will be indicated by similar variation in the nature of the color. In many ring compounds where no such arrangement of linkings, as in the benzoïd and quinoïd classes, is seen, almost no evidence of color can be found. Furfuran, pyrol, camphor and many others of a constitution exhibiting double linkings show only general absorption in their spectra. But whenever the benzoïd type is present, no matter whether the ring be composed entirely of carbon atoms or not, the conditions for isorropesis are at once favored so soon as unsaturated atoms or groups can be introduced. These groups by their unsaturated condition give rise to new linkings and then in turn undergo the make-and-break characteristic of substances showing selective absorption.
Indeed we come to the conclusion that isorropesis is the cause of color in the aromatic series as well as in the aliphatic series. In both series the two modifications which must always be in statu nascendi have actually been shown to exist. The change of linking, therefore, that must accompany the transformation of one into the other is certainly to be considered as the source of the oscillations which give rise to vibrations in the ether of a free period corresponding to those in the visible region of the spectrum, and hence the development of color in the substance. The application of these ideas to the interpretation of color among inorganic compounds is yet to be made. There seems, however, no doubt but that, where residual affinity exists, there may arise some form of oscillation, caused by the make-and-break of these induced linkings as brought about by the molecular movements, which will record itself in definite vibratory motions of the ether and consequently, if these vibrations are of low enough frequency, will indicate color in the compound. Not until something of a more definite nature is known as regards the true spatial arrangement of the atoms in these compounds, can anything of positive value be postulated concerning the disturbances which certain atoms may bring to bear upon other atoms or groups of atoms in the molecule. Consequently the periods of oscillation that correspond to many of our well known colored salts have received no explanation in terms of those periods so definitely established among the carbon compounds; periods which through spectroscopic evidence have been made to reveal so much concerning the internal vibrations of the atoms in the molecule and of the disturbances within the atoms themselves.