594 Scientific Proceedings, Royal Dublin Society. 
are not in a position to answer these queries. It cannot, however, 
be doubted that a grouping of the atoms is present which displays 
an unsymmetrical arrangement not to be brought to coincidence. 
Further it is equally certain that the arrangement of the atoms of 
laevo-tartaric acid is precisely the inverse of this unsymmetrical 
arrangement.” ? 
One very important support of the conclusion that the source 
of the property of rotation is found in some peculiar grouping of 
the parts of the molecule which affects its symmetry is found in 
the fundamental fact discovered by Pasteur that when the solution 
of a body is optically active it displays in the crystalline state 
non-coincident hemihedrism, as in the case of quartz, a relation 
between the sense of the hemihedrism and the sense of the 
rotating power being always present.” 
It is consistent with our data to suppose that for a group of 
balls or complexes of balls to permanently present an enantiomor- 
phous form, it must contain a sufficient variety of them to prevent 
the possibility of some slight modification in the arrangement 
taking place which would increase symmetry and make the arrange- 
ment identical with its own mirror-image; for where such a change 
is possible it will be likely to occur because it will almost certainly 
bring about closer packing. 
To compare with this suggestion we have the facts that every 
1 Recherches sur la dissymétrie moléculaire des produits organiques naturels. 
Lecons de chemie professées en 1860. Paris 1861, p. 25. 
Van’t Hoff originally made the assumption that the four atoms or atom-complexes, 
found in combination with a carbon atom, are situated at the corners of a regular 
tetrahedron whose centre is occupied by the carbon atom. The precise parallel in 
this investigation to such an assumption is found in the particular arrangement of this 
nature possible for a group in which the fouratoms are alike. Perhaps the facts which 
have been supposed to indicate the existence of the very specialized arrangement 
referred to may be found in most cases to agree equally well with a much more general 
hypothesis. Van’t Hoff himself suggests a less-specialized one as an alternative to 
his hypothesis just referred to, and probably a further widening of the conception will 
yet take place. See Stéréochimie, Nouvelle edition de “‘ Dix années dans l'histoire 
dune théorie,” par J. H. van ’t Hoff redigée par Dr. W. Meyerhoffer, Paris, 1892, 
pp. 11 and13. Compare post, p. 609. 
* This rule is not proved to apply in all cases, but the absence of proof of the 
existence of hemihedrism in some solitary instances is merely negative evidence, and 
the proof requisite in one of these instances, to transfer the body from a holohedral 
class of symmetry to a hemihedral one, may at any moment be discovered. 
