PYRITZ 



PYROPE 



507 



other minerals as an incrustation. Beautiful speci- 

 mens of globular pyrite are found in the chalk 

 of England. It is a very widely diffused and 

 plentiful mineral, occurring in many different kinds 

 of rock. It is too abundant in many coal-seams, 

 the action of water and air changing it into sul- 

 phate of iron (vitriol), during which change so 

 much heat is evolved that the coal is frequently 

 kindled by it, mines become unworkable, and the 

 progress of the fire can only be stopped, if at all, 

 by building up portions of them to cut off the 

 access of air, or by the admission of a plentiful 

 supply of water. Sandstones containing pyrite 

 ought not to be employed for building purposes, 

 as it is prone to oxidation. Sometimes it is 

 changed into sulphate of iron, but when other 

 bases are present in the rock the sulphuric acid 

 often unites with these in preference, leaving the 

 iron of the original sulphide free. The iron then 

 becomes oxidised, and appears as dark brown 

 blotches. The presence of pyrite thus leads to 

 corrosion and unsightly staining. The colour of 

 pyrite has often caused it to be mistaken for gold, 

 a mistake which its hardness and comparative 

 lightness should prevent, or its ready solubility in 

 nitric acid, and its burning before the blowpipe on 

 charcoal with bluish flame and smell of sulphur. 

 But it sometimes does contain a small proportion 

 of gold, occasionally even in visible grains. This 

 auriferous pyrite is found in Siberia and in South 

 America. Pyrite is never used as an ore of iron, 

 but it is much employed in the manufacture of 

 sulphuric acid, and sulphur is obtained from it by 

 sublimation. It is also used for the manufacture 

 of alum. A rather unstable variety of iron disul 

 phide of a very pale colour is called Marcafite ; it 

 crystallises in orthorhombic forms. Another sul- 

 phide of iron known as Pyrrotine (FejSg) is 

 magnetic. 



<'o]>l>er Pyrites, also called Yellmc Copper and 

 Chalcopyrite, is the most abundant of all the ores 

 of copper, and yields a large proportion ( perhaps a 

 third ) of the copper used in the world. It is brass- 

 yellow, the colour varying with the amount of 

 copper which it contains, a rich colour indicating 

 much copj>er, and a pale colour the presence of a 

 comparatively large amount of iron ; for this ore 

 IB not a sulphide of copper alone, but of copper and 

 iron. It occurs massive and disseminated in rocks 

 of almost every class, and is often found crystal- 

 lised in octahedrons and tetrahedrons, but generally 

 in very small crystals. It may at once be distin- 

 guished from iron-pvrites by its comparative soft- 

 ness, yielding readily to the knife, and by the 

 green colour of its solution in nitric acid. Before 

 the blowpipe, with borax and soda, it yields a bead 

 of copper. Cobaltite, an arsenio-sulplnde of copper, 

 is a principal ore of cobalt. It is generally of a 

 silver-white colour, and occurs massive, dissem- 

 inated, or crystallised in cubes, octahedrons, dodeca- 

 hedrons, and polyhedrons, in schistose rocks. 

 Nickelite, used as an ore of nickel, is a compound 

 of nickel and arsenic. It is generally found mas- 

 sive, and is of a copper-red colour ; hence it is 

 called by the German miners Kupfer-nickel, be- 

 cause they mistook it for an ore of copper. 



Pyritz, a manufacturing town of Pomerania, 

 25 miles SE. of Stettin by rail ; pop. 8062. 



Pyrmont. See WALDECK-PYHMONT. 



Pyrogallic Acid. See GALLIC ACID, PHOTO- 

 GRAPHY. 



Pyrollgneons Acid, or WOOD VINEGAR, a 



crude commercial form of Acetic Acid (q.v.). It is 

 made by the destructive distillation of wood, and, 

 besides acetic acid, contains tar, creasote, wood- 

 naphtha, and other products, which have to be 

 removed if it is required in a very pure state. 



