1932 



Journal of Applied Microscopy 



best results, and record the distances of lamp and microscope from the con- 

 denser for future reference. Now slide the lamp carrier slowly toward or from 

 the condenser, noting the point at which the best light is obtained. Keep the 

 arc as steady as possible by due attention to the feed. Time spent in thoroughly 

 testing the apparatus is not wasted, if records of the best results are made so 

 that the same arrangement of parts can be made at any subsequent time by 

 measurements. A. H. Cole. 



University of Chicago. 



MICRO-CHEMICAL ANALYSIS. 

 XX. 



SILVER GROUP CONTINUED— LEAD. 



///. Pfltassiitm Iodide added to solutions of salts of Lead precipitates Lead 

 Iodide. 



Pb(N03)2 + 2KI = Pbl,, -\- 2KNO3. 



Method. To the test drop, which should not be too dilute, add a tiny frag- 

 ment of the reagent. A bright yellow precipitate is at once formed in a circular 



band about the reagent fragment. The circle 

 gradually becomes larger and larger and at its 

 outside circumference beautiful hexagonal 

 plates appear. These plates and fiakes of lead 

 iodide appear greenish or brownish yellow by 

 transmitted light, sometimes even gray, ac- 

 cording to their thickness. By reflected light 

 lead iodide plates glow and glisten and display 

 the iridescent colors of thin films, an extremely 

 characteristic feature of this salt. 



These hexagons of lead iodide do not 

 belong, according to Behrens, to the hexagonal 

 system as usually stated, but are probably 

 orthorhombic, this view being based upon the 

 fact that the hexagonal plates of lead iodide are dichroic. 



The usual forms assumed by lead iodide are shown in Fig. 82. 

 Remarks. An excess of the reagent must be avoided, otherwise the precipi- 

 tate at first formed will be dissolved because of the formation of a double iodide 

 of the composition Pblo • 2KI • A'H^O.^ Not infrequently colorless crystals of 

 this double iodide will be seen in the immediate neighborhood of the reagent 

 fragment. The addition of a drop of water will usually cause the decomposi- 

 tion of the double salt and a precipitation of the normal iodide. 



Double iodides of lead with many elements are known, most of them crys- 

 tallizing readily-, but it is not often that there will be a sufficient separation of 



i M I I I 

 Fig. 82. 



1 Brooks, Chem. N., i8g8, 191. 



'-See Mosnier, Ann. cliim. phys. (7) 12, 374. 



Comptes rend. 1 20, 444. 



