CH. VI~\ MICRO-CHEMISTRY 157 



AN EXERCISE FOR PRACTICE 

 1. "Sodium chlorid. Isometric system 



a. Take a fragment of the salt the size of this period (.) or a little 

 smaller. Dissolve at the corner of a slide in a minute drop of distilled water. 

 Heat over the "micro" flame so as to cause rapid evaporation. When dry 

 examine residue (after cooling) with yi inch obj. and 2 in. eyepiece. 



Breathe on the preparation several times, allow the moisture to evaporate 

 spontaneously. There should now be obtained well developed crystals instead 

 of a mere crystalline mass. Too rapid crystallization is always to be avoided. 

 The object of the experiment is to emphasize this fact. 



Select one of the best crystals. Measure its angles. Try its behavior 

 between crossed Nicols. (§ 219. ) 



b. Place a small drop of water in the usual position add carefully Ferric 

 •Chlorid till the drop when held over white paper appears distinctly 

 yellow. Stir. Add two or three tiny fragments of Sodium Acetate at 

 the center of the drop. Place on the stage of the microscope. There is 

 formed Ferric acetate, Sodium chlorid and possibly a double chlorid of iron 

 and sodium. Notice the following points. 1. Tendency toward formation 

 of double salt. 2. That the type crystal of NaCl is a cube. 3, Cubes may 

 so grow as to present the appearance of a rectangular prism. 4. In certain 

 positions cubes have the appearance of a pyramid. 5. In other positions they 

 exhibit a hexagonal outline, thus simulating a polyhedron of many faces. 

 6. There is scarcely any tendency in this case toward the formation of the 

 hoppers observed in the first experiment (a). 7. The crystals often develop 

 fastest along the diagonal planes so that the regular faces are replaced by 

 pyramidal depressions." 



Fig. 127. CzapskVs Ocular Iris-diaphragm with cross 

 hairs for examining and accurately determining the axial im- 

 ages of small crystals. The iris diaphragm enables the observer 

 to make the field as large or small as desired. 



A. Lo7igitudinal section. 



B. Transection, showing the cross lines and the iris dia- 

 phragm with the projecting part at the left, by which the dia- 

 phragm is opened and closed. (Zeiss' Catalog). 



For directions and hints in micro-chemical work and crystallography, consult 

 the various volumes of the Journal of the Roy. Micr. Soc. ; Zeitschrift fur physio- 

 logische Chemie, and other chemical journals ; Wormly ; Klement & Renard ; 

 Carpenter-Dallinger ; Hogg ; Behrens, Kossel und Schiefferdecker ; Frey ; Dana, 

 and other works on mineralogy ; Davis. Behrens, T. H. — Anleitung zur micro- 

 chemischen Analyse der wichtigsten organischen Verbindungen. Hamburg, 

 1895-1897. Microchemische Technik, 2d edition, Hamburg, 1900. A manual of 

 microchemical analysis with an introdnctory chapter by J. W. Judd, London, 



