PHY3IOGBAPHY. CLASS u. 



According to BERZELIUS it is expressed by Ca F, and 

 composed of 72-14 of Lime, and 27'86 of Fluoric acid. Before 

 the blowpipe it decrepitates, and becomes phosphorescent, 

 but loses its colour, and melts at last into a rather opake 

 globule. It phosphoresces likewise, if thrown upon 

 ignited charcoal or heated iron. Several varieties, which 

 exhibit this phenomenon in particularly bright green colours, 

 have been called Chlorophane or Pyro-smaragdus. If ex- 

 posed to a too high degree of temperature, they lose the 

 property of again shewing this phenomenon. Sulphuric 

 acid decomposes the powder of the mineral ; fluoric acid 

 is disengaged in a gaseous state, and corrodes glass. Several 

 varieties, particularly the sky-blue ones, lose their colour, 

 on being exposed to light. 



3. This species does not enter as a regular constituent 

 into the composition of rocks. It is not very frequently 

 found in beds ; this, however, seems to be the case at Al- 

 stonmoor and Castleton ; some varieties occur in beds 

 of octahedral Iron-ore, pyramidal Copper-pyrites, &c. More 

 generally it is found in veins, with various minerals, in an- 

 cient and more modern rocks. Very seldom it is associated 

 with petrifactions, as the blue varieties of Derbyshire with 

 entrochites. 



4. Octahedral Fluor-haloide is common in some coun- 

 tries, while in others it is very rare. Among the first we 

 notice Saxony, some districts of the Hartz, and England, 

 among the last Scotland, Hungary, Transylvania, &c. 

 Most beautiful and interesting crystallized varieties are 

 found at Beeralston in Devonshire, at St Agnes and other 

 places in Cornwall, and at Zinnwald in Bohemia ; also 

 some rare colours, as for instance the sky-blue ones. Large 

 crystals, generally twins of the hexahedron, of beautiful 

 green and blue colours, occur at Alston in Cumberland, 

 which frequently contain water. Beautiful dark blue per- 

 fect crystals, of the form Fig. 148., have been found, in 

 small geodes, along with rhombohedral Quartz, near Gou- 

 rock in Renfrewshire. They here occur in porphy- 



