302 



SALTS. 



the change of color proceeds, the small scales of which 

 the yellow iodide is composed break up into octa- 

 hedrons. The change of color is regarded as a conse- 

 quence of the re-arrangement of atoms, which produces 

 the change of form. 



FLUORIDES. 



What is said 760. FLUOR-SPAR. The fluorides, with 

 of 'fluor-spar ? fa e exception of those of the alkalies, are 

 for the most part, white insoluble compounds. The 

 only one of especial interest, is the beautiful mineral 

 knoivn as fluor-spar. This mineral is a fluoride of 

 calcium. It is found of white, green, purple 

 and rose color, crystallized in regular cubes 

 or octahedrons. Hydrofluoric acid, which has 

 the remarkable property of etching glass, as 

 before described, is prepared from it. 



SULPHURETS. 



Define a sul- ?^- The compounds of the metals with 

 phuret. sulphur are called sulphides or sulphur ets. 



They are of various colors, and, for the 

 most part, insoluble. Iron pyrites, and ga- 

 lena or sulphuret of lead, are examples. 

 The figure represents a crystal of magnetic pyrites, 

 which is one of the sulphurets of iron. The form be- 

 longs to the sixth or hexagonal system. 



762. PREPARATION. Most of the sul- 



How are sul- 

 phurets gene- phurets may be produced by adding hydro- 



p a aredT~ sulphuric acid to solutions of the different 

 metals or their salts. Sulphur and metal 



