PORPEZITE. 



Transparent at the edges, or translucent. 

 Lustre vitreous ; on cleavage planes pearly. 

 Fracture uneven. H. 5-5. S.G. 2-65 to 268. 



Comp. 4A1 Si + 2Ca Si + Na Si + |KC] 

 (Schafhautl). 



Ayialysis, by Schafhautl : 



Silica . . . ... 49-2 



Alumina . . . . 2r3 



Lime 15-4 



Soda 6-5 



Chloride of potassium . .1-2 

 Water . . . . .1-2 



100-8 



BB fuses with tolerable facility, swelling 

 up and forming a colourless blistered glass. 



Decomposed by strong acids, without the 

 formation of a jelly. 



Locality. Obernzell, in Bavaria. 



Brit. Mus., Case 30. 



PoRPEZiTB. See Oro Pudre. 



PoRRicm. See Pyroxene. 



PoRTiTE, Meneghini, Bechi. Tn radiated 

 masses, with a very distinct rhombic cleav- 

 age. Colour opaque-white. Lustre vitreous. 

 H. 5. S.G. 2-4. 



Comp. (Mg Ca)3 Si3 + 4Al Sl2 + 7H, or 



A\ Si2 + 2H— if the protoxides are not taken 



into consideration. 

 Analysis, by Bechi : 



Silica 58-12 



Alumina .... 27-50 



Magnesia .... 4-87 



Lime 1-76 



Soda 0-16 



Potash 0-10 



Water 7-92 



100-43 



BB fuses "with much intumescence to a 

 milk-white enamel. 



Soluble in acids, forming a jelly. 



Locality. Tuscany, in gabbro rosso. 



Name. After Mons. Porte, of TuscauA^ 



PoRZELLANERDE, Werner. See Kaolin. 



Potash Alum. Found native in a few 

 minerals, but generally as an efflorescence. 

 Colour white. Transparent to opaque. Lus- 

 tre vitreous; of fibrous varieties somewhat 

 pearl3^ Taste astringent and sweetish. H. 

 2 to 2-5. S G. 1-75. 



Comp. Sulphate of alumina and potash, 



or (k S + 6H) + (Al S3 + 18H) Kane -sul- 

 phate of potash 18-4, sulphate of alumina 

 86-2, water 45-4 = 100. 



Soluble in from 16 to 20 times its weight 



POTSTONE. 297 



of cold water, and in slightly more than its 

 own weight of boiling water. When gra- 

 dually heated to a temperature just below 

 redness, it loses water, swells up very much, 

 and leaves a residue of anhydrous alum 

 (burnt alum), which forms a loose, friable, 

 porous mass. 



Localities. — English. Near Whitby in 

 Yorkshire, in Lias (alum-shale). Chudleiglx 

 in Devon, in clay, and in the Lower Bagshot 

 clays of Branksea Island, and Isle of Pur- 

 beck, Dorsetshire. — Scotch. Hurlet, near 

 Paisley ; in shale. — Irish. Along the 

 coasts of Clare and Kerry. Most of the 

 caves at Ballybunnion in Kerry are coated 

 with crystals of Alum of a whitish or 

 pale yellowish- white colour. (F. J. Foot.) 

 — Foreign. The volcanoes of the Lipari 

 Islands, Sicily, and the Azores. Hesse and 

 the Rhine, in the Brown Coal formation. 

 Silurian alum-slate of Sweden and Chris- 

 tiana in Norway. Cape Sable, Maryland, 

 U. S. 



Brit. Mus., Case 55. 



The alum of commerce is manufac- 

 tured from the aluminous earth of volcanic 

 regions, or from alum-shale, by disintegrat- 

 ing them by exposure to the air for a con- 

 siderable time, or by roasting, during which 

 process the sulphur becomes converted into 

 sulphuric acid. The sulphate of alumina 

 thus formed, together with the sulphates of 

 iron, is extracted by digestion in water, and 

 the latter being removed, potash is added to 

 the purified sulphate of alumina. 



Alum is made from the Dorsetshire and 

 other clays, by treating them with sulphuric 

 acid. The solution of sulphate of alumina 

 thus obtained is evaporated to a certain 

 point, and then mixed with ley from wood 

 ashes, crude potash, or other substances 

 containing potash. The potash-alum which 

 then crystallizes out may be afterwards 

 purified by repeated crystallization. 



Alum is used in medicine as an astringent, 

 and to prevent putrefaction : as a mor- 

 daunt in dyeing and calico printing ; in the 

 manufacture of paper and leather, and for 

 rendering wood incombustible. 



Potash Copperas. See Gelbeisenerz. 



Potash Harmotome. See Phillipsite. 



Potash Mica, Nicol. See Muscovite. 



Potasse Nitratee, Haiiy. See Nitre. 



PoTAssE SuLPHATEE, Haiiy. See Gla- 



SERITE. 



PoTSTONE. A coarse granular variety of 

 Steatite, or Soapstone, with a greenish-grey 

 or leek-green colour, and a feeble or pearly 

 lustre. Unctuous to the touch. Tough. 



