370 ELEMENTARY CHEMICAL MICROSCOPY 
Behrens, to the hexagonal system, as usually stated, but are 
probably only pseudohexagonal and in reality orthorhombic. 
From neutral solutions containing lead in the form of lead 
acetate, potassium iodide will generally precipitate, in addition 
to the normal iodide, basic iodides of variable composition, such 
as Pbl2 . PbO; Pbl2 • 2 PbO (?). 
Lead iodide can be recrystalHzed from hot water, best if acidi¬ 
fied with nitric acid. On cooling, large, beautifully formed 
hexagons separate. A large drop of water is necessary in order 
that good results may be obtained. 
Heated with hydrochloric acid lead iodide dissolves, and on 
cooling crystals of the normal iodide PbL, the normal chloride 
PbCb and a chloriodide PbCh.- PbL or 2 PbCb • PbL (or both) 
will separate. The chloriodides appear in the form of needles 
of a faint yellow color. 
Silver iodide separates as a yellowish amorphous mass insoluble 
in hot water and in hot nitric acid. 
Mercuric iodide takes the form of red rhombs. Mercurous 
salts acidified with nitric acid usually give in addition to the 
heavy precipitate of mercurous iodide the ruby colored rhombs 
of the mercuric salt. 
If cuprous salts are present a white granular precipitate of 
cuprous iodide is formed and iodine is set free. Cupric salts 
will behave similarly. 
Thallium is precipitated as an exceedingly fine granular pre¬ 
cipitate. 
Antimony and bismuth salts interfere with the reaction for 
lead. These elements yield with potassium iodide, double 
iodides which separate in neat, well-formed crystals. Solutions 
containing lead, antimony and bismuth, when treated with 
potassium iodide, yield a dark reddish brown, sandy precipitate 
wholly unlike in appearance anything obtained with the different 
elements alone. Boiling the mixed product with water will 
generally cause a partial decomposition, and on cooling hexagons 
and irregular plates of lead iodide will appear. In the presence 
of a little bismuth, lead iodide separates as orange red disks and 
plates, or the iodide scales may even appear crimson in color. 
