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Pclopldas, d. 864 B.C. Theban patriot, associate of Epaminondas. Banished 882, he overthrew the Spartan power and expelled the polemarchs from Thebes 379; led the Sacred Band ;it Leuctra371; was Ambassador to Susa 366, and was killed at CynocephalaJ.

Pcloponnesian War. 431-404 B.C.; between Athens and the s. Greek states, supported by their colonies. The destruction of the Athenian fleet and army at Syracuse 418 was fatal; and the war ended most unfortunately for civilization with the capture of Athens by the Spartans 404, Its history was brilliantly written by Thucydides.

Peloponnesus. Peninsula of Greece; now Morea.

Pclops. Son of Tantalus; king of Elis; father of Atreus and Thyestes; honored above all heroes at Olympia.

Peloria. Condition of monstrosity, especially in certain plants of the Snapdragon family, in which, instead of only one lobe of the corolla being extended into a spur, as is the normal state, the other four lobes are also spurred; also, more broadly, regular forms of flowers which are normally irregular.

Pelt), Andries, 17th cent. Dutch dramatist and poet.

Peltate. In Botany, leaves or other organs having the stalk inserted on the lower surface of the lamina, instead of at the base, so as to become shield-shaped.

Peltier's Effect. Thermo-electric phenomenon, first observed by Peltier. Consider a junction of two metals, A and B, such that, when this is made the hot junction in a thermoelectric circuit, the current is from A to B. If now a current from some external source be made to pass through that junction from A to B, the junction will be cooled; while if the direction be from B to A. it will be heated. A similar phenomenon was observed by Thomson to take place within a single metal, and in this case is known as the Thomson effect. E.g., hot iron is thermo-electricully positive to colder iron, and hence a current made to pass in either direction tends to exaggerate the existing differences of temperature. This is reversed in the case of copper.

Peltry. Skins of wild animals undressed except by drying; dressed skins being called furs.

Pelusluni. City on e. mouth of the Nile; key of Egypt. Here Cambyses defeated Psammetichus HI. 525 B.C. It surrendered to Alexander 333, was taken by Persians 309, by Antiochus 178, bv Mark Antony 55, by Octavian 81 B.C., and by the Saracens A.D". 688.

Pelvis. Lower part of the trunk; a basin-like cavity

formed by the union of the innominate bones with the lower end of the spinal column. It furnishes a foundation for the contents of the abdominal cavity and attachments for the muscles forming the abdominal walls.

Pemba. Coral island 80 m. n. of Zanzibar, belonging to the British protectorate of Zanzibar. It produces excellent „. < timber. Area 227 sq. 'm., pop. ab. 10,000. Pemberton, John Clifford, 1814-1881. U. S. officer 183761; Brig.-gen. C.S.A. 1861, Lieut.-gen. 1862-63; defender of Vicksburg against Grant May-July 1863.

Pemberton, Max, b. ab.1860. English romancer; ed. Cassell's Mag. 1896. A Puritan's Wife, 1896; Stories of Old Paris, 1897.

Peinmlcail. 1. Dried venison or buffalo meat, used by

Indians. 2. Preparation of beef for Arctic expeditions.

Pemphigus. Inflammatory disease of the skin marked by blisters from the size of a pea und upward, forming successive crops. The blisters may break and form scabs and leave a dark spot, rarely a scar, when healed. The disease is sometimes fatal, especially when of long duration.

Pen. Writing instruments of two general kinds, those which scratch and those which use ink. The former are the more ancient and consist of a stylus made of bone or metal and employed upon a leaf, a waxed tablet, a sheet of lead, a stone, etc. The pen with which ink was applied was made 40 centuries since of a reed, which continued until ab. 500,

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when the quill came into use. The pen was used to write on bark, leaves, papyrus, skins, and paper. Modern pens are made entirely of metals. A bronze nibbed pen was found at Pompeii. The steel pens are made by a succession of stamps and dies and carefully tempered. Birmingham, Eng., is a large manufacturer; U. S. has several factories, producing one and a half million gross, and imports half a million gross.

Penaiacea;. Natural family of flowering plants, of the class Angiospermm and sub-class Dicotyledons, comprising 4 genera and ab. 20 species, natives of s. Africa.

Penal Colonies. England sent convicts to Virginia 16191770, when the colonists refused to receive them longer; subsequently Botany Bay, Sydney, Tasmania, North and West Australia, and British Guiana were utilized in a similar way till about the middle of the 19th century. Russia sends the majority of her criminals to the mines in Siberia. France sent convicts to French Guiana 1853-64 and Communists to New Caledonia 1871. Other European states have gotten rid of many of their convicts by transportation.

Penal Servitude. Punishment introduced in lieu of transportation beyond the seas. See PRISONS.

Penally. Punishment imposed either by law or by contract; excessive penalties are forbidden by XL S. Constitution.

