236 Proceedings of Royal Society of Edinburgh . [sess. 
The production of the brown substance seems likewise to be 
independent of the state of the iodine. Iodine in solution in 
water, in aqueous potassium iodide, in alcohol, in chloroform, or 
in carbon disulphide at once unites with magnesia, staining it deep 
brown. The vapour of iodine, too, is equally effective. If two 
watch-glasses, containing iodine and magnesia respectively, are 
placed beneath an inverted glass dish, it will be seen that the 
magnesia rapidly acquires a coffee-brown colour, especially if the 
laboratory temperature is moderately high. The colour is not 
superficial, but penetrates uniformly into the interior of the powder. 
Some other oxides (for example, calcium oxide), when exposed to 
the vapour of iodine under the same conditions, acquire a super- 
ficial brownish tint, but this is by no means uniform even on 
the surface, and differs greatly in appearance from the brown of 
the magnesia. Zinc oxide is stained slowly to a dirty pink, the 
colour on the surface differing somewhat from the colour in the 
interior. Pure white salts, such as precipitated calcium carbonate, 
are quite unaffected. 
It is perhaps worthy of note that the coloration at once takes 
place when iodine and magnesia are simultaneously produced in 
the same solution. If to a solution of iodine in aqueous potassium 
iodide potash is cautiously added until the iodine has completely 
disappeared, the addition of a small quantity of magnesium 
sulphate will immediately produce a chocolate-brown precipitate in 
the pale yellow hypoiodite solution. Expressed in terms of the 
electrolytic dissociation theory the action which takes place is as 
follows. Hypoiodite ions can only exist in presence of iodide ions 
when hydroxyl ions are also present in certain proportions ; if the 
hydroxyl ions are removed, the other ions react with water to re- 
produce hydroxyl ions, iodine being at the same time liberated. 
There is, in fact, a balanced action expressible by the equation, 
i 2 + 20 H'^tr + or + h 2 o, 
and if this balance be disturbed by the removal of hydroxyl ions, 
more hydroxyl ions will be produced, and their production must be 
accompanied by the liberation of iodine. Now the addition of a 
magnesium salt removes hydroxyl ions from the solution in the 
hydroxide which is precipitated, and this hydroxide is at once 
