Wakeman—Pigments of Flowering Plants. 773 
5. The addition of hydrogen always results in raising the tint. 
6. The rise or fall of the tint (the passage of absorption 
from violet to red) by substitution of hypsochrome or batho- 
chrome groups, or by the addition or loss of hydrogen, is the 
greater the nearer the atoms affected by the change are to the 
chromophore group. From this it would appear that in the 
bi-derivitives of benzene the substituents in para position are 
nearer to each other than those in ortho position. 
7. These rules hold only for monochromophoric compounds 
and for symmetrical dichromophoric compounds. The color 
of an unsymmetrical dichromophoric compound, Y, A, X, A, Z, 
is approximately the same as that of a mixture of the two 
symmetrical compounds, Y, A, X, A, Y, and Z, A, X, A, Z. 
In general the term bathochrome group is interpreted to 
mean a group which swings the absorption toward the red, 
and a hypsochrome group, one which swings the absorption to¬ 
ward the violet. Among the former are the hydrocarbon 
radicles, the halogens, and the salt forming groups with the ex¬ 
ception of the amino group. Among the latter are the acetyl 
and the benzoyl groups, the alkyl-oxy groups, hydrogen, and 
the amino group. The sulpho group is sometimes one and 
sometimes the other. 
The effect of the introduction of bathochrome and hypso¬ 
chrome groups upon the color of the original substance de¬ 
pends upon the original molecule, the position of the group, 
and the number of groups introduced. It is easy to under¬ 
stand how the introduction of one bathochrome group might 
deepen the color by throwing the absorption into the red, 
while the introduction of two or three such groups would re¬ 
move it by throwing the absorption wholly outside the visible 
spectrum. Again the same two or three groups might be re¬ 
quired to render another molecule colored. Here again we 
find additional evidence that the color of a substance is not 
conditioned by the presence of a definite group, or groups, 
but by the entire structure of the molecule. 
In 1904 Baly and Desch 10 in the course of a study of the ultra¬ 
violet spectrum of certain enol-keto tautomerides brought forth 
evidence to support the view that the absorption bands ex¬ 
hibited by these substances are due to the equilibrium existing 
10 Proc. Chem. Soc., 85, p. 1029. 
