COLOR AND CHEMICAL CONSTITUTION 137 



colorless free nitrophenols present only the regular form of vibration 

 known to the benzo'id structure and hence can give no oscillation of a 

 frequency low enough to produce color. In the salts of these phenols, 

 however, the quinoid type is developed and, though always in equilib- 

 rium with a certain amount of the benzo'id type, isorropesis will be 

 present to an extent dependent upon the degree of unsaturation of the 

 atoms, and indicated by the appearance of color. In the case of meta- 

 compounds a measure of the persistence of their absorption bands indi- 

 cates a smaller amount of isorropesis and consequently they will be less 

 colored than the ortho- and para-derivatives. In all of these investiga- 

 tions care must be used in the selection of a proper solvent. Since 

 water is known to possess a large amount of residual affinity its action 

 upon the ethers of nitro-phenols will be quite apparent. Alcohol serves 

 the purpose here because it is well known to exert little or no ionizing 

 action upon ethers and esters. In general, the new free period of 

 oscillation — isorropesis — may be represented by the equilibrium : 



Ri— C — C— R, 1^—0 = 0—11, 



I t ** i-k 



These are conditions which accord entirely with certain known 

 chemical facts. 



In compounds of the benzol structure the cause of color begins with 

 the particular vibrations of the molecule itself. These oscillations, 

 however, as has been seen, are synchronous with light waves of a very 

 high frequency and give rise to absorption bands in the ultra-violet 

 region only. When some other influences can be brought to bear upon 

 these movements, as, for example, the introduction of a potential keto- 

 enol tautomerism, isorropesis is established and the oscillations, which 

 are now of a less frequency, may be low enough to show the beginning 

 of color. When the retardation of these oscillation frequencies is con- 

 tinued, as, for example, by the introduction of heavier atomic com- 

 plexes for the simpler and lighter hydrogen atoms, the absorption 

 curves due to oscillation will gradually be made to travel toward the 

 red end of the spectrum and the color, naturally, will travel into the 

 blue. A very well-known example of this is the increase in depth of the 

 blue color possessed by certain dyes which accompanies an increase 

 in the number of methyl groups introduced into the molecule. The 

 introduction of a chromophore group, one of a more or less unsaturated 

 nature, may in this light be considered as among the best to push back 

 the oscillation frequency. But with reference to the powerful effect 

 these chromophore groups have upon the retardation of the pulsations 

 of the molecule and the consequent establishment of a new type of 

 linking always in equilibrium with that of the original nucleus, the 

 interpretation of their influence seems best explained in the production 

 of an entirely new, free period of vibration — isorropesis — within the 



