The artificial preparation of theobromine and caffeine from xanthine, and guanine also show clearly their relations.
"Scientific American Supplement, No. 415, December 15, 1883" by Various
The xanthin group is almost without any excitatory action, and its metabolic end products are constant.
"All About Coffee" by William H. Ukers
Xanthin and catechol browns are pleasing in appearance, but their effect is less rich than that obtained with potash.
"The Decoration of Leather" by Georges de Récy