Page:EB1911 - Volume 22.djvu/712

 silicate. The compact variety of pyrophyllite is used for slate pencils and tailors' chalk (“ French chalk ”), and is carved by the Chinese into small images and ornaments of various kinds. Other soft compact minerals (steatite and pinite) used for these Chinese carvings are included with pyrophyllite under the terms agalmatolite and pagodite

Pyrophyllite occurs in schistose rocks, often associated with cyanite, of which it is an alteration product. Pale green foliated masses, very like talc in appearance, are found at Beresovsk near Ekaterinburg in the Urals, and at Zermatt in Switzerland. The most extensive deposits are in the Deep river region of North Carolina, where the compact variety is mined, and in South Carolina and Georgia.

 PYROXENE, an important group of rock-forming minerals, very similar in chemical composition and general characters to the s (q.v.). Although crystallizing in three different systems, they all possess distinct prismatic cleavages, the angles between which are about 87° (the cleavage angle in the amphiboles being 56°). They are metasilicates, but, as shown in the following table, the composition varies widely in the different species, with corresponding differences in the various physical characters. The name pyroxene was originally given by R. J. Haüy in 1796 to the black crystals of augite found in the lavas of Vesuvius and Etna: he derived the name from the Greek  (fire) and  (a stranger), because he thought that the crystals had been accidentally caught up by the lavas which contained them. As a matter of fact, the pyroxenes are, next to the felspars, the commonest constituents of igneous rocks of almost all kinds, being especially characteristic of those of basic composition. An igneous rock composed almost wholly of pyroxene is known as a pyroxenite. Besides being minerals of primary origin in igneous rocks. the pyroxenes are also of frequent occurrence in metamorphic rocks, for example, in crystalline limestones, being then of secondary origin.

At the present day the name pyroxene is used as a group name for all the minerals enumerated below, though sometimes it is also applied as a specific name to include the monoclinic members diopside, hedenbergite, schefferite and augite. For details respecting the special characters and modes of occurrence of most of these species reference may be made to the respective headings: others not so treated are briefly mentioned below. Hedenbergite, or calcium iron pyroxene, is a black mineral closely allied to (q.v) and, owing to the isomorphous replacement of iron by magnesium, there is no sharp line of division between them. Schefferite, or manganese pyroxene, is a brown mineral found in the manganese mines of Sweden. Pectolite is a secondary mineral occurring as white masses with a radially fibrous structure in the veins and cavities of basic igneous rocks. Babingtonite is found as small black crystals on felspar in the granite of Baveno in Italy, and in the Haytor iron mine in Devonshire. Rosenbuschite, hiortdahlite, and some other rare members containing zirconium and fluorine, occur as accessory constituents in the nephelinesyenite of southern Norway.

PYROXENITE, a rock consisting essentially of minerals of the pyroxene group, such as augite and diallage, hypersthene, bronzite or enstatite. Names have been given to members of this group according to their component minerals, e.g. pyroxenite (augite), diallagite (diallage), hypersthenite (hypersthene), bronzitites (bronzite), websterite (diallage and hypersthene). Closely allied to this group are the hornblendites, consisting essentially of hornblende. The term perknite (Gr. , dark) has also been used to designate the whole series.

 PYRRHO OF ELIS (c. 360–270 ), a Greek sceptic philosopher and founder of the school known as Pyrrhonism. Diogenes Laërtius (ix. 61), quoting from Apollodorus, says that he was at first a painter, and that pictures by him were in existence in the gymnasium at Elis. Later he' was diverted to philosophy by the works of Democritus, and became acquainted with the Megarian dialectic through Bryson, pupil of Stilpo. With Anaxarchus, he went to the East in the train of Alexander, and studied in India under the (q.v.) and under the Magi in Persia. From the Oriental philosophy he seems to have adopted a life of solitude. Returning to Elis, he lived in poor circumstances, but highly honoured by the Elians and also by the Athenians, who gave him the rights of citizenship. His doctrines are known mainly through the satiric writings ( ) of his pupil Timon of Phlius (the Sillographer) The main principle of his thought is expressed in the word acatalepsia, which implies the impossibility of knowing things in their own nature. Against every statement the contradictory may be advanced with equal reason ( ). Secondly, it is necessary in view of this fact to preserve an attitude of intellectual suspense (évroxh), or, as Timon expressed it,  (i.e. no assertion more valid than another). The same idea is expressed also by the terms eippexkia' (equilibrium) and dqbarria. (refusal to speak, non-committal silence). Thirdly, these results are applied to life in general. Pyrrho concludes that, since nothing can be known, the only proper attitude is imperturbability (ataraxia). The impossibility of knowledge, even in regard to our own ignorance or doubt, should induce the wise man to withdraw into himself, avoiding the stress and emotion which belong to the contest of vain imaginings. This drastic scepticism is the first and the most thorough exposition of agnosticism in the history of thought. Its ethical results may be compared with the ideal tranquillity of the Stoics and the Epicureans. (For its relation to the New Academy and to scepticism in general see and .)

PYRRHOTITE, a mineral species consisting of iron sulphide and crystallizing in the hexagonal system. The formula is Fe$3$S$3$ where n may vary from 5 to 16; usually it is Fe$3$S$3$ or Fe$2$S$3$, the latter being also the composition of the artificially prepared compound. Small amounts of nickel and cobalt are often present.