MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 289 



results a fine granular deposit. This granular or sandy deposit 

 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 BaSC>4 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 Ag 2 SO 4 in the form of colorless, highly 

 refractive, orthorhombic prisms, rhombs or crystallites of char- 

 acteristic appearance. 



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. 



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 ag- 

 gregates 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 

 minutes will usually permit the crystals to develop and their 

 appearance will then be such as to avoid error. If, however, the 

 analyst is still in doubt he may proceed as follows : After allowing 

 sufficient time for the separation of almost all the calcium as 

 CaSO4 2 H 2 O, draw off the supernatant liquor, add to the residue 



