P YR 



P YR 



PURPURIC ACID. An acid obtained 

 from uric or lithic acid, remarkable for 

 its tendency to form red or pwrp^e coloured 

 salts with alkaline bases. Its salts are 

 called purpurates. 



PURPURI'N^. A sub-family of the 

 Muricidce, named from the typical genus 

 Purpura, including shells whose general 

 form resembles that of the whelks. 



PUTA'MEN {puto, to prune). A syn- 

 onymous term for the endocarp, or 

 innermost layer of the pericarp of osseous 

 fruits. 



PUTREFA'CTION. The spontaneous 

 decomposition of animal or vegetable 

 matters, attended with foetor ; a species 

 of fermentation. 



PUZZOLA'NO. A substance of vol- 

 canic origin, composed principally of 

 pumice, of which a stratum is excavated 

 in the neighbourhood of Pozzuoli, near 

 Naples. The mortar which it makes 

 with lime is called Roman cement. 



PYCNITE (TTuKi/or, thick). A sub- 

 stance formerly considered as a variety 

 of beryl, but now referred to topaz. 



PY'RAMID. A solid figure, contained 

 by planes which are constituted betwixt 

 one plane and one point above it in 

 which they meet. It differs from a cone, 

 in having for its base a right-lined, in- 

 stead of a circular figure. (See Cone). 

 The ancients derived the term nvpanh 

 OT pyramid, sometimes from irvp, fire, be- 

 cause of its pointed shape ; sometimes 

 from TTupdf , as if the pyramids had been 

 granaries ! No doubt the word, as well 

 as the thing, is Egyptian. 



PYRITES (ttup, fire). A compound 

 of sulphur and iron, usually occurring in 

 yellow shining crystals like brass, and in 

 almost every rock, stratified and unstra- 

 tified. The shining metallic bodies fre- 

 quently seen in common roofing-slate 

 afford a familiar example of the mineral. 

 The term originally denoted a fire-stone, 

 a sort of stone out of which fire could be 

 struck ; because, under particular circum- 

 stances, the stone produces spontaneous 

 heat, and even inflammation. 



PYRO- {-nvp, fire). Words compounded 

 "with this term denote some change pro- 

 duced by tbe action of fire : — 



1. Pyr-acid. An acid produced by the 

 destructive distillation of an organic acid, 

 as the pt/ro-citric, produced by decompo- 

 sition of the citric by fire. 



2. Pyr-allolite. A silicate of magnesia ; 

 a mineral belonging to the talc family, 

 found in a lime-quarry in Finland. Its 

 name is derived from its singular pro- 



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perty of blackening before the blowpipe 

 at a low-red heat, and afterwards becom- 

 ing white at a higher temperature. 



3. Pyr-argillite. A silicate of alumina 

 and other matters, found in granite in 

 Finland, emitting an argillaceous odour 

 when heated. 



4. Pyro-chlore. A new mineral found 

 in Norway, in zircon-syenite. It is so 

 called because it becomes yellow before 

 the blowpipe, whereas polymignite re- 

 tains its black colour. 



5. Pyr-eneite. A greyish-black mineral, 

 occurring in primitive limestone, and 

 consisting of silica, alumina, lime, and 

 iron. 



6. Pyro-ligneous Acid. An acid ob- 

 tained by distillation from wood ; in its 

 strongest form it is acetic acid. Pyro- 

 ligneous spirit is a substance, more vola- 

 tile than alcohol, produced during the 

 distillation of Wood. It is more correctly 

 named pyro-xylic spirit. 



7. Pyro-lusite. The most abundant 

 ore of manganese, occurring crystallized 

 and massive, in Devonshire, Warwick- 

 shire, Thuringia, Brazil, and other 

 places. It is the binoxide of peroxide of 

 manganese. 



8. Pyro-meter {nerpov, a measure). An 

 instrument for measuring all degrees of 

 heat above those which can be indicated 

 by a mercurial thermometer. Wedge- 

 wood's pyrometer acts by the contraction 

 of cylinders of fine white clay. The in- 

 dications of Daniell's pyrometer depend 

 on the difference in the expansion and 

 contraction of a platinum bar, and a 

 tube of black lead ware in which it is 

 contained. 



9. Pyro-morphite {p,6p<f>r\, form). Phos- 

 phate of lead, generally divided into 

 brown lead ore and green lead ore. 

 When heated before the blowpipe, it 

 fuses into a globule, which assumes a 

 polyhedral crystalline form as it cools. 



10. Pyr-ope. Chrome garnet, gene- 

 rally called Bohemian garnet, occurring 

 in rounded grains, and also imbedded in 

 serpentine, &c. 



11. Pyr-orthite. A silicate of oxide of 

 cerium, iron, &c., found imbedded in 

 granite or quartz, near Fahlun in Swe- 

 den. It resembles orthite, but is differ- 

 ently affected by heat. 



12. Pyr-osmalite. A mineral occurring 

 in magnetic ironstone, giving out vapours 

 of chlorine before the blo^vpipe, and be- 

 coming a magnetic oxide of iron. 



13. Pyr-oxene. The name given by 

 Haiiy to augite, a silicate of lime and 



