Wakeman—Pigments of Flowering Plants. 
769 
of color, might result in one case in the change of a colorless 
substance to a colored one, while in another case it might have 
the opposite result of taking away the color from a colored 
substance. It shows, in fact, that color in a substance is not a 
function of a particular group of elements but of the struc¬ 
ture of the entire molecule. 
The first attempt to show the relationship between constitu¬ 
tion and color in organic compounds appears to have been that 
of Graebe and Liebermann 1 in 1868. These investigators laid 
down the rule as of general application that, if the colored 
metallic salts of colorless organic acids are excepted, all col¬ 
ored organic compounds are rendered colorless by reducing 
agents, and that in this reduction the compound adds on hy¬ 
drogen without the elimination of any other elements from the 
molecule. 
As illustrations they quote quinone, C 6 H 4 
°\ 
o/ 
1 011 
reduced to hydroquinone, C 6 H 6 l ; azobenzene, C 6 H 5 N=N— 
JOH 
etc. 
C 6 H 5 ; reduced to hydrazobenzene, C 6 H 5 N 
-N—C 6 Hj 
l 
i 
H 
From these reactions they infer that colored compounds either 
contain elements with incompletely saturated affinities, or that 
some of the atoms.are more intimately bound than is neces¬ 
sary for their retention in the molecule; furthermore, that the 
physical property of color depends upon the manner in which 
the oxygen or nitrogen atom is combined, in the colored com¬ 
pounds these elements being in more intimate combination 
than in the colorless compounds. In the case of colored nitro 
and nitroso compounds, which are rendered colorless by re¬ 
duction to amido compounds, it is the intimate association of 
the oxygen and nitrogen to form a group which renders the 
substance colored. 
Graobe and Liebermann’s theory was formulated before the 
present diketone formula for quinones was accepted, their con¬ 
ception of a quinone being that of a benzene nucleus with two 
oxygen atoms linked together, hence the idea of “more inti¬ 
mate, or internal, combination. ’ ’ 
In 1876 Witt 2 advanced an entirely different explanation. 
1 Ber., 1, p. 106. 
a Ber., 9, p. 522. 
49—S. A. L. 
