Page:EB1911 - Volume 21.djvu/191

 PERLES, JOSEPH (1835–1894), Jewish rabbi, was born in Hungary in 1835, and died at Munich in 1894. He was one of the first rabbis trained at the new type of seminary (Breslau). Perles’ most important essays were on folk-lore and custom. There is much that is striking and original in his history of marriage (Die jüdische Hochzeit in nachbiblischer Zeit, 1860), and of mourning customs (Die Leichenfeierlichkeiten im nachbiblischen Judenthum, 1861), his contributions to the sources of the Arabian Nights (Zur rabbinische Sprach-und Sagenkunde, 1873), and his notes on rabbinic antiquities (Beiträge zur rabbinischen Sprach-und Altertumskunde, 1893). Perles’ essays are rich in suggestiveness, and have been the starting-point of much fruitful research. He also wrote an essay on Nachmanides, and a biography and critical appreciation of Rashba (1863).

PERLITE, or, a glassy volcanic rock which, when struck with a hammer, breaks up into small rounded masses that often have a pearly lustre. The reason for this peculiarity is obvious in microscopic sections of the rock, for many small cracks may be seen traversing the glassy substance. These mostly take a circular course, and often occur in groups, one within another. The circular cracks bound the little spheres into which the rock falls when it is struck, and the concentric fissures are the cause of the pearly lustre, by the reflection of light from enclosed films of air. Longer straight cracks run across the sections separating areas in which the circular fissures preponderate. By decomposition the fissures may be occupied by deposits of limonite, which make them more obvious, or by other secondary minerals. The glass itself often undergoes change along the cracks by becoming finely crystalline or devitrified, dull in appearance and slightly opaque in section. In polarized light the perlitic glass is usually quite isotropic, but sometimes the internal part of some of the spheres has a slight double refraction which is apparently due to strain. The glass found on the waste-heaps of glass-furnaces is sometimes very coarsely perlitic.

Perlitic structure is not confined to glass, but may be seen also in that variety of opal which is called hyalite. This forms small transparent rounded masses like drops of gum, and in microscopic section exhibits concentric systems of cracks. Hyalite, like perlitic obsidian, is amorphous or non-crystalline. It is easy to imitate perlitic structure by taking a little Canada balsam and heating it on a slip of glass till most of the volatile matters are driven out, then drop it in a basin of cold water and typical perlitic structure will be produced. The reason is apparently the sudden contraction when the mass is chilled. In the glaze on tiles and china rounded or polygonal systems of cracks may often be seen which somewhat resemble perlitic structure but are less perfect and regular. Many rocks which are cryptocrystalline or felsitic, and not glassy, have perfect perlitic structure, and it seems probable that these were originally vitreous obsidian's or pitchstones and have in process of time been changed to a finely crystalline state by devitrification. Occasionally in olivine and quartz rounded cracks not unlike perlitic structure may be observed.

Many perlitic rocks contain well-developed crystals of quartz, feldspar, augite or magnetite, &c., usually more or less corroded or rounded, and in the fine glassy base minute crystallites often abound. Some of the rocks have the resinous lustre and the high percentages of combined water which distinguish the pitchstones; others are bright and fresh obsidians, and nearly all the older examples are dull, cryptocrystalline felsites. According to their chemical compositions they range from very acid rhyolites to trachytes and andesites, and the dark basaltic glasses or tachylytes are sometimes highly perlitic. It is probable that most perlites are of intrusive origin, and the general absence of steam cavities in these rocks would support this conclusion. but some perlitic Hungarian rhyolites are believed to be lavas.

Very well known rocks of this kind are found in Meissen, Saxony, as dikes of greenish and brownish pitchstone. Other examples are furnished by the Tertiary igneous rocks of Hungary (Tokai, &c), the Euganean Hills (Italy) and Ponza Island (in the Mediterranean).

