780 Wisconsin Academy of Sciences, Arts, and Letters. 
many instances, however, the influence of both the presence and 
the position of these groups, especially of the hydroxy group, 
is evident. For example, the substitution of hydroxy groups 
for hydrogen in the xanthone or flavone pigments usually in¬ 
tensifies both the color and the dyeing properties of these sub¬ 
stances, while the removal of such groups, either by replace¬ 
ment with hydrogen or by methylation, usually diminishes both. 
In no instance does the presence of the chromophorous group 
explain the color. In most cases it will be seen that it is not 
the mere presence of these so-called chromophorous groups but 
their relation to each other and to the rest of the molecule which 
postulates color in a substance. Color, in other words, appears 
to be a function, not of certain groups or elements but of the 
entire molecule. 
3. The existence of homologous series of plant pigments, or 
more accurately, of pigments referable to homologous series of 
hydrocarbons is worthy of note. This homology is manifest in 
connection with every degree of saturation where a sufficiently 
large number of pigments, or of pigment forming substances, 
to admit of comparisons adequate to justify the drawing of con¬ 
clusions, has been isolated. 
Under the degree of saturation C n H 2n _ 4 we find evidence of 
the existence of quinone, methyl quinone and of methyl 
p-isopropyl quinone. Similarly, under the formula of satura¬ 
tion C n H 2n _ 6 the pigment forming substances hydroquinone, 
methyl hydroquinone, and methyl-p-isopropl hydroquinone 
are found. A similar homology is found to exist in connection 
with pigments referable to hydrocarbons of other degrees of 
saturation, especially to C n H 2n „ 16 where we find pigments 
referable to homologuus of diphenyl ethene, diphenyl propane, 
etc., as well as to homologous series of dihydroanthraeenes. 
A condition quite similar in many respects to homology, and 
sometimes confused with it, exists among the' pig¬ 
ments falling under the degrees of saturation C n H 2n __ 14 
and C n H 2n _ 16 . This is the existence of pigments referable to 
closely related series of hydrocarbons, not truly homologous yet 
differing from one another by CH 2 , such as diphenyl, diphenyl 
methane, diphenyl ethane, diphenyl propane, among the former; 
and diphenyl ethene, diphenyl propene, and diphenyl butene 
among the latter, as well as alkyl substitution products of these 
