*urex*

n. [ From Murexide. ] (Chem.) A complex nitrogenous substance obtained from murexide, alloxantin, and other ureids, as a white, or yellowish, crystalline which turns red on exposure to the air; -- called also uramil, dialuramide, and formerly purpuric acid. [ 1913 Webster ]

n. [ L. murex the purple fish, purple. ] (Chem.) A crystalline nitrogenous substance (C8H8N6O6, 5-5'-nitrilodibarbituric acid monoammonium salt) having a splendid dichroism, being green by reflected light and garnet-red by transmitted light. It was formerly used in dyeing calico, and was obtained in a large quantities from guano. It is now synthesized from alloxan. Formerly called also ammonium purpurate. [ 1913 Webster +PJC ]