More than one hypothesis has been proposed to account for these cases of
isomerism, but no one has shown itself to be entirely satisfactory.
Quite recently Johannes Wislicenus, Professor of Chemistry in the
University of Liepsic, has made what has the appearance of being an
important contribution toward the solution of the problem referred to.
The author shows that many of the facts known in regard to the relations
between maleic and fumaric acids, and the other substances which
furnish examples of "abnormal isomerism," may be explained by the aid of
an extension of the Le Bel-Van't Hoff hypothesis. It is difficult
without the aid of models to give a clear idea concerning the hypothesis
of Wislicenus, but some idea of it may be gained from the following. If
we suppose a carbon atom to exert its affinities in the directions of
the solid angles of a tetrahedron, as is done in the Le Bel-Van't Hoff
hypothesis, then, when two carbon atoms unite, as in ethane, the union
will be between two solid angles of two tetrahedrons. If the two carbon
atoms unite by the ethylene kind of union, the union will be along a
line corresponding to one of the edges of each tetrahedron. In the
former case, in which single union exists, the two parts of the molecule
represented by the two tetrahedrons can be supposed to be capable of
revolving around an axis either in the same direction or in opposite
directions.
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