In mineralogical collections rounded nodules of brown glass varying from the size of a pea to that of an orange may often be seen labelled Marekanite. They have long been known to geologists and are found at Ockotsk, Siberia, in association with a large mass of perlitic obsidian. These globular bodies are, in fact, the more coherent portions of a perlite; the rest of the rock falls down in a fine powder, setting free the glassy spheres. They are subject to considerable internal strain, as is shown by the fact that when struck with a hammer or sliced with a lapidary’s saw they often burst into fragments. Their behaviour in this respect closely resembles the balls of rapidly cooled, annealed glass which are called Prince Rupert’s drops. In their natural condition the marekanite spheres are doubly refracting, but when they have been heated and very slowly cooled they lose this property and no longer exhibit any tendency to sudden disintegration.

In Great Britain Tertiary vitreous rocks are not common, but the pitchstone which forms the Scuir of Eigg is a dark andesitic porphyry with perlitic structure in its glassy matrix. A better example, however, is provided by a perlitic dacitic pitchstone porphyry that occurs near the Tay Bridge in Fifeshire. The tachylytic basalt dikes of Mull are occasionally highly perlitic. At Sandy Braes in Antrim a perlitic obsidian has been found, and the Lea Rock, near Wellington in Shropshire, is a devitrified obsidian which shows perlitic cracks and the remains of spherulites.

 PERM, a government of east Russia, bounded S. by the governments of Orenburg and Ufa, W. by Vyatka, N.W. by Vologda, and E. by Tobolsk (Siberia). It has an area of 128,173 sq. m. Though administratively it belongs entirely to Russia in Europe, its eastern part (about 57,000 sq. m.) is situated in Siberia, in the basin of the Ob. The government is traversed from north to south by the Ural Mountains, 30 to 45 m. in width, thickly clothed with forests, and deeply excavated by rivers. The highest summits do not rise above 3600 ft. in the northern section of the range (the Vogulian Ural); in the central portion, between 59° and 60° 30′ N., they once or twice exceed 5000 ft. (Denezhkin, 5360 ft.); but the chain soon sinks towards the south, where it barely attains an elevation of 3000 ft. Where the great Siberian road crosses it the highest point is 1400 ft.

The government is very well drained by rivers belonging to the Pechora, Tobol (affluent of the Ob) and Kama systems. The Pechora itself rises in the northern corner of the government, and its tributary the Volosnitsa is separated by a distance of less than 3 m. from the navigable Vogulka, a tributary of the Kama, a circumstance of some commercial importance. The chief river of Perm, is however, the Kama, whose navigable tributaries the Chusovaya, Sylva and Kolva are important channels for the export of heavy iron goods to Russia. The government is dotted with a great number of lakes of comparatively trifling size, their total area being 730 sq. m., and with marshes, which are extensive in the hilly tracts of the north. Granites, diorites, porphyries, serpentine and Laurentian gneisses and limestones, containing iron, copper and zinc ores, constitute the main axis of the Ural chain; their western slope is covered by a narrow strip of Huronian crystalline slates, which disappear in the east under the Post-Tertiary deposits of the Siberian lowlands, while on the West narrow strips of Silurian limestones, quartzite’s and slates, and separate islands of Devonian deposits, appear on the surface. These in their turn are overlain with Carboniferous clays and sandstones, containing Coal Measures in several isolated basins. The Permian deposits extend as a regular strip, parallel to the main ridge, over these last, and are covered with the so-called “variegated marls,” which are considered as Triassic, and appear only in the western corner of the territory.

Perm is the chief mining region of Russia, owing to its wealth in iron, silver, platinum, copper, nickel, lead, chrome ore, manganese and auriferous alluvial deposits. Many rare metals, such as iridium, osmium, rhodium and ruthenium, are found along with the above, as also a great variety of precious stones, such as diamonds, sapphires, jaspers, tourmalines, beryls, phenacites, chrysoberyls, emeralds, aquamarines, topazes, amethysts, jades, malachite. Salt-springs occur in the west; and the mineral waters, though still little known, are worthy of mention. No less than 70% of the total area is occupied with forest, but the forests are distributed very unequally, covering 95% of the area in the north and only 25% in the south-east. Firs, the pine, cedar, larch, birch, alder and lime are the most common; the oak appears only in the south-west. The flora of