Agar-agar is a gelatinous substance prepared from Gelidium corneum, Lam., G. cartilagineum, Gaill., Sphaerococcus compressus, Ag., and other Algae (N.O. Rhodophyceae). It occurs in transparent strips, about 6 decimetres long, and of the thickness of a straw, or more frequently in yellowish-white pieces, 3 decimetres long, 2.5 to 25 millimetres thick, and more than 25 millimetres wide. The latter variety is most suitable for the preparation of culture media for bacteria. Agar-agar is odourless and tasteless. It consists of a carbohydrate, gelose, which is converted into galactose on boiling with diluted sulphuric acid. An aqueous solution gives no precipitate with solution of tannic acid (absence of gelatin), and no blue colour with solution of iodine (absence of starch).
Insoluble in cold water, soluble in hot water, the solution gelatinising on cooling.
Action and Uses.—Agar-agar is not attacked by the digestive secretions, and passes through the intestinal canal almost unchanged. It, however, absorbs and retains moisture, increasing very largely in bulk. On account of this property it has been used with success in chronic constipation with intestinal atony. It renders the faeces soft and bulky, and promotes peristalsis. For this purpose agar-agar is usually administered in small shreds, mixed with fruit, milk, or any convenient vehicle. It is preferably not given in powder, as this has been stated to give rise to irritation in some cases. Agar-agar is employed for preparing culture media for use in bacteriology; it has also been recommended as a substitute for gelatin in making suppositories, and for use as an emulsifier. A jelly for invalids is prepared by dissolving 1 part of agar-agar in 200 of boiling water. Preparations containing agar-agar (sometimes with cascara (These were professionals? Cascara is just Spanish for "bark". The usual plant meant is Cascara sagrada (Rhamnus purshiana), but there's also Cascara amarga, which is the bark from a Picrasma or Quassia—in addition to all the other barks out there. Using just one part of a name is bound to cause confusion.—Henriette) are known under the trade-names Regulin and Agarase.
Dose.—15 to 30 grammes (½ to 1 ounce).