234 Proceedings of Royal Society of Edinburgh . [sess. 
this way long colourless crystals were obtained, the properties of 
which are described as follows : — 
“ Once prepared, it is somewhat stable. When heated it fuses 
with separation of iodine. Water, alcohol, and ether dissolve the 
di-iodide, again with separation of iodine ; hence the explanation 
of its non-formation in solution. On warming the aqueous solu- 
tion, or adding an alkali, sulphur is liberated 
“ Dilute hydrochloric acid does not decompose the di-iodide in 
the cold, as the latter substance is nearly insoluble in it ; but, on 
warming, iodine separates in considerable quantity, without libera- 
tion of sulphur. This shows that the di-iodide decomposes more 
easily into iodine and thio-urea than into hydriodic acid, sulphur, 
thio-urea, and cyanamide. 
“ If a drop of alcohol and, immediately, some nitric acid are 
added to a portion of di-iodide, there instantly results a precipitate 
of the di-nitrate.” 
Recently I had occasion to examine a product, obtained from 
thio-urea, which appeared to be one of the above series of salts. 
The results obtained on adding potassium iodide to its aqueous 
solution seemed anomalous, as the amount of iodine liberated by a 
given quantity of substance varied greatly, and in an erratic 
manner, in different experiments. It was also found that in cases 
where the amount of iodine liberated was comparatively small, 
slight warming of the solution deepened the colour quite distinctly, 
while subsequent cooling changed it back again. This behaviour 
indicated that the action was a reversible one, and that what 
McGowan described as slower decomposition in aqueous solution 
might really be less complete decomposition. In that case the forma- 
tion of the di-iodide from thio-urea and iodine should be effected 
more easily in the presence of water than in presence of alcohol. 
Experiment showed that this was the case, and the preparation is 
exceedingly simple. An examination of the properties of the di- 
iodide obtained in this experiment had most interesting results, as 
its behaviour in solutions of various kinds is very peculiar. So far, 
only qualitative experiments have been performed, but it has been 
considered advisable to publish a description of them in a prelim- 
inary communication, as it is probable that a full investigation of 
the points involved will take some time. 
