182 ELEMENTARY CHEMICAL MICROSCOPY 



Cleavage dependent upon crystal species. 

 Biaxial. 



Extinction parallel in two positions, oblique in the third. (This does not apply 

 to sections through a crystal.) 

 Three indices of refraction. 



TRICLINIC SYSTEM. 



Three crystallographic axes, all of unequal length and oblique to one another. 

 There are no axes of symmetry. 



Cleavage dependent upon crystal species. 



Biaxial. 



Extinction oblique in all three positions. 



Three indices of refraction. 



EXPERIMENTS DEALING WITH CRYSTAL FORMS AND OPTICAL 



PROPERTIES. 



Center the stage and test graduations and accuracy of polar- 

 izing apparatus. 



Study the following salts, make sketches and note their be- 

 havior between crossed nicols: 



Isometric System. 



Sodium chloride; potassium iodide; barium nitrate; ammonia alum; chrome 

 alum; arsenic trioxide; sodium chlorate (circular polarization). 

 Hexagonal System. 



Lead iodide; iodoform; cadmium iodide; normal sodium phosphate; strontium 

 chloride; strontium antimonyl tartrate; sodium nitrate. 

 Tetragonal System. 



Potassium arsenate; mercuric cyanide; potassium copper chloride; urea; 

 strychnine sulphate. 

 Orthorhombic System. 



Ammonium sulphate; mercuric chloride; potassium antimonyl tartrate; po- 

 tassium nitrate; potassium sulphate; sodium nitroprusside; zinc sulphate; uranyl 

 acetate. 

 Mono clinic System. 



Potassium ferrocyanide; potassium ferri cyanide; sodium ferric oxalate; am- 

 monium persulphate; potassium chlorate; barium chloride; nickel chloride; 

 tartaric acid; saccharose. 

 Triclmic System. 



Copper sulphate; potassium bichromate; potassium persulphate; boric acid; 

 manganous sulphate. 

 Pleochroic Salts. 



Copper acetate; iodoquinine sulphate; potassium (or sodium) ferric oxalate; 

 potassium cobalt sulphate; silver bichromate; potassium chromium oxalate. 



In a watch glass place a few drops of benzene, add a few crystals of quinone, 

 stir until dissolved. Add a few crystals of resorcin, stir. Remove a drop to a 

 slide and allow it to deposit crystals by spontaneous evaporation. The crystals 

 will be found to be strongly pleochroic. 



Note. The experiments outlined above have dealt with normal well-formed 

 crystals whose habits could be ascertained with but little difficulty. Abnormal 



