molecule itself. Now the term isorropesis is used to define the oscilla- tion that takes place between the residual affinities of atoms in juxta- position. The idea, however, may have already presented itself that in the case of the benzol structure the presence of keto-enol tautomerism with its particular period of vibration together with the oscillations occurring in the benzol nucleus, might, by a mutual combination of these two periods, give a period of greater wave-length and thus coincide with light rays in the visible region of the spectrum. If this were true, then these two conditions just stated might be looked upon as potential color systems. The actual presence of the conditions for isorropesis in the aliphatic series argues most strongly for the same sort of oscillation in the aromatic series wherever circumstances are favorable for its existence. In fact it seems highly probable that its presence alone will account for all the color-formations in the aromatic series. Other vibratory centers may exist and in fact do exist in the various compounds, but their presence only influences the amount of isorropesis that can take place and does not altogether destroy this particular form of oscillation.
In the quinoïd type of compounds the actual existence of the two distinct modifications which underlie isorropesis has already been shown. The change of one of these forms into the other and vice versa necessitates a change in manner of linking throughout the molecule which accompanies the oscillation in question. The fact that no one arrangement of atoms, no matter what their method of linking, can be made to show an absorption band is sufficient in itself to argue for the make-and-break in the two forms of the quinone as the cause of the color that exists among members of this class. For many years the quinoid linking has been supposed to be the source of color in compounds of quinone formation. It was not until recently, how- ever, that Gomberg has been able to prove conclusively that the quinoid type of linking actually exists in colored compounds of this nature. Not alone the presence of the quinoïd type, but also the benzoïd type has been shown to be present. In fact he has been able to interpret the conditions which determine the equilibrium always existent between these two forms and thus has succeeded in establishing by purely chemical means the amount of quinoid formation and con- sequently of isorropesis possible among aromatic derivatives. The spectroscopic evidence, therefore, on the existence of just such a type of oscillation as may be present in equilibria of this nature is corrob- orated. Upon the amount of isorropesis shown — a factor always dependent upon the relative unsaturated condition of the atoms coming into juxtaposition — we arrive at the depth of color in any given equi- librium. The presence of other groups may augment or retard the influence of these unsaturated atoms undergoing isorropesis, and con- sequently the corresponding variations in the oscillation frequency
