COLOR AND CHEMICAL CONSTITUTION 127 



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 alto- 

 gether separate existence from that of the negative ion. Accordingly 

 we may regard the Faraday tubes of force as stretched, but not neces- 

 sarily 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 in which the labile atom has been replaced by an alkyl group there is 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 transi- 

 torial state or the measure of the extent to which the labile atoms are 

 separated from the molecule proper. Wherever the tautomeric com- 

 pounds 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. Eecently it has been demon- 

 strated 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 



