PYBOLUSITE. 577 PYROPE. is found in Thuringia, iloravia, Bohemia, and Transylvania ; also in Australia and India, and abundantly in the United .States in Vermont, Massachusetts, Connecticut, Virginia, Arkansas, and California. Pyrolusite is the common ore of manganese, and is extensively worked for that product, being used in the manufacture of alloys, such as ferromanganese and manganese bronze ; as an oxidizing agent, as in the manufacture of chlorine and oxygen; and as a coloring material in the manufacture of glass, pottery, and paints. PYK'OMA'NIA ( Xeo-Lat., from Gk. vOp, pyr, lire + fiavla, mania, madness). A variety of pe- riodic insanity, classed with the degenerative in- sanities, i.e. the insanities depending upon an hereditary or acquired constitutional condition. During certain periods the patient manifests an uncontrollable desire to commit arson, and feels relief and pleasurable sensation when watching flames. Between these attacks the patient is ap- parentlv sane, but in reality the interval is only su1)-lucid. The patient is capable of attend- ing to his all'airs, perhaps, but is nervous or hys- terical, and evinces morbid irritability. Attacks of pyromania are generally sudden, though some- times preceded bj' brief depression. The impulse, or imperative conception, is strong and the pa- tient obeys. The morbid impulse to incendiarism occurs not infrequently in epileptics and men- struating girls, rarely in jjregnant and hysterical women, frequently in any lunatic with destructive tendencies. See Dipsomania ; Kleptomania ; In- sanity. PYKOM'ETER ( from Gk. irGp, pyr. fire + niTfiov, iiKlrvH, measure). An instrument used for measuring high temperatures where the or- dinary mercurial thermometer is not available, as mercury boils at 358° Centigrade. The first pyrometer was that invented by the Dutch physi- cist ilusschenbroek, about 1725, and consisted of a metallic bar which expanded under the influence of heat. Wedgwood's pyrometer, which was de- vised aljout 1780, employed cylinders of fire-clay which were exposed to a high temperature and then allowed to cool. The degi'ee of heat was as- certained by measuring the amount of contraction in length experienced liy the clay. This method, however, was found unreliable, and the apjjaratus invented in 1821 by Daniell formed the first really serviceable pyrometer. Here the temperature was measured by the relative expansion of a bar of metal such as iron or platinum contained in a tube of black-lead earthenware. The metal rod was in contact with a lever which moved an indi- cator over a graduated scale. This instrument was subsequently imjiroved by the substitution of graphite for the platinum rod, and could be used for the measurement of temperature as high as 1000° Centigrade. Numerous other pyrometers have been devised depending on the property pos- sessed by metals expanding with an increase in temperature, but the most accurate instrument is the air thermometer (q.v. ), which is the ultimate standard of reference in all high-temperature measurements. It is usually constructed with a bulb of porcelain or platinum containing either air or hydrogen, and the expansion of the gas under the influence of heat increases its pi-essure and raises the level of a column of mercury. As the pressure of a gas is proportional to its tem- perature, it follows that by determining the height of the mercury column supported by the air at the different temperatures, a determination can be made with considerable accuracy, though there are numerous practical difficulties necessitating an experienced .observer. A form of pyrometer much used is that invented by Dr. C. W. Siemens, in which the temperature is measured b}- the in- crease in the electrical resistance of platinum wire exposed to the heat. This wire is wound on a cylinder of refractory fire-clay inclosed in a shield of platinum wire, which can be determined easily and quickly with a galvanometer and re- sistance coils b}' the application of the Wheat- stone bridge method. The platinum thermom- eter, now made in many forms, can be standard- ized by direct comparison with an air thermom- eter, and a reference table constructed enabling an observer to ascertain readily the temperature of a furnace or substance with a high degree of accuracy. The pyrometer which is considered the most useful for extremely high temperatures is that used by Le Chatelier, and may Iw taken as typical of those making use of the thermo-couple. This instrument consists essentially of two pieces of wire of but slightly difl'erent composition, which are inclosed in a long tube of porcelain or fire-clay. These wires are platinum and an alloy of 90 per cent, platinum and 10 per cent, of the rare metal rhodium, and the current pi'oduced at au increase in temperature is measured by a gal- vanometer. The instrument measures readily and accurately temperatures in the neighborhood of 1200° Centigrade. To Le Chatelier is also due a photometric method of determining high temperatures by measuring the intensity of the light emitted by a glowing n)etal. Tlie spectro- scope can also be used to measure high tempera- tures as well as the calorimeter, in which case the temperature of a given piece of metal whose specific heat is known is determined by noting the increase in temperature of a mass of water in which it is immediately placed upon removal from the furnace. This principle is em- ployed in a numljer of instruments, but it does not afford especially accurate results, as the spe- cific heat of metals is difl'erent at difl'erent tem- peratures. The thermo-couple and the platinum are on the whole the most useful forms of pyrom- eters, as they can be used over an extended range of temperature, from that of liquid air in hydrogen almost to the melting point of platimun, and readings can be made without inidue diflicul- ties either of manipulation or of calculation. Con- sult : Preston. Theory of Heat ( Xew York. 1894) ; Barns. "Thermo-Elcctrie ileasuremcnt of High Temperatures," Bulletin, 5.) United States Geolog- iciil (Purvey; id.. Report on High Temperature Measurements, International Congress of Physi- cists (Paris, 1901). PYR'OMOR'PHITE ( from Gk. wOp, pyr, fire -- /ioprp-^, »i or/;/).', form). A mineral lead phos- phate and chloride that crystallizes in the hexag- onal system, has a resinous lustre, and in color is of various shades of green, yellow, and brown. It occurs with lead ores, usually in veins. The principal localities where it is found are in Sax- ony, Bohemia. Nassau, Siberia, Derbyshire and Cumberland. England: and in the United States in JIaine, IMassachusctts, New York, Phenix- ville. Pa., and David.son County, N. C. PYROPE, pir'6p (from Gk. iri/poiwis, pyropos, sort of red bronze, fire-eyed, from irCp, pyr, fire + w^, ops, eye). A name applied to the magne-
