The doctrine of menstruums, as laid down by Boerhaave, seems most intelligible and applicable to our purpose. He says, the solutions of bodies in general are the effect only of attraction and repulsion, between the particles of the menstruums and those of the body dissolved, the whole action depending on the relation between these two; of consequence, there cannot be any body, natural or artificial, which, without distinction, will dissolve all bodies whatsoever; nor is the cause assignable why certain menstruums dissolve certain bodies: the effects of alcaline, acid, neutral, fixed, or volatile salts, any more than those of oils, water, alcohol, fire, or air, are not to be accounted for by any general rule, that universally holds true; nor even, in many cases, doth the dissolution of a body depend on the purity or simplicity of the menstruum: the nearest path then to success, is cautiously to apply every menstruum we know of to the body whose solvent we want to discover. The elements of fire and air greatly promote the action and effect of menstruums, and in this light they are admitted as such. Water dissolves most salts, all the natural sapos of plants, and the ripe juices of fruits; for in these, the oils, salts, and spirit of the vegetables, are accurately mixed and concreted together, and malts, having the same constituent parts with them, this element becomes a proper menstruum to extract this grain: though malts, by being dried with heats which greatly exceed what is necessary to bring barley to a state of maturity, do, from hence, require greater, though determinate heats, yet inferior to that at which water boils; but such heats must be applied in proportion to their dryness, to extract their necessary parts. Even earths, by the intervention of acids, dissolve in water; but having treated of the four elements already, as far as we conceived was requisite for the art of brewing, we shall, in this chapter, confine ourselves to oils and salts, and view these acting as menstruums only. To the definition already given of oils, it may be necessary to add, in general, they contain some water, and a volatile acid salt; that they receive different appellations, and have different properties in proportion to their respective spissitudes. Oils from vegetables are obtained by expression, infusion, and distillation; in either of which methods, a too great heat is to be avoided, as this gives them a prejudicial rancidness, and where water does not interpose, alters their color until thereby they are turned black. In general oils unite with themselves, but, excepting alcohol, not with water, unless when combined with salts, for salts attract water, and so they do oils: hence arises many elegant preparations both natural and artificial, from which wines are formed.