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.  This axis corresponds to the straight line joining the two carbon atoms.  In the case in which double union exists no such revolution is possible.  Again, if, by addition to an unsaturated compound like ethylene, a saturated compound is formed, the kind of union between the carbon atoms is changed, and the possibility of revolution of the two parts of the compound is given.  Whether such revolution take place or not will be determined largely by the structure of the compound.  The tendency will be for those parts of the molecule which have the greatest specific affinity for one another to take those positions in which they are nearest to one another.  Thus, suppose that chlorine is added to ethylene.  By following the change on the model, it is seen that in the resulting figure the two chlorine atoms in ethylene chloride are situated at angles of the two tetrahedrons which are nearest each other.  But chlorine has a stronger affinity for hydrogen than it has for chlorine, and therefore each chlorine atom would tend to get as near a hydrogen atom as possible.  This involves a partial revolution of the two tetrahedrons in opposite directions around their common axis.  So also hydrogen would tend to take a position as near as possible to hydroxyl and to carboxyl, while hydroxyl would avoid hydroxyl, and carboxyl would avoid carboxyl.  These views are suggested as a result of a careful application of the original Le Bel-Van’t Hoff hypothesis, and are, of course, of little value unless they can be shown to be in accordance with the facts.