882 Wisconsin Academy of Sciences , Arts, and Letters. 
Purpurin, — TriJiydroxy-1, 2, 4-anthraquinone. 
O 
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o 
Purpurin exists in Rubia tinctorium 1 and other species of 
Rubia, 2 probably as a glucoside, along with alizarin. Purpurin 
crystallizes in long orange yellow crystals which melt at 253°. 
It is soluble in water with a deep yellow color, soluble in ether 
and carbon disulphide, acetic acid and hot benzene; but almost 
insoluble in alkaline solutions. It imparts to fabrics mordanted 
with aluminum a violet red color, with iron a violet blue, and 
with chromium a reddish brown color. These colors are not 
so permanent as those given by alizarin. 
II. B.) Pigments referable to the homologues of dihydroan¬ 
thracene. 
The plant pigments referable to the homologues of dihydro¬ 
anthracene are derivatives of two different monomethyl ethers 
of dihydroanthracene, the methyl-l-anthraquinone and the 
methyl-2-anthraquinone. Of the former five representatives 
and of the latter three representatives are found in plants. 
1. Methyl-l-anthraquinone. 
a.) D'ihydroxy methyl-l-anthraquinones. 
Rubiadin. 
Chrysophanic acid. 
b.) Trihydroxy methyl-l-anthraquinones. 
Emodin. 
Aloeemodin. 
c.) Penthydroxy methyl-l-anthraquinones. 
Bhein. 
1 Ann., 2, p. 34; Jr. prakt. Chem., 5, p. 366; Ann., 66, p. 351. 
3 Jr. Chem. Soo., 63, p. 1157. 
