appear to be of the same order as light waves and, consequently, by absorption of their wave-lengths white light should show the absorption spectra we have noted. The sodium and aluminium derivatives of ethyl aceto-acetate may be described as equilibrium mixtures of the enolic and ketonic forms. The fact that the sodium salt is so easily hydrolyzed in an aqueous solution need not enter into the discussion of the absorption spectra. The evidence in all these cases goes to show that the metallic ion still exerts its influence and does not lead an altogether separate existence from that of the negative ion. Accordingly we may regard the Faraday tubes of force as stretched, but not necessarily broken, by the action of the solvent. On this basis, an ionizing solvent is to be considered as one that will bring about this lengthening of the Faraday tubes. Among the best examples, we may cite water, liquid sulphur dioxide, and liquid ammonia, or those substances which possess in reality a certain amount of "residual affinity"—affinities that may yet be exerted. Tautomeric changes in solution receive their interpretation, then, in the lengthening of the Faraday tubes of force to that point where the labile atom comes so far under the influence of a neighboring atom that a break occurs, which in turn gives rise to the oscillatory disturbances already discussed. With tautomeric compounds in which the labile atom has been replaced by an alkyl group there is absence of tautomerism due to the non-formation of alkyl ions, in which case it is seen that water and alcohol have not sufficient power to lengthen the Faraday tubes of force. The persistence of an absorption band may be defined now as the measure of the atoms in this transitorial state or the measure of the extent to which the labile atoms are separated from the molecule proper. Wherever the tautomeric compounds display the phenomenon of fluorescence a second free period of vibration is present. The latter must depend upon the former since a compound does not fluoresce except when exposed to light rays of the frequency of the first free period. Recently it has been demonstrated that a fluorescent substance gives two bands in its absorption spectrum, one for each of these periods of vibration. The band from the incident light is well marked, but the band from the fluorescent light is so faint that it can be made fairly visible only when the light of the first free period is strengthened; a fact that substantiates the dependence of the second free period upon the first.
As the origin of absorption bands in the ultra-violet spectrum have received an explanation in the change of linking brought about by the shifting of a labile atom, so clearly represented in the examples of keto-enol tautomerism, we may rightly expect to find absorption bands in the spectra of other compounds in which some change of linking is exhibited. No more beautiful example can be found than that of the compound known as benzol, where six carbon atoms, unchangeable in their order, are bound together in a single ring. To each carbon is