240 Proceedings of Royal Society of Edinburgh. [sess. 
asbestos felt was in the end adopted as giving the most satisfactory 
results in the filtration. The filtrate was titrated with sodium 
thiosulphate solution equivalent in strength to the original solution 
of iodine. The quantity of thiosulphate required gave the amount 
of free iodine remaining in the solution. The liquid was next 
acidified in order to liberate the iodine contained in the solution 
as iodate and iodide, and again titrated. Lastly, the precipitate 
was dissolved in hydrochloric acid, with addition of potassium 
iodide to effect the solution of the liberated iodine, which was then 
titrated with thiosulphate. 
With freshly calcined magnesia it was found that the quantity 
of iodine remaining in a given solution diminished as the time 
during which the solution was agitated increased. Thus 50 c.c. 
of decinormal iodine solution, on being shaken with 1 g. of magnesia, 
gave the following numbers — 
Time of Agitation. 
Titre of Filtered Solution. 
15 minutes 
29’4 c.c. 
30 „ 
27*0 c.c. 
60 „ 
20’9 c.c. 
As we thought that the absorption might proceed more rapidly 
and uniformly if precipitated magnesia were used instead of finely 
divided calcined magnesia, a seminormal solution of caustic soda 
was prepared, and measured quantities of it were added to a slight 
excess of magnesium sulphate solution. The magnesia was thus 
precipitated in a loose flocculent state and very rapidly absorbed 
iodine, prolonged agitation increasing the amount absorbed, as the 
following numbers show. In the first set the solution was filtered 
immediately after the addition of the iodine ; in the second set 
after shaking for fifteen minutes. 
Iodine taken. 
Iodine 
remaining. 
Iodine absorbed. 
Iodine 
transformed. 
Error. 
5 
2*28 
1*42 
1*20 
o-io 
5 
1-89 
1-81 
1-21 
0-09 
