Source : Webster's Revised Unabridged Dictionary (1913)

Menstruum \Men"stru*um\, n.; pl. E. {Menstruums}, L. {Menstrua}.
   [L. menstruus. See {Menstruous}.]
   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.