i 3 8 POPULAR SCIENCE MONTHLY 



molecule itself. Now the term isorropesis is used to define the oscilla- 

 tion that takes place between the residual affinities of atoms in juxta- 

 position. The idea, however, may have already presented itself that 

 in the case of the benzol structure the presence of keto-enol tautomerism 

 with its particular period of vibration together with the oscillations 

 occurring in the benzol nucleus, might, by a mutual combination of 

 these two periods, give a period of greater wave-length and thus 

 coincide with light rays in the visible region of the spectrum. If this 

 were true, then these two conditions just stated might be looked upon 

 as potential color systems. The actual presence of the conditions for 

 isorropesis in the aliphatic series argues most strongly for the same 

 sort of oscillation in the aromatic series wherever circumstances are 

 favorable for its existence. In fact it seems highly probable that its 

 presence alone will account for all the color-formations in the aromatic 

 series. Other vibratory centers may exist and in fact do exist in the 

 various compounds, but their presence only influences the amount of 

 isorropesis that can take place and does not altogether destroy this 

 particular form of oscillation. 



In the quinoid type of compounds the actual existence of the two 

 distinct modifications which underlie isorropesis has already been 

 shown. The change of one of these forms into the other and vice versa 

 necessitates a change in manner of linking throughout the molecule 

 which accompanies the oscillation in question. The fact that no one 

 arrangement of atoms, no matter what their method of linking, can 

 be made to show an absorption band is sufficient in itself to argue for 

 the make-and-break in the two forms of the quinone as the cause of 

 the color that exists among members of this class. For many years 

 the quinoid linking has been supposed to be the source of color in 

 compounds of quinone formation. It was not until recently, how- 

 ever, that Gomberg has been able to prove conclusively that the 

 quinoid type of linking actually exists in colored compounds of this 

 nature. Not alone the presence of the quinoid type, but also the 

 benzo'id type has been shown to be present. In fact he has been able 

 to interpret the conditions which determine the equilibrium always 

 existent between these two forms and thus has succeeded in establishing 

 by purely chemical means the amount of quinoid formation and con- 

 sequently of isorropesis possible among aromatic derivatives. The 

 spectroscopic evidence, therefore, on the existence of just such a type 

 of oscillation as may be present in equilibria of this nature is corrob- 

 orated. Upon the amount of isorropesis shown — a factor always 

 dependent upon the relative unsaturated condition of the atoms coming 

 into juxtaposition — we arrive at the depth of color in any given equi- 

 librium. The presence of other groups may augment or retard the 

 influence of these unsaturated atoms undergoing isorropesis, and con- 

 sequently the corresponding variations in the oscillation frequency 



