Aqua Regia: 1 Volume of Nitric Acid to 4 of Hydrochloric Acid (Thorpe 1912)

Aqua regia, the alchemists' solvent for gold, is simply nitric and hydrochloric acids mixed; together they set free chlorine, which does the dissolving. Result: a liquid at first colourless, turning deep orange-yellow, especially when warmed. Source: Thorpe, Dictionary of Applied Chemistry, vol. 1 (1912), Aqua regia. Not given: the strength of the acids. Danger: both acids burn, and the mixture gives off chlorine. Picture drawn by Youblob.