1901-2.] Dissociation of Compound of Iodine and Thio-urea. 237 
is intermediate between water and ethyl alcohol as regards ionising 
power, whilst acetone is generally inferior to the latter in this 
respect. Experiments with the di-iodide gave results entirely in 
agreement with Carrara’s experience. Equal parts of an aqueous 
•solution of the di-iodide were diluted to the same volume with 
water, methyl alcohol, ethyl alcohol, and acetone, respectively. 
The solution which was diluted with water became nearly colour- 
less ; that with methyl alcohol assumed a deeper tint than formerly, 
that with ethyl alcohol still deeper, and that with acetone was 
darker than any of these. 
When ether was added to an aqueous solution in quantity in- 
sufficient to form a separate layer, a deepening of the colour was 
observable; when the quantity was sufficient to form a separate 
layer, the latter was dark brown. On removing the layer and 
adding more ether, another dark layer was formed ; this could be 
repeated several times without a very marked diminution in the 
depth of colour of the ethereal layer, in accordance with the 
assumption of a moderate dissociation of the compound in the 
aqueous solution. 
According to the ionisation hypothesis, the addition, to a solution 
of a salt, of any other salt having an ion in common with the 
first one results in diminishing the extent to which that salt is 
ionised. Such was found to be the case with the di-iodide ; the 
addition of potassium or sodium iodide, solid . or dissolved, to its 
solution resulted at once in the production of a dark brown 
colour. The corresponding chlorides and bromides had no appre- 
ciable effect ; but, on the other hand, the di-bromide corresponding 
to the di-iodide produced a distinct deepening of the colour, 
though the effect was very much less than that caused by a 
metallic iodide. 
Dilute hydrochloric acid has no appreciable effect on a solution 
of the di-iodide, but concentrated acid produces a dark coloration. 
This may result from various causes, one of which probably is the 
union of the hydrochloric acid with the thio-urea in solution to 
form a definite compound, the equilibrium consequently being 
affected. 
The phenomena here described are, so far as I can discover, quite 
novel, and are of particular interest from their bearing on the 
