288 ELEMENTARY CHEMICAL MICROSCOPY 



B. ANISOTROPIC. 



Hexagonal. Carbonate (H or 0) ; chloride. 



Tetragonal. Oxalate. 



Orthorhombic. Ar senate (0 or M); chroma te 



(0 or M); tartrate. 

 Monodinic. Nitrate; sulphate; double sulphates 



of calcium and sodium or potassium. 

 Triclinic. Ferrocyanide. 



DETECTION. 



A. By Means of Dilute Sulphuric Acid. 

 Apply the reagent by Method /, page 251. 



If calcium is present, monoclinic crystals of calcium sulphate 

 will rapidly appear near the circumference of the drop of the 

 substance. These crystals take the form of exceedingly slender, 

 colorless, transparent needles, either singly, in sheaves, in 

 bundles or in star-like clusters. When in tiny sheaves near the 

 edge of the drop the crystals have often a more or less brownish 

 tint when seen by transmitted light. Shortly after the appear- 

 ance of the bunches of needles at the periphery, long, thin, 

 slender and plate-like prisms with obliquely truncated ends are 

 formed throughout the drop. These prisms are frequently 

 twinned, yielding so-called arrowhead or swallow-tailed and 

 X-like twins. These twin crystals are the most characteristic 

 of the forms assumed by calcium sulphate of the formula 

 CaS0 4 2 H 2 0. 



If no crystals are visible after waiting a short time, the prepa- 

 ration may be cautiously concentrated. This procedure (evapo- 

 ration) may, however, lead to the separation of such an amount 

 of other salts as to render difficult the detection of the crystals 

 of calcium sulphate. A better plan is to hasten the separation of 

 the calcium salt by exposing the test drop to the vapor of 

 alcohol; see page 257, Method VI. 



Salts of strontium may, under exceptional conditions (if the 

 preparation be examined at once), yield a precipitate which 

 closely resembles that given by calcium. These crystals of 

 strontium sulphate rapidly disintegrate, however, and there 



