770 Wisconsin Academy of Sciences, Arts, and Letters. 
Color in a substance, according to Witt, is due to the presence 
of a chromophore group in the molecule. Such a substance, 
though generally colored, is not a dyestuff and is called by 
Witt a chromogen. It becomes a dye by the introduction of a 
salt forming auxochrome group. According to Witt the 
principal chromophore groups are. the nitro_> the ni- 
Wo 
troso, —N = O, the carbonyl —C = 0, and the allied group 
the thio carbonyl, = C = S; and also the azo menthin group 
—C = N —, the azo group — N — N —. The principal auxo¬ 
chrome groups are the hydroxyl, the amino, and the mono- 
and di- alkyl amino groups. Witt’s theory, while it proved 
of great value in the synthesis of artificial dyes, as stated be¬ 
fore, ha^ been of comparatively little use in the study of plant 
pigments, since only the corbonyl group among the so called 
chromophore groups and the hydroxy among the auxochrome 
groups are of at all frequent occurrence in the plant pigments 
of known constitution. Moreover the mere presence of these 
two groups, or of multiples of one or both, is not sufficient to 
explain the phenomenon of color in the plant pigment mole¬ 
cule. 
From time to time other chromophore groups have been 
added to those named by Witt. Principal among these are the 
—N—N— —N=N— 
ethylene 3 group =C=C—, the azoxy 4 group ^ / or || 
o o 
—N = N— 
or \^/ , and combinations of some of the above named groups. 
In 1888 Armstrong 0 introduced his quinone theory of color. 
Since dyestuffs in general can, by the addition of hydrogen, be 
reduced to the corresponding leuco bases, Armstrong consid¬ 
ered all colored compounds to be quinones and the correspond¬ 
ing colorless compounds to be hydroquinones. Using the 
Fittig diketone formula for quinones, Armstrong attempted to 
show that the structure of colored compounds in general may 
be represented by a quinoidal formula. Under the term quin- 
oidal formula Armstrong included all structures containing 
3 Ber., 33, p. 666. 
«Ber., 31, p. 1361; 33, p. 123; 29, p. 2413. 
6 Proc. Chem. Soc., 4, p. 27; 8, pp. 101, 143, 189, 194. 
