110 
ORIGINAL COMMUNICATIONS. 
"Take of solution of potassa two pints; iodine a sufficient 
quantity. Apply a gentle heat to the solution, and add, by 
degrees, sufficient iodine to saturate the potassa, and to impart 
a brown colour to the liquid. Then pass hydro-sulphuric 
acid through the solution, in a proper vessel, till it loses its 
brown colour, and retains the odour of the acid. Filter 
through paper, and having poured hot water upon the residue, 
again filter. Boil the filtered liquors for a short time, that 
the hydro-sulphuric acid may be driven off ; then, if sulphur 
has been precipitated, remove it, and saturate any acid that may 
be present with solution of potassa. Lastly, boil the liquor 
to dryness. Hydro-sulphuric acid is obtained from sulphuret 
of iron, by the addition of sulphuric acid diluted with four 
times its weight of water." 
The rationale of this process, on the assumption that the 
iodide of potassium becomes hydriodate of potassa by solution, 
may be explained as follows. On adding iodine to a solution 
of potassa, water is decomposed; one atom of iodine unites 
with five atoms of oxygen, forming one atom of iodic acid, 
whilst five atoms of iodine unite with five atoms of hydrogen, 
forming five atoms of hydriodic acid; these acids, then, seve- 
rally unite with the potassa, forming iodate and hydriodate of 
potassa. The proportion of the former salt to the latter, 
(adopting the atomic weights as given by Turner,) is as 
1423 to 5815, or as one to four and one-twelfth nearly. On 
passing hydro-sulphuric acid through a solution of these salts, 
it is decomposed, its hydrogen uniting with the iodine and 
oxygen of the iodate, forming hydriodic acid and water, whilst 
its sulphur is precipitated. The excess of iodine, which the 
solution at first contains, is at the same time converted into 
hydriodic acid, which is directed to be saturated with solution 
of potassa. The solution, having by these means been con- 
verted into one exclusively of hydriodate of potassa, when 
evaporated to dryness, yields iodide of potassium, in conse- 
quence of the hydriodate losing its oxygen and hydrogen. 
If instead of adding an excess of iodine (as indicated by its 
