Scientific American 7, 6.11.1847
Boil four parts of nitric acid on one part of indigo: the solution becomes yellow, and a resinous matter appears on the surface. Stop the boiling; let it cool: remove the resinous matter, and gently evaporate the solution to the consistency of honey. Re-dissolve it in hot water: filter; a solution of potash will throw down yellow spicular crystals coasisting of bitter principle, combined with potash. These crystals have the curious power of detonating with a purple light when wrapped in paper and struck by the hammer. The resin by being treated again with nitric acid, is converted into the same bitter principle.