776 Wisconsin Academy of Sciences, Arts, and Letters. 
tntion. Other groups or elements when introduced in place 
of hydrogen may diminish the oscillation frequency, but the 
effect in such cases must be slight and the general character 
of the absortion would remain unaltered. It is conceivable 
that such groups introduced into the molecule of a substance 
colorless in the ordinary sense, might, if its absorption occurs' 
just outside the visible spectrum, render the substance colored 
by a slight shifting of the absorption band; but when a radical 
change in the color takes place on salt formation the salt is con¬ 
stituted differently from the parent substance. 
In 1900 Hewett 15 advanced the idea that symmetrical com¬ 
pounds capable of equal tautomeric displacements in either of 
two directions should be fluorescent, for the molecule would 
swing between the two extremes like a pendulum, the energy 
absorbed in one wave length being degraded and given out with 
slower frequency. 
CH 
O 
CH 
CH 
O 
CH 
COH 
CH 
In 1910 Porai-Koschitz 16 summed up the more recent views 
of the oscillation theory of the cause of color in organic com¬ 
pounds as follows: The change in color of a compound is due 
to the retarding of the oscillations or the setting up of a new 
16 Zeitschr. physikal. Chem., 34, p. 1. (See also Jr. Phys. Chem. 10, p. 375 : 
Jr. Chem. Soc., 87, p. 7.68.) 
18 Jr. Russ. Phys. Chem. Soc., 42, p. 1237. (Jr. Chem. Soc., A. II, p. 3.) 
