Note 23 — Chapter III
Reading edition
Professor Carey Lea (American Journal of Science, 3, ix., 121) prefers to mix a weak solution of ammonio-ferrous sulphate with a little ammonio-ferric citrate, to acidify with hydrochloric acid, then to place two or three drops of this on a white plate, and to add a few drops of the suspected solution. A blue cloudiness indicates HCN. This method, he says, is capable of detecting 1/5000 of a grain of HCN. But I do not think it more delicate than the old method if properly performed, and it does not so easily admit of comparative experiment as to quantity.