236 Proceedings of Royal Society of Edinburgh. [sess. 
the quantity of free iodine ought to increase, corresponding to' 
increase in the degree of dissociation, with diminution of the con- 
centration. As the balance shifts in the opposite direction, we 
must assume that the undissociated di-iodide is being used up by 
some other reaction, which does not result in the liberation of 
iodine, so that the dissociated products will reunite to restore the 
equilibrium. The nature of such a second reaction is suggested 
by the saline character of the compound, and the phenomena are 
explicable on the assumption that in aqueous solution the di-iodide 
undergoes ionisation, and that the increased ionisation produced by 
dilution is the cause of the diminished dissociation. In addition 
to the first balanced action we may therefore assume the second 
one, represented by the equation 
(CSN 2 hM 2 =^ (CSN 2 H 4 y • + 21' . 
This assumption that the di-iodide and its analogue are true 
salts is justified by the readiness with which they undergo double 
decomposition with other salts, as exemplified by the precipitation 
of the sparingly soluble di-nitrate. Further, the aqueous solution 
of the di-iodide dissolves free iodine, like solutions of metallic 
iodides ; it precipitates lead iodide and silver iodide from solutions 
of lead and silver salts ; with mercuric chloride it gives a pre- 
cipitate of mercuric iodide, soluble in excess of the di-iodide 
solution. 
If the above explanation of the diminished dissociation is a 
correct one, it is evident that any diminution in the amount of 
ionisation will result in an increased liberation of iodine. A number 
of experiments bearing on this point were tried in order to test the 
theory, and in all cases the results were of a confirmatory nature. 
In the first place, the experience of previous investigators with 
organic solvents, already referred to, was satisfactory in that 
respect, for alcohol and ether are deficient in ionising power. 
Other organic solvents which dissolve the di-iodide give dark 
solutions ; those tried included methyl alcohol, acetone, and glacial 
acetic acid. 
It has been shown by Carrara * that, as a rule, methyl alcohol 
* Journ. CJiem. Soc. (Abs.), 72, ii. "471 (1897). 
