356 HISTORY OF CHEMISTRY [LECT. xvi.
the accuracy of their results.1313 Pope and Peachey sub-
sequently prepared optically active compounds of sulphur 13:J
and of tin.134
Another subject that was much discussed was the signi-
ficance of racemism, about which a clear understanding only
became possible upon the introduction of the conception of
the transition temperature, and upon the recognition of the
analogy between racemic substances and double salts. The
most important method of splitting racemic substances—that
by means of optically active substances—remained a standing
enigma as long as the existence of partially racemic sub-
stances was denied.135 Every difficulty was removed, how-
ever, after Ladenburg had shown that such substances do
without doubt exist,136 and after a transition temperature
had been recognised in their case also.137
Furthermore, the much debated question as to how a
truly racemic substance (inactive by intra-molecular com-
pensation) can be distinguished from the mixture of the
active components, may now be looked upon as practically
settled.138
It is beyond doubt that the founding and development
of stereo-chemistry (a name which originated with Victor
Meyer139) is the most important thing that was accomplished
in organic chemistry during the last two decades of the
nineteenth century. Stereo-chemistry possesses a signi-
ficance for this period similar to that which the foundation
and introduction of the theory of aromatic compounds pos-
sessed for the twenty years preceding. There are besides,
ia'2 Le Bel, Comptes Rendus. 129, 548 ; Ladenburg, Berichte. 29, 2706 ;
34, 3416; compare further Wedekind, Ibid. 32, 511 and 722; Pope and
Peachey, Journ. Chem. Soc. 75, 1127; 79, 828. m Ibid. 77* 1072.
m Proc. Chem. Soc. 16, 42. m E. Fischer, Berichte. 27, 3226 ; Landolt,
Das Optische Drehungsverm8gen, Second Edition, Braunschweig, 1898, 85.
136 Ladenburg and Herz, Berichte* 31, 937; Ladenburg and Doctor, Ibid.
31,1969. 187 Ladenburg and Doctor, Ibid. 32, 50* m Roozeboom, Z.
physik. Chem. 28, 494; Berichte. 32, 537 ; Ladenburg, Journ. Chem. Soc.
• 75, 465; Berichte. 32, 864; Pope and Peachey, Joura Chem* Soc. 75,
11II. 139 Berichte. 23, $68,