ORDEaVIl. DODECAIIEDUAL CORUNDUM. 297 



Silica 6-45 15-50 2-00. 



Magnesia 14-63 8-25 12-00. 



Oxide of Iron 4-26 1-50 16-00. 



Lime 0-00 BERZ. 0*75 KLAPR. 0-00. DESCOT. 



VAUQUELiy discovered 6-18 per cent, of chromic acid in 

 the red Spinelle. The pure varieties are designated by 

 BERZELIUS with Mg Al 4 , corresponding to 83-25 of alu- 

 mina and 16-75 of magnesia. Pleonaste contains, besides, a 

 proportion of oxide of iron. The red varieties, exposed to 

 heat, become black and opake ; on cooling, they appear first 

 green, then almost colourless, and at last re-assume their red 

 colour. They are difficultly fusible with borax, a little more 

 easily with salt of phosphorus. Pleonaste yields a deep 

 green globule. Dodecahedral Corundum assumes positive 

 electricity by friction. 



3. The original repositories of several varieties of the 

 present species are certainly rocks or mountain masses ; but 

 it is not known with certainty whether this be everywhere 

 gneiss, as it has been quoted for several localities. Crys- 

 tals from Ceylon have been observed imbedded in lime- 

 stoe, mixed with mica, or in rocks containing adularia, 

 which seem to have belonged to primitive rocks. Other va- 

 rieties, commonly called Pleonaste, occur in the drusy ca- 

 vities of rocks ejected by Vesuvius. The greater part, 

 however, of the varieties of dodecahedral Corundum are 

 found in more recent deposits, formed by diluvial or allu- 

 vial action, along with crystals of rhombohedral Corundum, 

 pyramidal Zircon, and other gems, also with octahedral 

 Iron-ore. 



4. Dodecahedral Corundum is principally brought from 

 Ceylon, where it occurs in isolated crystals in the sand, or 

 also imbedded in gneiss. All the regular forms mentioned 

 above, have been observed in this variety. In Sudermanland 

 in Sweden, blueish and pearl-grey varieties occur imbedded 

 in granular limestone. Pleonaste occurs in Ceylon in sand, 

 and in implanted crystals in Vesuvius. 



5. Clear and finely coloured red varieties of the present 



