The following investigation was undertaken in connection with a series of experiments on absorption at present being carried out in this laboratory.

In Gmelin's Handbook (Cavendish Edition, vol. 3, p. 240) it is stated, on Gay-Lussac's authority, that magnesia, when shaken up with a solution of iodine, absorbs the latter and assumes a reddishbrown colour. The production of this brown “hypoiodite of magnesium” is accompanied by the formation of small quantities of magnesium iodide and magnesium iodate, which pass into solution. When the brown compound is boiled with water, magnesium iodido and magnesia are formed, and when it is heated by itself, iodine is given off and magnesia remains.

The same compound is produced when magnesia is precipitated in presence of iodine, and its characteristic appearance makes it an excellent means of identifying a magnesium salt. The test is described as follows in the Notes on Reactions of Salts, used in the Practical Chemistry Class in the University of Edinburgh. “When a solution of iodine in potassium iodide is added to a magnesium salt, and a few drops of caustic potash are added to the mixture, a reddish-brown precipitate is produced; excess of caustic potash decolorises the precipitate.”

The state of the magnesia has apparently little effect on the production of the brown substance. It is formed indifferently with freshly precipitated magnesia, magnesia dried at the ordinary temperature, magnesia dried at 100°, and magnesia ignited over the blow-pipe.