This is a fluid obtained from mare's-milk by fermentation, and constitutes the principal part of the food of the people inhabiting a portion of Tartary. It contains alcohol, lactic acid, sugar, casein, fat, salts, carbonic acid, and water. In addition to these constituents, ascertainable by chemical analysis, koumiss possesses fragrant compounds, volatile, the product, probably, of the decomposition of the fat and the reaction of the acids on the alcohol, forming ethers. Koumiss of good quality may also be prepared from cow's-milk by the process of fermentation, but, as mare's-milk is more nearly allied to human milk in composition, it is to be preferred in the preparation of this aliment. By variations in the method of preparation, different kinds of koumiss are produced, as, for example, thick koumiss, whey-koumiss, skimmed-koumiss. According to the different stages to which the process of fermentation is carried, there result three degrees of quality, No. 1, No. 2, and No. 3. No. 2 differs from No. 1 in containing more alcohol and carbonic acid, and less sugar and casein. These constituents, especially the carbonic acid, impart a liveliness to the fluid, so that it effervesces like champagne. In No. 3, the fermentation having proceeded further, butyric, succinic, and acetic acids are produced, and the sparkling quality is enhanced.

Koumiss is prepared from milk, by the addition of a ferment— some koumiss obtained from a previous fermentation or dried koumiss. It is allowed to ferment three days at a temperature of from 70° to 80° Fahr. It is then a bluish-white liquid, having a sharp, acidulous taste, and none of the characteristics of ordinary milk. If heated to 100° Fahr., fermentation is definitely arrested. If before being heated it is bottled, products corresponding to 1, 2, and 3, named above, are the result. Allowed to stand after three days' fermentation, it separates into three layers: the inferior, caseous; the middle, an acid water; and the uppermost, a whitish fluid, the best koumiss. The alcoholic strength is of course determined by the stage of fermentation. The koumiss of two days' fermentation is feeble in strength, and hence the product of three days' fermentation is preferable for medicinal use.

The quantity of koumiss administered depends on the condition of the patient. In cases of feeble digestion, this being the only article of food, an ounce every hour will be a sufficient quantity. "With increased facility in its digestion and assimilation, from a quart to a gallon a day may be taken. When it is used in connection with other food, a tumblerful may be administered after each meal. It is estimated that each quart of koumiss contains four ounces of solid food.

The tolerance of the stomach to koumiss is remarkable, even in cases of gastralgia. It improves the appetite, and excites the action of the kidneys. The patients experience a pleasing exhilaration, due probably to the combined action of the carbonic acid and the alcohol. Decided intoxication undoubtedly may result from the use of a large quantity by any one unaccustomed to it. It also causes somnolence during the day, and favors sleep at night without leaving any after-headache. Its most important action is the increase of the body nutrition; and hence its utility in the treatment of phthisis, indigestion, and the various cachexiae.Jagielsky says that he has had patients gain as much as ten pounds a month when no other food was taken.