296 ELEMENTARY CHEMICAL MICROSCOPY 



Tetragonal. 



Orthorhombic. Chromate (O or M). 



Monoclinic. Chloride; chlorate; bromide; ferro- 



cyanide; acid-oxalate. 

 Tridinic. Acetate. 



DETECTION. 



A. By Means of Sulphuric Acid. 



Read fully the directions and comments under Calcium 

 and Strontium, pages 289 and 290, and 293 and 294. 



The amorphous or semicrystalline precipitate first obtained 

 must be recrystallized from concentrated sulphuric acid before 

 identification is possible. The recrystallized salt appears at 

 first as tiny rectangular plates and X-like crystallites. In this 

 stage of development it may be mistaken for strontium sulphate. 

 Continue breathing upon the drop of acid; under the influence 

 of the moisture absorbed the crystallites grow rapidly, still 

 retaining their X-like shape but the arms of the X's become 

 feathered. There is a marked tendency for two adjacent arms 

 of the X to develop much more rapidly than the other two. 

 These crystallites grow relatively large and are constant and 

 peculiar to barium. 



In the presence of certain acids or acid salts, especially from 

 hot solutions, crystallites of barium sulphate may sometimes be 

 obtained immediately upon the addition of dilute sulphuric acid. 



In the event of a heavy precipitate being obtained with the 

 reagent, it is wise to remove a small portion to another slide for 

 crystallization, rather than attempt to dissolve the whole mass. 



Recrystallization in the presence of much calcium is to be 

 avoided. First extract the calcium sulphate with hot water. 



In the presence of moderate amounts of strontium the crys- 

 tallites of barium sulphate are generally not well formed. If 

 strontium is in excess, the crystals separating from the hot sul- 

 phuric acid have the general type of strontium sulphate, 'but are 

 not well developed and exhibit an inclination to approach the 

 X-forms of barium sulphate. For this reason it is advisable to 

 remove any strontium which may be present by repeatedly 



