1896 - 97 .] On the so-called “ Hypoioditc of Magnesium .” 237 
coloured by the iodine set free at the same time. That this view 
of the process is essentially correct may be seen from the following 
experiment. A solution of hypoiodous acid, made by agitating a 
pure aqueous solution of iodine with mercuric oxide and filtering, 
is rapidly neutralised with potash and a magnesium salt added. 
The precipitate obtained is white. Addition of a few drops 
of potassium iodide solution will now stain the precipitate brown, 
showing that the presence of iodide ions is essential to the action. 
The precipitate is also brown when the hypoiodite and iodide 
solutions are first mixed and then treated with magnesium sulphate. 
When the precipitate obtained from an aqueous solution is 
dried either at the ordinary temperature or at 100°, it is very dark 
in appearance, being almost black, whilst the substance prepared 
by colouring the magnesia with iodine in chloroform solution has 
approximately the colour of magnesia stained with iodine vapour. 
Regarding the stability of the dried substances prepared by the 
various methods, it may be said that they all give off iodine 
vapour at the ordinary temperature. The amount of iodine given 
off is undoubtedly very small, but the presence of iodine vapour 
may be made evident as follows. The sample to be tested is placed 
in a closed space alongside a small quantity of magnesia. After a 
little time the magnesia becomes brown, the depth of tint varying 
considerably, however, with different specimens. This process 
forms a convenient mode of testing for the presence of iodine 
vapour, especially where the absence of moisture is essential. The 
material suspected to give off iodine is placed near a small quantity 
of magnesia on a white tile, both substances being then covered by 
a watch-glass. After 5-10 minutes the magnesia may be examined, 
and will then reveal the presence of even minute traces of iodine. 
The following example of its application may be mentioned. 
Magnesium iodide as purchased is of a brown colour. It was 
suspected that this might be due to partial decomposition of the 
iodide with formation both of magnesia and of iodine, which could 
then unite to give the “ hypoiodite.” A little of the iodide was 
accordingly brought into juxtaposition with magnesia, when it at 
once imparted a brown colour to the latter, showing that it gave 
off iodine vapour. That our surmise was correct became further 
evident on dissolving the iodide in water. The bulk of the solid 
