murexide

 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]