Definition of "menstruum"

Whenever any menstruum or other additament is employed, together with the fire, to obtain a sulphur or a salt from a body, we may well take the freedom to examine, whether or no the menstruum do barely help to separate the principle obtained by it...

1661, Robert Boyle, The Sceptical Chymist

[T]hat combustible sulphureous Body is presently prey'd upon and devoured by the aereal incompassing Menstruum, whose office in this Particular I have shewn in the Explication of Charcole.

1665, Robert Hooke, Micrographia

The analysis of mineral bodies... refined by the application of acid and alkaline menstrua, by Margraaf, Bergman, Bayen, and Achard, received still greater improvements from the labours of Klaprothk, Vauquelin, and Hatchett.

1812, Humphry Davy, The Elements of Chemical Philosophy, Introduction