324 TRANSACTIONS OF SECTION B. 
to the question of molecular configuration. In 1815 Biot observed that certain 
liquid organic substances deflect the plane of polarisation of a transmitted ray 
of light either to the right or to the left; half-a-century later Pasteur and 
Paternd pointed the obvious conclusion, namely, that the right- or left-handed 
deviation thus exerted must be due to a corresponding right- or left-handedness 
in the configuration of the chemical molecule. A scheme representing such right- 
or left-handedness, or enantiomorphism, was first enunciated by van ’t Hoff and 
Le Bel upon the basis of the previously established doctrine of chemical con- 
stitution ; briefly stated, the idea suggested was that the methane molecule, CH,, 
was not to be regarded as extended in a plane in the manner represented by the 
Frankland-Kekulé constitutional formula, but as built up symmetrically in three- 
dimensional space. The carbon atom of the methane molecule thus occupies the 
centre of a regular tetrahedron, of which the apices are replaced by the four 
hydrogen atoms. A methane derivative, in which one carbon is separately 
attached to four different univalent atoms or radicles of the type CXYZW, 
should thus exist in two enantiomorphous configurations, one exhibiting right- 
and the other left-handedness. The inventors of this daringly mechanistic inter- 
pretation of the far less concrete constitutional formule were able to interpret 
immediately a large number of known facts, previously incomprehensible, by 
means of their extension of the Frankland-Kekulé view of constitution. They 
showed that every substance then known, which in the liquid state exhibited 
so-called optical activity, could be regarded as a derivative of methane in which 
the methane carbon atom was attached to four different univalent atoms or 
groups of atoms; a methane carbon atom so associated is termed an asymmetric 
carbon atom. It is of interest to note that the van ’t Hoff-Le Bel deduction 
resulted from the discussion of the behaviour of organic substances of some 
molecular complexity; the optically active substances then known were 
mostly the products of animal or vegetable life, and among them none occurs 
which contains less than three carbon atoms in the molecule. Lactic acid, 
CH, . CH(OH).CO.OH, is practically the most simple optically active sub- 
stance of natural occurrence; it contains twelve atoms in the molecule, and it has 
only recently been found possible to associate optical activity with a much 
more simply constituted substance, namely, chloriodomethanesulphonic acid 
CHCII.SO,H, the molecule of which contains less than 5 per cent. of carbon 
and only nine atoms, four more than the minimum number, five, which 
theoretically can give rise to optical activity.* 
The working out of the practical consequences of the doctrine of the tetra- 
hedral configuration of the methane carbon atom by von Baeyer, Emil Fischer, 
and Wislicenus is now a matter of history; the acquisition of masses of experi- 
mental data, broad in principle and minute in detail, placed the van *t Hoff- 
Le Bel hypothesis beyond dispute. The rapid growth of Organic Chemistry as 
a classified subject contrasted strongly with that of Inorganic Chemistry, in 
which the collection of a great variety of detailed knowledge incapable of far- 
reaching logical correlation formed the most striking feature; in fact, the exten- 
sion of the conclusion, proven in the case of carbon compounds, that the 
Frankland-Kekulé constitutional formule must be translated into terms of three- 
dimensional space, to compounds of elements other than carbon, did not imme- 
diately follow the application of the theory to this element. Twenty years ago, 
indeed, the idea prevailed that carbon compounds differed radically from those 
of other elements, and we were not prepared to transfer theoretical conclusions 
from the organic to the inorganic side of our subject. In 1891, however, Le Bel 
stated that he had found optical activity associated with asymmetry of a 
quinquevalent nitrogen atom; although the experimental work upon which this 
conclusion was founded is now known to be incorrect,” the conception thus put 
forward was important, as suggesting that the notion of space-configuration could 
not be restricted logically to methane derivatives. When it was proved in 1899 
that benzylphenylallylmethylammonium iodide could exist in a right- and left- 
handed configuration, it became necessary to admit that the spacial arrangement 
of the parts of a chemical molecule, previously restricted to methane derivatives, 
must be extended to ammonium salts.* 
1 Pope and Read, Trans. Chem. Soc., 1914, 105, 811. 2 Tbid., 1912, 101, 519. 
3 Pope and Peachey, Trans. Chem. Soc., 1899, '75, 1127. 
