876 Wisconsin Academy of Sciences , Arts, and Letters. 
cule of benzoic acid for one of gallic acid anthragallol results, 
and by substituting benzoic acid or its homologues, and various 
hydroxy benzoic acids for the gallic acid molecules any of the 
anthraquinone derivatives might be produced, just as any of 
the xanthone derivatives might be obtained by condensation 
of a molecule of benzoic acid or its derivatives with a phenol or 
of two molecules of phenols with one of carbonic acid. 
II. A. Pigments referable to dihydro anthracene. 
Six plant pigments of known constitution are referable to 
dihydroanthracene as the underlying hydrocarbon. These are 
all anthraquinone pigments, being mono- di- and tri- hydroxy 
derivatives of anthraquinone. The relation of anthraquinone 
to dihydroanthracene is shown on p. 870. The position of the 
hydroxy groups is here indicated by numbers in the usual way. 
1. ) Monohy droxyanthraquinones 
Monohydroxy -2- anthraquinone 
2. ) D ihy dr oxy anthraquinones 
a. ) Alizarin 
b. ) Hystazarin 
c. ) Xanthopurpurin 
3. ) Trihy droxyanthraquinones. 
a. ) Anthragallol 
b. ) Purpurin 
