45:7 · This structure has been used in alchemy to denote aqua fortis or nitric acid. It was used to differentiate between gold and silver since it dissolved silver, but not gold.
    In other alchemical contexts it has meant amalgam, i.e. alloys with mercury.
    Compare with the synonymous .
    In the US, British, French and Swedish systems of hobo signs (as well as the entry sign below) means prison or avoid this place-risk of being arrested.