MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 277 



unless these are obtained the conclusion that sodium is present 

 is unwarranted. 



Potassium sulphate yields plates having a hexagonal or coffin- 

 like outline or six-pointed stars and rosettes. When first formed 

 these plates appear as circular disks but they rapidly acquire six 

 sides or grow into rosettes. Ammonium, rubidium and cesium 

 form similar hexagons and rosettes. 



When both sodium and potassium are present, the rod-like 

 crystals of the sodium double salt and the hexagons of the potas- 

 sium salt each appear, permitting a simultaneous detection of 

 sodium and potassium. 



The addition of a very minute quantity of glycerine to the 

 preparation before heating usually yields better crystals and 

 more reliable results. 



Precautions. 



Tufts of fine radiating needles appearing greyish or brownish 

 by transmitted light must not be regarded as indicating the 

 presence of sodium; neither should stout prisms or elongated 

 plates with forked or broken ends. 



It is always best to remove members of the calcium group by 

 means of sulphuric acid before applying the bismuth sulphate 

 test. Calcium is especially to be guarded against since calcium 

 sulphate may assume forms which simulate tlje sodium double 

 salt; for although the crystals CaSO4 2 H 2 O are monoclinic and 

 usually lie in positions yielding oblique extinction, the extinction 

 angle is small and unless care is exercised the student may credit 

 them with parallel extinction. 



Free mineral acids (especially nitric) greatly retard the sepa- 

 ration of sodium bismuth sulphate. 



In the absence of bismuth sulphate the reagent may be pre- 

 pared as follows: At the corner of a slide place a drop of dilute 

 sulphuric acid; add to this drop a little basic bismuth nitrate 

 and stir until the bismuth salt has completely dissolved. Heat 

 carefully until the water has been mostly expelled, and crystal- 

 lization of the bismuth sulphate takes place; then add a rather 

 large drop of water and a very minute drop of 'dilute nitric acid. 



