Wakeman—Pigments of Flowering Plants. 775 
pounds are colorless. They consider this principle to be the 
key to Armstrong’ theory of color and as an explanation of the 
colors of many compounds which are difficult of interpretation 
by Armstrong’s quinoid linking alone, for though Armstrong 
was perfectly right in concluding that color is due to quinoid 
linking, this formula gives no reasons why color is thus pro¬ 
duced. 
In 1907 13 Hale drew practically the same conclusions as 
those of Baly and his associates, namely that isorropesis is the 
cause of color in both the aromatic and the aliphatic series. By 
isorropesis is meant the making and breaking of contact between 
atoms thus giving them marked activity. This change of 
linkage which must accompany the transformation of one modi¬ 
fication of the compound to the other is the source of the oscilla¬ 
tions producing the absorption bands. If these oscillations are 
synchronous with light waves of a high frequency they give 
rise to absorption bands in the ultra violet and the compound is 
colorless. If, however, they are less frequent, the absorption 
band appears in the visible portion of the spectrum and this 
absorption of colored rays results in the compound taking on 
the complementary color. 
In 1907 Hewett and Mitchell 14 pointed out that in every 
case of colored compounds the molecules contain not jnerely 
double linkages but chains of alternate double and single link¬ 
ages. Generally speaking the longer this chain of conjugate 
double linkages the slower the oscillation frequency of the mole¬ 
cule. This explains why a benzene nucleus gives absorption 
bands in the ultra violet and is colorless while a quinone 
nucleus gives absorption bands in the violet and is therefore 
colored yellow. In estimating the number of such alternate 
double and single linkages, when a benzene nucleus is encount¬ 
ered, one is justified in following the structure around one side 
of the ring only until para position is reached. The chain in 
the benzene nucleus, therefore, contains at best only two double 
linkages, while that of the quinone contains three. 
Hewett and Mitchell also conclude that a radical change in 
the absorption spectrum of a compound when it undergoes salt 
formation generally means the radical alteration of its const! 
13 Pop. Sci. Mo., 72, p. 116. 
14 Jr. Chem. Soc., 91, p. 1251. 