The best woods for the distiller are ' hard ' woods, 

 although all will yield it. Oak branches stripped 

 of their bark are cut into short billets, which are 

 placed in cast-iron retorts, and a sufficient heat 

 applied to drive off the volatile constituents 

 and carbonise the wood. This acid is of great use 

 in the arts, especially in making the acetates used 

 by dyers and calico-printers ; and it is also, when 

 very carefully purified and properly diluted with 

 water, used extensively as a substitute for common 

 vinegar in pickling, and even for table use. It is 

 also used in the preservation of fish, giving them a 

 ' smoked ' flavour. 



Pyromancy. See DIVINATION, Vol. IV. p. 21. 



Pyrometry, the measurement of tempera- 

 tures beyond the compass of the mercurial Ther- 

 mometer (q.v.). The leading methods are ocular, 

 calorimetric, and pyrometric. The eye alone is 

 often sufficiently accurate, and can distinguish 

 dull red, 525" C. (say 975 F.) ; cherry red, 800" C. 

 (say 1450 F.) ; orange, 1100 C. (2000 F.) ; white, 

 1300 C. (2350 F.) ; dazzling white, 1500 C. (2700 

 F.). Or we may use cobalt glass as a means of 

 more sharply discriminating the changes of visible 

 colour. Calorimetric : a lump of heated metal is 

 thrown into a known quantity of water ; the rise 

 of temperature is measured ; the temperature of 

 the heated metal is next calculated from its 

 weight, its specific heat, and the rise of tempera- 

 ture and the quantity of the water. This method 

 admits errors from loss of time and radiation ; 

 hence only rough results are attained, comparable 

 with one another, but not numerically reliable. 

 Of pyrometric methods may be named expansion 

 of air, hydrogen or nitrogen (only suited for labora- 

 tory purposes, for glass melts, metals become 

 permeable, and porcelain is fragile), or of mercury 

 vapour ; dilatation of solids porcelain, platinum, 

 or iron ( Professor J. F. Daniell, 1821 ) whose ex- 

 pansions are very small and difficult to measure, as 

 they generally take up a new set or form when alter- 

 nately heated and cooled ; "the shrinkage of clay 

 (Wedgwood's pyrometer) giving variable results; 

 the actual fusion of definite metals, alloys, or enamels 

 whose melting-points have been previously ascer- 

 tained ; the temperature acquiren by water made 

 to flow uniformly through a tube partially exposed 

 to the heat to be explored ; the speeds of outflow 

 of air through an aperture at the atmospheric and 

 at the furnace temperature (Barns, American 

 Journal of Science, 1889); Siemens' electric pyro- 

 meter, which measures the change in the resist- 

 ance of platinum wire exposed to the furnace heat ; 

 Becqnerel's thermo-electric pyrometer, in which a 

 thermo-electric couple (platinum-palladium) is ex- 

 posed to the heat. When Le Chatelier's thermo- 

 electric couple, consisting of platinum and platinum- 

 jilus-ten-per-cent. -of-rhodium, is used, the readings 

 of a thermo-electric pyrometer may be consistent 

 with one another. 



The whole subject of pyrometry was carefully discussed 

 by M. Le Chatelier before the French Societe Technique 

 de 1'Industrie du Gaz at its annual meeting in 1889; 

 and a summary of his address will be found in the Gat 

 World, March 15, 1890. See also Poggendorfs Aitnalcn, 

 vol. xxix. ; and for Ericsson's .Solar Pyrometer, see 

 Nature, vol. xxx. 



Pyrope, a gem, often called Carbuncle and 

 Hyacinth by lapidaries, which is nearly allied to 

 garnet. Composed of silica, alumina, magnesia, 

 lime, and the protoxides of iron, chrome, and man- 

 ganese, it is always of a deep red colour, and is 

 transparent, or at least translucent. It generally 

 occurs in roundish grains, but rarely in imperfectly 

 cubical crystals. Pyropes are found chiefly in 

 Saxony and Bohemia, also at Elie in Fife (where 

 they are called Elie Rubies). 



