MANNA, in the materia medica, the juice of certain trees of the ash-kind*, either naturally concreted on the plants, or exsiccated and purified by art. There are several sorts of manna in the shops. The larger pieces, called flake manna, are usually preferred; though the smaller grains are equally good, provided they are white, or of a pale yellow colour; very light, of a sweet, not unpleasant taste, and free from any visible impurities. Some people injudiciously prefer the fat honey-like manna to the foregoing; this has either been exposed to a moist air, or damaged by sea or other water. This kind of manna is said to be sometimes counterfeited by a composition of sugar and honey mixed with a little scammony; there is also a fictitious manna, which is white and dry, said to be composed of sugar, manna, and some purgative ingredient, boiled to a proper consistency. This may be distinguished by its weight, solidity, untransparent whiteness, and by its taste, which is different from that of manna.
Manna is a mild, agreeable laxative; and may be given with safety to children and pregnant women: nevertheless, in some particular constitutions, it acts very unkindly, producing flatulencies and distensions of the viscera: these inconveniences may be prevented by the addition of any grateful warm aromatic. It operates so weakly, that it does not produce the full effect of a cathartic, unless taken in large doses; and hence it is rarely given in this intention by itself. It may be commodiously dissolved in the purging mineral waters, or joined to the cathartic salts, senna, rhubarb, or the like. Geoffroy recommends acuating it with a few grains of emetic tartar: by this management, he says, bilious serum will be plentifully evacuated, without any nausea, gripes, or other inconvenience. It is remarkable, that the efficacy of this drug is greatly promoted (if the account of Vallisnieri is to be relied on) by a substance which is itself very slow of operation, viz. casia. See CASIA.