Menstruum, in general, signifies all liquors employed as solvents of other bodies, with the minute particles of which the former combine, so as to produce a new, uniform compound : they are chiefly used for extracting the virtues or ingredients of matters more solid than themselves, by infusion, decoction, distillation, etc.

Water is the solvent of all salts, vegetable gums, and animal jellies. Rectified spirit of wine is the menstruum of the essential oils and resins of vegetables, of soap, etc. Oils dissolve vegetable resins and balsams, wax, animal fat, mineral bitumens, sulphur, and certain metallic substances, particularly lead : yet, for this purpose, the expressed oils are more powerful menstrua than the distilled ; because the former are not so liable as the latter to be volatilized in a strong heat, which in most cases is required for enabling them to produce the desired effect.

All acids act as solvents of alkaline salts and earths, as well as metallic bodies ; but their action greatly varies on different metals : thus, the vegetable acids dissolve a large proportion of zinc, iron, copper, tin, and antimony, but particularly lead, if previously corroded by their steam.—The marine acid, or spirit of salt, dissolves zinc, iron, and copper; and, if combined with the nitric acid, or aqua fortis, a proper menstruum is obtained for gold and antimony.—The vitriolic acid, or oil of vitriol, acts upon zinc, iron, and copper: it also corrodes or imperfectly dissolves most other metals.

Alkaline lixivia, or leys, dissolve oils, resinous substances, and sulphur : by adding quick-lime, they become more powerful, as is evident in the preparation of common soap. By such addition, the flesh, skin, and bones of animals may be reduced to a jelly.

Solutions effected in water, and spirit of wine, possess the virtues of the substances dissolved; but oils generally sheathe their strength, while acids and alkalies change their qualities. Thus, water and distilled spirits are the proper menstrua of vegetable and animal matters, the efficacy of which is to be preserved.

Most of the solutions mentioned are easily made, by pouring the menstruum on the substance to be dissolved, and exposing both, for some time, to a proper degree of warmth.—Oils and alkaline liquors generally require a strong heat to increase their solvent power ; and acids, likewise, do not act on some metals without this aid. Watery and spirituous menstrua may be rendered more expeditious by a moderate heat; and the quantity they hold in solution, will be greater than without this assistance; but, on becoming cold, that proportion of soluble matter which was, in a manner, kept suspended by heat, again subsides.—As the action of acids on metallic bodies is generally attended with heat, effervescence, and a copious discharge of fumes, which are highly inflammable, such as those arising from the solution of iron in the vitriolic acid, the operator ought never to approach the vessel with a candle, or other burning substance; as the exhaling vapour would thus instantly be set on fire, and cause an explosion.

Lastly, there is another species of solution, in which the moisture of the atmosphere is the menstruum. If fixed alkaline salts or earths, for instance, pot-ash, as well as the neutral salts composed of the former, and the vegetable or any other acids (except the vitriolic, and some metallic salts), be exposed for some time to a moist air, they gradually absorb humidity, and at length become liquid; a process which is termed deliquation.