What does "menstruum" mean?

Menstruum: Any substance which dissolves a solid body; a solvent. The proper menstruum to dissolve metal. Bacon. All liquors are called menstruums which are used as dissolvents, or to extract the virtues of ingredients by infusion or decoction. Quincy. Note: 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.