MICROCHEMICAL REACTIONS OF CALCIUM 335 



is the form assumed by strontium sulphate under the conditions 

 which ordinarily obtain in this test. Barium is immediately pre- 

 cipitated in an exceedingly finely divided condition, amorphous 

 in appearance, but occasionally BaSO-t separates in crystalline 

 form (see Barium). 



Any lead which may be present will also be precipitated as a 

 dense white amorphous powder. Occasionally, however, lead 

 will yield a precipitate consisting of orthorhombic crystals. 



Silver will separate as AgoSOi in the form of colorless, highly 

 refractive, orthorhombic prisms, rhombs or crystallites of char- 

 acteristic appearance. 



Bismuth sometimes gives a crystalline sulphate closely resem- 

 bling that of calcium. The acicular crystals are larger, however, 

 and sheaves are usually absent. 



When the drop of sulphuric acid flows into the drop to be 

 tested which contains mercurous nitrate or other soluble mercu- 

 rous salts, the mercurous sulphate produced often assumes at 

 first the form of acicular needles, closely resembling those of 

 calcium sulphate; they are, however, blackish by transmitted 

 light and rapidly take the shape of rod-like prisms quite distinct 

 from the prismatic forms of the calcium salt. 



Bismuth may yield needles closely resembling those of CaS04 • 

 2 H2O, but there also appear hair-like, curving forms (trichites) ; 

 moreover the prisms and needles fail to exhibit truncated ends, 

 so characteristic of calcium sulphate. 



Precautions. 



Before applying the sulphate test, add a drop of dilute hydro- 

 chloric acid to assure the absence of lead, silver and mercurous 

 salts. If a precipitate is formed decant. 



It is not always wise to conclude that calcium is present when 

 crystals, which apparently resemble the star- and sheaf-like 

 aggregates of calcium sulphate, separate at once on the addition 

 of sulphuric acid, even if the crystals exhibit oblique extinction. 

 It sometimes happens that other compounds, not calcium sul- 

 phate, separate in forms not to be distinguished, at first sight, 

 from the crystals of the calcium salt. Such instances are for- 

 tunately very rare. Allowing the preparation to stand a few 



