334 
ELEMENTARY CHEMICAL MICROSCOPY 
B. ANISOTROPIC. 
Hexagonal. — Carbonate (H or O); chloride. 
Tetragonal. — Oxalate. 
Orthorhombic. — Arsenate (O or M); chromate 
(O or M); tartrate. 
Monoclinic. — 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 I, page 299. 
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 appearance 
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, yield¬ 
ing 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 CaS04 • 2H2O. 
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 305, 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 
results a fine granular deposit. This granular or sandy deposit 
