 231 



On the other hand, it is found, both theoretically and in prac- 

 tice, that bodies containing but a single asymmetric carbon atom 

 can appear in only one inactive form, namely, the divisible, bi- 

 molecular para- form. 



139. The Division of the Raeemic Compounds. 



The correctness of the assumption that the racemic modifica- 

 tions of stereoisomeric bodies are actually double compounds, 

 consisting of an equal number of dextro-rotatory and levo-rotatory 

 molecules, can be demonstrated in two ways. The first is by 

 synthesis. If, for example, equal quantities of equally strong 

 (nearly saturated) solutions of dextro- and levo-tartaric acid be 

 mixed together, combination of the two, attended with the evolu- 

 tion of heat, occurs, and the mixture solidifies in consequence of 

 the crystallisation of a racemic acid which can be proved to be 

 identical with natural racemic acid. Again, if equal quantities of 

 dextro-lactate and levo-lactate of zinc be dissolved in water and 

 mixed, the optically inactive zinc salt of "fermentation lactic 

 acid" possessing all the properties of this substance will 

 crystallise out. This synthetic construction of the inactive double 

 molecule from the two optically active components is, however, of 

 merely theoretical interest, but is without any practical value, no 

 new and hitherto unknown compounds being thereby obtained. 



As a rule, it is the inactive para- form with which we have to 

 deal, and this has to be split up into its constituent molecules of 

 the dextro- and levo- modifications. This division can be effected 

 in several ways. In some cases the inactive substance decomposes 

 spontaneously on crystallising out from solution, and the com- 

 ponents which differ in their crystalline habit can be separated 

 by hand (selecting the crystals according to their structure). The \ 

 earliest example of this was given by PASTEUR (XI.). If a solu- 

 tion of sodium-ammonium racemate (C 4 H 4 (NH 4 )Na0 6 + 4 aq.) be 

 allowed to evaporate slowly, hemfliedral crystals are obtained. 

 Pasteur found that the hemihedral surfaces did not occupy the 

 same position in all the crystals, but that the one class of crystals 

 formed, as it were, the reflected image of the other, one being 

 dextro-hemihedral but levo-rotatory, and the other levo-hemi- 

 hedral but dextro-rotatory. On separating the two kinds, and 

 preparing the acid from each, he obtained dextro-tartaric acid in 

 the one case and levo-tartaric acid in the other, but in no instance 

 was the inactive racemic acid produced. 



The most usual method of division consists in allowing the 

 inactive substance to unite with an optically active one. If 

 the substance to be split up is an acid, then an active alkaloid 

 (e.g. morphine, quinine, strychnine, &c.) is used ; if a base, it is 

 combined with dextro-tartaric acid. Experience teaches that the 

 resulting compounds have very different degrees of solubility, on 



