CH. VI} MICRO-CHEMISTRY 157 



Frequently a chemically pure salt cannot be made to yield satisfactory 

 crystals on the evaporation of its solution, but beautifully formed crystals will 

 result when in the presence of other compounds. A striking example is found in 

 Ammonium chlorid. This salt fails to yield other than dendritic masses when 

 preparations are made from the pure salt, but if formed by metathesis and 

 especially if in the presence of a difficultly crystallizable salt, well formed isome- 

 tric crystals (cubes) are seen. 



AN EXERCISE FOR PRACTICE 



Take a fragment of ammonium chlorid, dissolve in a tiny drop of water on a 

 slide and try to obtain distinct well formed crystals. Neither slow nor rapid 

 evaporation nor recrystallization by breathing on the preparation will yield satis- 

 factory crystals. 



Place a small drop of water on a glass slide, add Ferric chlorid until the drop 

 is distictly yellow. Stir. At the centre of the drop add two or three tiny frag- 

 ments of Ammonium acetate. The preparation must not be warmed. There is 

 formed Ferric acetate, Ammonium chlorid and double chlorids of ammonium and 

 iron. Study the preparation and observe the following points, i. Tendency 

 toward formation of double salt. 2. That the type crystal of NH 4 C1 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 

 dendritic masses observed in the first experiment. 7. The crystals often develop 

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

 pyramidal depressions. ' ' 



FIG. 127. Czapski's 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. Longitudinal 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 fiir physio- 

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

 Carpenter-Dalliuger ; 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 

 michrochemical analysis with an introductory chapter by J. W. Judd, London. 



