IODIDE OF POTASSIUM. 
Ill 
producing a brown colour in the liquid) just sufficient be 
added to neutralize the potassa, upon passing hydro-sulphuric 
acid through the solution, there will be no free hydriodic 
acid formed, and the subsequent addition of potassa will not 
be necessary. 
One great objection to this process is the employment of 
hydro-sulphuric acid, which, besides the trouble and expense 
of forming it, has such an exceedingly offensive odour, as to 
make it desirable to dispense with it. 
The formula adopted by the Dublin College is as follows: 
" Take of iodine one part; sulphuret of iron, in coarse 
powder, five parts; sulphuric acid, seven parts; distilled water 
forty-eight parts; water of carbonate of potassa, a sufficient 
quantity; rectified spirit, six parts. Mix the iodine by tri- 
turation with sixteen parts of the water, and put the mixture 
into a glass vessel. Pour the acid, previously diluted with 
thirty -two parts of the water, on the sulphuret, contained in a 
matrass; and by means of a tube adapted to the neck of the 
matrass, and reaching to the bottom of the vessel containing 
the iodine and water, transmit the gas through the mixture, 
until the iodine entirely disappears. Filter the liquor, and 
immediately evaporate it, by a superior heat, to one-eighth 
part, and again filter it. Then gradually add as much water 
of carbonate of potassa, as will be sufficient to saturate the 
acid, which is known by the cessation of the effervescence. 
Then expose the mixture to heat, until the residual salt is dry 
and of a white colour. On this pour the spirit, and dissolve 
by the aid of heat. Lastly, from the remaining salt, pour off 
the solution, evaporate it to dryness, and keep the residuum 
in a close vessel.'^ 
The first step in this process is to obtain a solution of hy- 
driodic acid. This is done by passing a stream of hydro- 
sulphuric acid through the water in which the iodine is 
diffused. The affinity of iodine for hydrogen being greater 
than that of sulphur for the same element, the acid is decom- 
