The reaction is due to the decomposition of oil of vitriol by iodohydric acid, water and sulphurous acid being formed, and iodine set free.

The iodides in solution are decomposed by chlorine, iodine being precipitated, the smallest quantity of which in solution is instantly detected by its imparting to starch an intensely blue color.
Iodide of Potassium.*--This compound is easily made in the following manner: Subject to a moderate heat a mixture of 100 parts of iodine, 75 of carbonate of potash, 30 of iron filings, and 120 parts of water.
This mass must be thoroughly dried and then heated to redness; the resulting reddish powder is to be washed with water, and the solution obtained filtered, and evaporated to dryness.

It is found that 100 parts of iodine yield 135 parts of very white, but slightly alkaline, iodide of potassium.
* I shall present the preparation of only a few iodides, and such as are more intimately connected with the Daguerreotype.
Experiment .-- On projecting dry pulverized iodide of potassium into fused anhydrous phosphoric acid, a violent disengagement of iodine takes place, attended by a transient ignition; fused hydrate of phosphoric acid liberates iodine abundantly from iodide of potassium; this reaction is accompanied by the phenomenon of flame and formation of a considerable quantity of hydriodic acid.
Iodide of Mercury .-- For the preparation of iodide of mercury, Dublanc recommends to cover 100 grms.

of mercury with 1 kilogrm.

of alcohol, to add 124 grms.