Put three ounces of rock alum and one ounce of honey or sugar into a new earthen dish, glazed, and which is capable of standing a strong heat; keep the mixture over the fire, stirring it continually until it becomes very dry and hard; then remove it from the fire and pound it to a coarse powder. Put this powder into a long-necked bottle, leaving a part of the vessel empty; and having placed it in the crucible, fill up the crucible with fine sand and surround it with burning coals. When the bottle has been kept at a red heat for about seven or eight minutes, and no more vapor issues from it, remove it from the fire, then stop it with a piece of cork; and, having suffered it to cool, preserve the mixture in small bottles, well closed.

If you unclose one of these bottles and let fall a few grains of this powder on a bit of paper, or any other very dry substance it will first become blue, then brown, and will at last burn the paper or other substance on which it has fallen.