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VII. On the Laws of Connexion between the Conditions of a Chemical Change and its 
Amount. By A. Vernon Harcourt, M.A., Student of Christ Church, and Demon- 
strator of Chemistry jn the University of Oxford, and William Esson, M.A., Fellow 
of Merton College, Oxford. Communicated by Sir B. C. Brodie, Bart., F.B.S., 
Professor of Chemistry in the University of Oxford. 
Received July 13, — Read November 22, 1866. 
II. On the Beaction of Hydric Peroxide and Hydric Iodide. 
The reaction of hydric permanganate upon hydric oxalate, which formed the subject of 
the first part of this inquiry*, having proved to be of a complex character, consisting in 
fact of several distinct reactions, it became necessary to seek for investigation a simpler 
case of chemical change. The reaction selected must at the same time combine all the 
other qualifications before enumerated, that it might be possible successively to vary its 
conditions and to measure its conditions and its amount. 
After making trial of several reactions which appeared suitable, and being as often 
foiled by some practical difficulty in the proposed methods of investigation, we at last 
succeeded in devising for a very simple case of chemical change a method of investigation 
at once easy and exact. The reaction is that of hydric peroxide and hydric iodide, 
H 2 0 2 +2HI=2H 2 0+I 2 . 
When solutions of potassic iodide and sodic peroxide are brought together in presence 
either of an acid or an alkaline bicarbonate, a gradual development of iodine takes 
place. If sodic hyposulphite be added to the solution it reconverts the iodine, as soon 
as it is formed, into iodide, but appears in no other way to affect the course of the 
reaction. Consequently, if the peroxide be in excess over the hyposulphite, the whole 
of the latter is changed by the action of nascent iodine into tetrathionate, while the 
amount of iodide remains always constant ; and after this conversion of the hyposulphite 
is complete, free iodine makes its appearance in the solution f. The moment at which 
* Philosophical Transactions, 1866, p. 193. 
t A solution of sodic hyposulphite may be mixed with a large volume of a dilute solution of potassic iodide 
and hydric sulphate or chloride without undergoing any decomposition. It is not oxidized to sulphate, nor acted 
upon in any way in this solution by hydric peroxide ; for its decomposition is accompanied by a formation of 
sulphur, which even in very minute quantity would produce a perceptible opalescence in the liquid under obser- 
vation. When hydric chloride has been employed to acidulate the solution, the addition of barium chloride after 
or during the set of experiments produces no precipitate. The quantity of sodic hyposulphite in the solution 
varies in each experiment from the maximum quantity to zero ; the progress of the reaction is unaffected by 
this variation. 
MDCCCLXYI1. 
R 
