In alchemy or chemistry, relating to a substance or process combining the properties of water and mercury, or a combination of the two.
From Latin 'aqua' (water) combined with 'mercurial' (relating to mercury, the liquid metal named after the Roman god Mercury). This is a specialized alchemical/chemical term.
Alchemists were fascinated by mercury because it looks like liquid silver—calling something 'aqueomercurial' meant it combined water's flowing nature with mercury's mysterious heaviness and ability to dissolve other metals, which made them think it was transformative.
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