A crystalline glucoside obtained by treating the bark of Salix alba, and other species of Salix, and the bark of various species of Populus, with hot water, removing tannin and colouring matter from the decoction, evaporating, purifying, and recrystallising.

Characters. - Colourless shining crystals with a very bitter taste.

Solubility. - Soluble in about twenty-eight parts of water or a similar quantity of spirit at common temperatures; insoluble in ether. It is much less readily dissolved by putting it into cold water than by dissolving it in. hot water and allowing the solution to cool.

Reactions. - Sulphuric acid colours it red. A small quantity heated with a little red chromate of potassium, a few drops of sulphuric acid and some water, yields vapours of salicylic aldehyde, C6H4.OH (CHO), having the odour of meadow-sweet. The crystals melt when heated, and emit vapours having the odour of meadow-sweet. On ignition in air it leaves no residue.

The aqueous solution of salicin should not be precipitated by tannic or picric acids, nor by iodide of mercury and potassium (absence of and difference from alkaloids).

Dose. - 3 to 20 grains.

Action. - Its action is similar to that of salicylic acid (p. 820). Salicin is one of the sources of salicylic acid, which may be prepared from it by heating with caustic potash and treating the mass with hydrochloric acid. The salicylic acid prepared from salicin, or from oil of wintergreen, is generally purer than that made artificially (p. 820), and may frequently be tolerated by patients when the artificial salicylic acid disagrees. Salicin appears to be decomposed in the body, and is eliminated in the urine partly as salicin and partly as salicylic acid, as salicyluric acid, and as saligenin. Its action is less powerful than that of salicylic acid, and its depressing effect on the circulation less marked.

Uses. - It is used as an antipyretic, and has been given with success instead of salicylic acid in the treatment of acute rheumatism. It is useful also in headaches.