234 
SELECTED ARTICLES. 
the mother waters, to the state of sulphates. It is very easy 
to understand that these compounds are transformed into 
sulphates by taking a small quantity of the calcined matter, 
and pouring upon it sulphuric acid in excess. It ought not to 
give rise either to disengagement of sulphuretted hydrogen or 
a deposition of sulphur when the transformation is complete. 
If, during calcination, violet fumes are disengaged, it is 
necessary to moderate the action of the heat in order to avoid 
the loss of the iodine. 
When the sulphurets are entirely decomposed, the residue 
is dissolved in a sufficient quantity of water to obtain a solu- 
tion at 36° of the areometer. 
Into this solution a current of chlorine gas is made to pass, 
taking care to agitate it continually with a glass rod; the 
liquid becomes deeply coloured, then turbid, and allows the 
iodine to deposit under the form of a black powder; it is 
collected and distilled in a glass retort, in order to obtain it 
in the crystallized state, as it is found in commerce. The 
only difficulty which this preparation presents, is that of pro- 
perly seizing the point when the action of the chlorine should 
be arrested, so as not to make it pass in excess and re-act upon 
the precipitated iodine. This excess of chlorine is to be 
especially guarded against, when it is desirable to extract from 
the same mother waters the bromine they contain. It is re- 
quisite in order to avoid adding an excess of chlorine, to allow 
the liquid to settle when it is supposed to have reached the 
point of saturation, to withdraw the current of chlorine, and 
direct the gas upon the surface of the liquid; for as long as 
the iodide is held in solution, a pellicle of iodine will be seen 
to form upon it, which effect no longer takes place when all 
the iodine is precipitated; in this last case the liquid becomes 
clear rapidly, and only retains a slight reddish tint. 
The extraction of bromine, which is ordinarily practised, 
also presents great difficulties which can be avoided by the 
following method: 
This method is very analogous to the preceding. It is, like 
it, based upon the elective affinity of chlorine, and upon the 
