338 ELEMENTARY CHEMICAL MICROSCOPY 



changing to crystals of a great variety of forms, hexagonal 

 plates, six-sided plate-like prisms and stout prisms with obliquely 

 truncated ends. 



In the presence of stannic chloride Behrens has shown that 

 calcium oxalate assumes the form of tiny oval grains exhibiting 

 an octahedral tendency while strontium yields large clear-cut 

 beautifully developed tetragonal octahedra and barium gives 

 short stout prisms singly, in crosses and in radiating masses, 

 or if much barium is present, fusiform crystals and bundles of 

 radiating needles are seen. 



Precautions. 



Oxalic acid, under favorable conditions, can cause the separa- 

 tion of oxalates of the following elements: Gl, Ca, Sr, Ba, Mg, 

 Zn, Cd, Tl; rare earths; Sb, Bi, Sn, Pb, U, Mn, Fe, Ni, Co, 

 Cu, Ag. 



In the event of a precipitate of doubtful composition being 

 obtained, draw off the supernatant liquid, or separate by means 

 of the centrifuge, and add to the residue a tiny drop of dilute 

 sulphuric acid; calcium oxalate is dissolved and in a few seconds 

 the characteristic crystals of CaSO 4 2 H 2 O make their appear- 

 ance. 



Owing to the minute size of the crystals, testing for calcium 

 with oxalic acid is not always satisfactory. As an offset to this 

 disadvantage, chlorides of the trivalent metals, unless in con- 

 centrated solution, and boric acid have no effect other than a 

 retardation of the reaction. A small amount of free nitric acid 

 merely greatly retards the separation of the oxalates of calcium 

 and strontium, but prevents the formation of barium oxalate. 



EXPERIMENTS. 



a. Try reaction after the manner given above, on a salt of Ca in a neutral solution. 

 Try again in the presence of free HC1, then in the presence of free HNO 3 . 



b. Precipitate CaC2O 4 - 3 H2O,draw off the supernatant liquor and treat the residue 

 with dilute H 2 SO4. After examining the preparation, add more acid, and heat 

 until white fumes appear; cool; breathe upon the preparation and examine again. 



STRONTIUM. 



Crystal Forms and Optical Properties of Common Salts 

 of Strontium. 



