1 
112 ORIGINAL COMMUNICATIONS. 
posed, its hydrogen unites with the iodine, and its sulphur 
is precipitated. The sulphur having been separated by 
filtration, the solution is evaporated to one-eighth part, and 
then again filtered. This second filtration is, however, 
unnecessary, unless there be a new deposition of sulphur, 
which, perhaps, is rarely the case. The hydriodic acid 
having been considerably concentrated, is neutralized with 
carbonate of potassa, and then evaporated to dryness. The 
residue is then dissolved in the rectified spirit, and again 
evaporated. This last operation is intended to remove any 
carbonate or iodate of potassa with which the previous product 
may have been contaminated. 
This College directs a matrass to be used as the vessel in 
which the gas is to be generated, and the whole of the sul- 
phuric acid to be poured on the sulphuret of iron at once; but 
a better way is to use a Wolfe's bottle, fitted with a safety 
tube, and a tube to convey the gas into the water containing 
the iodine. The sulphuret of iron and water having been 
introduced into the bottle, the sulphuric acid should be poured 
through the safety tube by small portions at a time. By 
these means we may avoid producing more hydro-sulphuric 
acid than would be necessary to form the hydriodic acid. 
The formation of the hydriodic acid, the slow evaporation, 
and the solution in alcohol, render this process so troublesome 
and dilatory, that it is, perhaps, seldom employed. 
Both Pharmacopoeias direct the solution to be evaporated 
to dryness; but the article has a much better appearance when 
in crystals, in which slate, also, it is not so liable to contain 
impurities or adulterations. 
Another process which is recommended for procuring the 
iodide of potassium, is, first to form a solution of hydriodate of 
zinc or of iron, and then to precipitate the oxide or carbonate 
of these metals by a quantity of potassa or carbonate of potassa, 
exactly sufiicient for combining with the hydriodic acid. 
The precipitate having been removed by filtration, the solu- 
tion of hydriodate of potassa is evaporated to dryness or 
crystallized, as before. The greatest difficulty of this process 
