File a clean spot upon the metal to be tested, so that any gilding or outside colouring may be removed. Apply a small drop of pure nitric acid to this spot, and watch it closely. If the metal is brass, it will boil up a bright green immediately. If an imitation gold alloy, it may go black in a few seconds. If 9-ct. gold, it will turn a pale brown tint. If 15-ct. or over, it will remain unaltered, and the acid will stand upon it like water. Now press the tip of one finger hard upon it and wipe the finger again immediately. If 15-ct., the spot will turn a pale brown, as 9-ct. did before pressing with the finger. If 18-ct. or over, the acid will still stand upon it like water; 22-ct. can be told by its colour by an expert.