This is an officinal newly introduced into the U. S. Pharmacopoeia. it is prepared by passing hydrosulphuric acid gas (sulphuretted hydrogen) through iodine suspended in distilled water, until the whole of the iodine has disappeared, then boiling so as to drive off the excess of hydrosulphuric acid, and finally filtering, and adding enough distilled water, through the filter, to make the resulting liquid of the proper strength. in the process, iodine takes the place of an equivalent quantity of the sulphur of the hydrosulphuric acid, forming with the hydrogen hydriodic acid, while the sulphur is precipitated, and afterwards separated by filtration.

When pure, hydriodic acid is gaseous, but, in its officinal form, it is a liquid, consisting of the acid combined with a large proportion of water; much more than is necessary for the absorption of all the gaseous acid contained in it. in this diluted state, it is colourless, of a sour taste, a feeble odour analogous to that of muriatic acid, and of the sp. gr. 1.112. On exposure to the air it becomes coloured, in consequence of the separation of iodine, of which it acquires the peculiar smell.

Diluted hydriodic acid has been occasionally used as a means of bringing the system under the influence of iodine; being thought capable of producing this result as effectually as any other preparation of that medicine, while less unpleasant to the taste and more acceptable to the stomach than most of them. Each fluidrachm of it contains ten grains of iodine; and thirty minims, largely diluted with water, may be given three times a day. if discoloured and irritant through the separation of iodine, it should be administered in some amylaceous liquid, as rice or barley water.