POT 



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P R E 



the whole. Potash may he re- 

 garded as constituting between six 

 and seven per cent, of granites, 

 greenstones, and rocks of that class. 



POTA'SSITJM. A metal discovered by 

 Sir H. Davy in 1808. At a tem- 

 perature of 32 potassium is hard 

 and brittle, with a crystalline tex- 

 ture ; at 50 it becomes malleable, 

 with a lustre like that of polished 

 silver; and at 150 it fuses. Po- 

 tassium is lighter than water, its 

 specific gravity being 0'85 ; to 

 preserve it unchanged, it should be 

 kept in a phial with pure naptha. 



POTEEIOCEINI'TES. (from TroTrjpiov, 

 a cup, and Kplvov, a lily, Gr.) 

 Vase-like, lily-shaped animal. A 

 genus of Crino'idea, belonging to 

 the division crinoi'dea semiarticu- 

 lata, according to Miller's arrange- 

 ment. That author thus describes 

 the generic characters of Poterio- 

 crinites. " A crinoidal animal, 

 with a round column composed of 

 numerous thin joints, having in 

 their centre a round alimentary 

 canal, and articulating by surfaces 

 striated in radii. Hound auxiliary 

 side arms, proceeding at irregular 

 distances from the column. Pelvis 

 formed of five pentagonal plate- 

 like joints, supporting five hexa- 

 gonal intercostal plate-like joints, 

 and five plate-like scapulae, having 

 on one of the intercostal s an inter- 

 scapulary plate interposed. An 

 arm proceeding from each of the 

 scapulae. Base, probably fascicular, 

 and permanently adhering." 



PO'TSTONE. The Lapis ollaris of Pliny. 

 A variety of steatite, nearly equal 

 in hardness to common steatite ; it 

 is, however, more tenacious, and 

 though it may be turned with the 

 lathe, it breaks with difficulty. It 

 is smooth and unctuous to the 

 touch. It is usually of a greenish- 

 grey colour, with various shades, 

 and often spotted. Its fracture 

 curved, and, sometimes, almost 

 foliated. Specific gravity from 



2-8 to 3-2. It emits an argillaceous 

 odour when breathed on. From 

 its being formed into culinary 

 vessels it has obtained its name. 

 From an analysis by "Weigleb, the 

 potstone of Corno, in Lombardy, 

 where it occurs in great abundance, 

 consists of magnesia 38, silica 38, 

 alumina 7, iron 15, carbonate of 

 lime 1, fluoric acid 1. 



PO'TTEES' CLAY. A variety of clay, 

 of a reddish or grey colour, which 

 becomes red when heated. That 

 used in our potteries for making 

 coarse red ware comes chiefly from 

 Devonshire. It is exceedingly 

 infusible, and contains a large 

 proportion of alumine. Potter's 

 clay, mixed with ground flints, is 

 employed in the manufacture of 

 the finer kinds of pottery in 

 Staffordshire. 



POZZUOLA'KA. Scoriae or volcanic 

 ashes, brought from Pozzuoli, a 

 town in the bay of Naples, and 

 named therefrom. Pozzuolana is 

 used, mixed with lime, for making 

 Roman, or water-setting, cement. 



PEASE, (from irpaaov, Gr. a leek; 

 so called from its colour.) The 

 Prasem of "Werner ; Quartz hyalin 

 vert obscur of Haiiy ; Quartz prase 

 of Brongniart. A leek-green trans- 

 lucent variety of rhombohedral 

 quartz ; lustre vitreous ; fracture 

 splintery. Specific gravity 2'5. 

 Prase appears to be common quartz, 

 coloured by actynolite or epidote. 



PBA'SINOUS. (prasinus, Lat.) Of a 

 light green colour, inclining to 

 yellow. 



PEE'HNITE. A mineral thus named 

 after Colonel Prehn, who brought 

 it from the Cape of Good Hope. 

 Prehnite is of a green, grey, or 

 white colour. It occurs crystal- 

 lised ; in granular, scopiform, and 

 stellular fibrous distinct concretions; 

 massive and reniform. Its texture 

 is foliated. Fracture uneven. In- 

 ternal lustre pearly. It scratches 

 glass, though feebly, and gives 



