This expression is usually found in the form of passing or undergoing the acid test. Since the late 19th century, the acid test has meant a decisive, rigorous examination of virtually anything in order to determine its validity or soundness. Its origin, however, in the 17th century, was much more specific. Gold coinage was in wide circulation then, but so was counterfeit. Real gold was found to be impervious to aquafortis or nitric acid, which would decompose other metals. Therefore, the original acid test was to dip gold into nitric acid, quality control at its most rigorous.