1901 - 2 .] Dissociation of Compound of Iodine and Thio-urea. 235 
The di-iodide was prepared by gradually adding 5 parts of 
iodine crystals to 3 parts of powdered thio-urea (1 mol. I 2 to 2 mols. 
CSNoH 4 ) mixed with about 25 parts of water. The iodine rapidly 
dissolves in the solution of thio-urea, forming at first a colourless 
solution, and the liquid becomes warm ; as more iodine is added, the 
whole of the thio-urea goes into solution, and prismatic crystals of 
the di-iodide soon separate, especially if the liquid is cooled — a 
precaution which is advisable in view of the slight stability of all 
these salts. After all the iodine has been added and the liquid 
thoroughly cooled, the mother liquor possesses only a pale brown 
colour, unless excess of iodine has been employed. It is now only 
necessary to filter, draining thoroughly, and dry the crystals 
between filter paper ; they are practically colourless. On standing 
for some time the mother liquor deposits sulphur, along with 
clear yellow crystals of some compound which has not yet been 
analysed. 
When water is added to the crystals of di-iodide, they dissolve, 
forming a yellowish solution, which is distinctly acid to litmus. 
Whether this acidity is the result of hydrolysis, or is due to a 
slight amount of decomposition of a more profound character, it is 
impossible to say at present. The yellow-brown coloration, due 
to the liberation of iodine, is not very strongly marked, and 
becomes less intense on dilution of the solution with more water. 
This is not merely an apparent diminution, due to the increased 
quantity of solution ; for, if a concentrated solution is divided into 
two equal portions in similar tubes, and one of them is diluted 
with water, the colour of the latter appears much feebler than the 
other when viewed along the axis of the tube. If the solution is 
gently warmed, the colour is intensified and changes back on 
cooling ; but this experiment must be carried out cautiously and 
quickly, otherwise decomposition takes place. The addition of 
thio-urea to the solution diminishes or destroys the colour. 
The decrease in the quantity of free iodine, which takes place 
on dilution, points to the occurrence of some action other than 
mere dissociation into iodine and thio-urea, when the di-iodide 
is dissolved in water ; for, if we represent the latter action by the 
equation 
(CSN 2 H 4 ) 2 I 2 ^ 2CSK,H 4 + 1 2 
