menstruum

n.; pl. E. Menstruums L. Menstrua [ L. menstruus. See Menstruous. ] Any substance which dissolves a solid body; a solvent. [ 1913 Webster ]

The proper menstruum to dissolve metal. Bacon. [ 1913 Webster ]

All liquors are called menstruums which are used as dissolvents, or to extract the virtues of ingredients by infusion or decoction. Quincy. [ 1913 Webster ]

☞ The use is supposed to have originated in some notion of the old chemists about the influence of the moon in the preparation of dissolvents. Johnson. [ 1913 Webster ]