colorless free nitrophenols present only the regular form of vibration known to the benzoïd structure and hence can give no oscillation of a frequency low enough to produce color. In the salts of these phenols, however, the quinoïd type is developed and, though always in equilibrium with a certain amount of the benzoïd 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 indicates a smaller amount of isorropesis and consequently they will be less colored than the ortho- and para-derivatives. In all of these investigations 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:
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 ketoenol 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 continued, as, for example, by the introduction of heavier atomic complexes 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
