220 



lioness — Etching Figures of the 



crystal is allowed to lie in the cold 25-percent citric acid 

 for twenty minutes, Ihe base is well etched with very dis- 

 t inct figures (fig. 18) ; these are very much like those pro- 

 duced by the hot citric acid, but differ by possessing' sharp 

 angles. The sides of the figures are_yery straight and 

 oriented parallel to the edge 0001/1010. Like the HC1 

 figures they are regular, triangular pyramidal pits and by 



0112 



0001 



^ 



1120 



^7 



1010 



1120 



Dla.A 



Dia.B 



their form and position indicate the three vertical planes 

 of symmetry characteristic of the calcite type. Diagram 

 B represents calcite as etched by citric acid. 



Thus five of the seven possible forms of calcite have 

 been etched with three or four different solvents, — sul- 

 phuric acid and acetic acid having been tried and found 

 less desirable, and although there has been a great varia- 

 tion in the form of the figures produced, each shows with 

 unerring accuracy the symmetry of the face upon which it 

 occurs, and the symmetry requirements of a calcite crys- 

 tal as determined geometrically have been fully met in 

 the etch figure. 



