1896- 97.] On the so-called “ Hypoiodite of Magnesium!’ 235 
On the so-called “ Hypoiodite of Magnesium.” By James 
Walker, D.Sc., Ph.D., and Sydney A. Kay, B.Sc., Univer- 
sity College, Dundee. 
(Read December 7, 1896.) 
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 reddisli- 
hrown 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 iodide 
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. It is true that the freshly precipitated compound 
is more active than any of the other forms, the difference being 
possibly due to the loose flocculent nature of the precipitate, or 
perhaps to its being the hydroxide and not the oxide. The ignited 
oxide is least readily coloured by the iodine. 