Penance. Self-mortification of any kind, inflicted as expiation of sin; in R. C. Ch., a sacrament. In the Middle Ages penances were various, heavy, and constantly in evidence.

Penances, or FORFEITS. Children's game. Penances are properly the sequence of forfeits. The players are required to perform certain feats, and failing, give forfeits. These are later redeemed, one player acting as judge and one as forfeitcrier, who stands behind the judge and holds up the forfeit. The judge, without seeing the forfeit, passes sentence, to which the owner of the article must submit in order to redeem his property.

Penang, or Prince Of Wales Island. W. of Malay peninsula, at n. end of Strait of Malacca; held by Britain since 1785. It is covered with forests of palms and other tropical



plants. It produces rice and sugar. Tin ore is also found. Georgetown or P. is the only town. It is situated on the strait which divides the island from the mainland. Area 107 sq. m., pop. ab. 100,000.

Penates. Gunrdian deities of the Roman household, and of the state as a union of households. Their images were kept in the penetralia, or central part of the house; the lire on the hearth was sacred to them. See Lares.

Pclia y Pefla, Manuel De La, 1789-1850. Member of Mexican Supreme Court 1824. and long its head; acting Pres. of Mexico 1847-48.

Pencil. Collection of rays, either of radiant energy or other form of wave motion. It may be convergent or divergent.

Pencil. Plumbago inclosed in a small wooden cylinder, used as early as 1565. The plumbago is mixed with clay and water to form a dough, pressed through perforations in a metal cylinder, dried and baked. The rods thus formed are then incased in cedar. Colored pencils are made by mixing pigments or aniline colors with clay: 250 million pencils are made annually at Nuremberg, Germ., Faber being a noted maker. There are large makers in U. S.

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Pencil, Axial. Figure composed of planes, all embracing the same straight line, which is the axis of the pencil.

Pcnoll, Flat. Figure formed by straight lines in the same plane, proceeding from a common point.

Peilda. King of Mercia. England, 636-866; and champion of heathenism. He subdued East Anglia, Deira, and Wessex, defeated and killed Edwine of Northumbria 634, and was Bretwalda. The extinction of heathenism followed his death; slain by Oswes.

Pend d'Oreille, Lake. In n. Idaho, on Clark's Fork of the Columbia. Area 180 sq. m.

Pendent. In Gothic Architecture, the keystone of a vault when projected from the ceiling and decorated; also applied to the protrusion of the hammer-beam in timber construction.

Pendentlne. Curved and inward sloping spandrel between two arches which support a circular or polygonal dome. It is a feature of frequent occurrence in the Byzantine, Saracenic, and Renaissance styles; in fact, a necessary feature in all architecture which admits a classical construction.

Pendeiit-Poftt. In timber roofs, the straight pieces placed against the wall to receive the hammer-beams.

Pender, William Dorsey, 1834-1863. Major-gen. C.S.A. 1863; mortally wounded at Gettysburg.

Peudergraat, Garrett Jesse, U.S.N., 1803-1862. Captain 1855, Commodore 1862. He commanded the Cumberland off Norfolk 1861, and saved the U.S. vessels there from capture by threatening the city.

Pendleton, Edmund, 1721-1803. Delegate to Congress 1774-75: Pres. Va. Chancery Court 1778 and Court of Appeals from 1779.—His nephew, Nathaniel, 1756-1821, wns a Judge in Ga. and N.Y.—His grandson, George Hunt, 1825-1889, was M.C. from Ohio 1857-65, Democratic candidate for Vice-Pres. 1864, U.S. Senator 1879-85, and U.S. Minister to Germany 1885.

Pendleton, Louis Beauregard, b. 1861. American novelist.

Pendulum. A simple pendulum consists of a heavy particle, called the bob, suspended by a weightless cord from a fixed point, about which it can oscillate without friction. If the bob have a uniform circular motion in a horizontal plane, it constitutes a conical pendulum. If T be the time of a complete oscillation to and fro, 1 the length of the pendulum, and g the

acceleration of gravity; then T—2>r-^2(l). A compound pendulum is any rigid body fixed at only one point o, and capable of oscillating about this point under the action of gravity. The point o is called the Center Of Suspension (q.v.), and a straight line through o perpendicular to the plane of vibration is called the axis of suspension. If m be the mass of a compound pendulum, a the distance from the center of suspension o to the center of gravity c, and M its moment of inertia about o,

then T-=2ff .^JL (2). Evidently the length L of a simple pendulum that oscillates in the same time is —u or equal to the

ma n

moment of inertia divided by the static moment. The point o1 in a compound pendulum in the line of o and c at a distance L from o, is called the Center Of Oscillation (q.v.); the straight line through o', parallel to the axis of suspension, is called the axis of oscillation. The points oand o1 are mutually interchangeable without affecting T, a principle discovered by Hiiyghens; the product of their distances from the center of gravity c, is k», where k is the radius of gyration. The point o' is sometimes called the Cknter Of Percussion (q.v.), since there is no reaction at o if the pendulum be struck at o1. L is sometimes termed the radius of oscillation. If in equation (1),

we substitute T—2 seconds, then 1=——, which is the leng-th

of a pendulum that makes a single vibration in one second. This is called a seconds pendulum. A cycloidal pendulum is one the bob of which, by simple mechanical means, is made to describe the arc of a cycloid instead of that of a circle. It has the advantage of being strictly isochronous, while a circular pendulum is not even approximately so except for small amplitudes.

