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The Collaborative International Dictionary
rheic acid

Chrysophanic \Chrys`o*phan"ic\, a. Pertaining to, or derived from, or resembling, chrysophane.

Chrysophanic acid (Chem.), a yellow crystalline substance extracted from rhubarb, yellow dock, sienna, chrysarobin, etc., and shown to be a derivative of an anthracene. It is used in the treatment of skin diseases; -- called also rhein, rheic acid, rhubarbarin, etc.

Wiktionary
rheic acid

n. (context organic compound obsolete English) An acid found in rhubarb; chrysophanic acid.