The dipole moments of organic substances represent the resultants of a number of distinct moments within the molecule form which they must, as was pointed out by Sir J. J. Thomson, be obtainable by vector summation. This view was extended and further examined by Eucken and Meyer in 1929. It has been generally accepted and confirmed for a large variety of substances. The figures for
para
-disubstituted benzene derivatives of the type X. C
6
H
4
. Y are of particular importance for two reasons. First, it was anticipated by Ingold on the ground of the electronic theory of aromatic reactivity, that some deviation from the additive law might occur in this class of compound when Y was a group such as —NR
2
as a result of electronic displacements in the aromatic nucleus. When the necessary data became available by the measurements of Højendahl, a gross discrepancy of this kind appeared probable for
p
-nitraniline and
p
-nitroanisole, and this was specifically emphasized at that time. Secondly, a number of workers have recently made use of dipole moment data for aromatic ethers and sulphides of this same general type in order to compute the angle between the valencies of the characteristic atom (oxygen and sulphur) of the group Y.
Studies on the Mechanism of Organic Reactions by means of the Electronic Theory. II