324 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 

 asPbI 2 .PbO; PbI 2 . 2 PbO (?). 



Lead iodide can be recrystallized 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 ioolide dissolves, and on 

 cooling crystals of the normal iodide Pbl2, the normal chloride 

 PbCl 2 and a chloriodide PbCl 2 - PbI 2 or 2 PbCl 2 - PbI 2 (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. 



