| Bartow—A Mechanical Cause of Homogeneity of Crystals. 608 
‘ties of opposite sense, present in an optically inactive mixture 
composed of the two, constitutes one of the most difficult of prob- 
lems, the isomers presenting as they do, the completest identity of 
their chemical attributes. Racemic acid, which we can with 
certainty regard as a mixture of the two active tartaric acids, 
furnishes with sodium and ammonium a salt of the formula 
C,H,0,NaNH,. This salt can be obtained in crystals of a rhombic — 
form which display the hemihedral modification, and as a result 
allow the recognition of enantiormorphism. Now it has been 
found that the solution of the crystals of one sort brings about a 
right-handed rotation of the plane of polarization, while that of 
the other sort, enantiomorphous to them, brings about a left- 
handed one. The acids separated from the two sorts of crystals 
behave themselves in a similar manner, the one proving to be 
identical with dextro-tartaric acid, the other with laevo-tartaric 
acid. Thus the crystallization of the double salt referred to effects 
the separation of the optically inactive racemic acid into its com- 
ponents, the two active tartaric acids.’”? 
Such a separation by means of crystallization is, however, one 
of the rarest occurrences.? On the contrary, the separation of the 
optically-opposite components of an inactive mixture appears to be 
rendered more difficult by the circumstance that the molecules 
which are oppositely endowed as to their optical -activity, display 
a certain mutual readiness to combine. 
Racemic acids and other compounds whose enantiomorphous 
isomers display this readiness to combine are known as racemic 
forms. Hantzsch speaks of them as follows:—“ The inactive forms 
thus obtained have sometimes been regarded not as mere mixtures 
but as definite, although unstable compounds, of two mirror-image 
molecules. This kind of combination was first observed in the 
ease of racemic acid = acide racémique, and has therefore been 
designated racemation.” 
He further remarks :—“ The formation and continuance of a 
1 Van’t Hoff’s ‘‘ Lagerung der Atome im Raume,”’ 1877 ed.,p. 41. Comp. 1894 ed. 
of same work, p. 28. Comp. “‘ Grundriss &c.,’’ pp. 31 and 32; also ‘‘ Stéréochimie ”’ 
par van ’t Hoff and Meyerhoffer, p. 73. 
2 Die ‘‘Lagerung der Atome im Raume’’ 1894 ed., p. 24. Compare ‘* Grundriss 
der Stereochemie,”’ pp. 31 and 32. 