Pendulum, Ballistic. Instrument invented about 1765 by Robins for determining the initial velocity of a projectile. It consists of a pendulum having a strong and heavy bob to receive the impact of the projectile. From the relation of the momentum of the projectile before impact to that of the pendulum and projectile after impact the velocity of the projectile at the instant of striking is readily deduced.

PenelOpe. Wife of Ulysses, King of Ithaca; during his absence of 20 years, besieged by importunate suitors, whom she put off by saying she must first finish a shroud for Laertes. Every night she undid the work of the previous day.

Pencil*. Chief river of Thessaly. It flows through the vale of Tempe.

Pengelly, William, 1812-1894. English geologist, engaged for 13 years in exploring Kent's Cavern (q.v.), Torquay. His reports conclusively established the coexistence of man with many of the extinct mammalia.

Pengllinfl. Natatorial birds of the Antarctic Ocean. They show many features of a primitive type. The vertebrae are opisthoccelous, the wings tin-like, the feathers scale-like; the three short metatarsals are partly separate; the hallux turns forward and is webbed with the other three toes; the feathers are narrow and cover the body uuiformlv. The leg is short and placed far back; the foot is plantigrade. On land they sit

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bolt upright and progress by a waddling gait, except the Rockhopper, which moves both feet at a time. In diving the feet are used as rudders: they have no quills in their wings, which thev use in water as paddles, moving them alternately. One white egg, 3 by 4J in., is laid in Oct. and incubated by both sexes for six weeks. They blow and scream like geese. A heavy layer of blubber exists beneath the skin. The voice of the Jackass Penguin resembles a bray. The King Penguin is 3 ft. high.

Penhallow, Samuel, 1665-1726. Colonist at Portsmouth, N. H., ah.1690; Chief-justice from 1717. Indian Wars, 170S-26, 1726.

Peniclllate. In Botany and Zoology, organs or organisms tipped or margined by tufts of slender processes or hairs.

Penlniiular and Oriental Company. Chartered by Great Britain 1840 to carry mails and passengers between Great Britain and Australia, China, and India. It has about

60 vessels.

Penlnnnlar War. 1808-14, between the French and the combined forces of Spain, Portugnl. and England, mainly under Wellington; ended by the victory of Toulouse, the French being driven out of the peninsula. It was caused by Napoleon's attempt to seat his brother Joseph on the throne of Spain.

Penitential INalm». 6, 82, 88, 51, 102, 180, and 143; so designated from ab. 200.

Penitentiary. See Prisons.

Penn, John, 1741-1788. Delegate to Congress from N. C. 1775-77 and 1779; signer of the Declaration of Independence.

Penn, William, 1644-1718. Founder of Pa. He received the grant of the province 1681 in compensation for a debt due his father. Admiral Penn, and was made proprietor of the colony, which he twice visited, 1682 and 1699. His administration was distinguished by the magnanimity and justice of his treatment of the native Indians. He was twice imprisoned as a Quaker, once in the Tower, 1668, where he wrote A'o Cross, No Crown, and 1671 in Newgate, where he composed The Great

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The Fenn Mansion, Philadelphia later writers.—Two of his grandsons, John, 1729-1795, and Richard, 1735-1811, were lieut.-governors of Pa.

Pen n He. Contour feathers; they have stiff shafts and a firm vexillum or vane.

Pennant, Thomas, LL.D., F.R.S., 1726-1798. "Welsh traveler, antiquarian, and scientist. British Zoology, 4 vols., 176577; Tours in Scotland, 3 vols., 1771-75; Tours in Wales, 1778-81; Arctic Zoology. 1785-87; Literary Life, 1793.

Pennatulldae. Sea-feathers or Sea-pens. Family of alcyonarian Polyps, aggregated in colonies, supported on a common stem, and having a horny, flexible skeleton. Renilla belongs here; it, however, has no axis. It is a pinkish, heartshaped disk with a short fleshy stem; the upper surface is covered by the white polyps, among which a certain amount of differentiation of structure and function obtains.

Penncll, Mrs. Elizabeth Robins, b. 1855. American author and traveler. With her husband. Joskph Pennel, the artist, she has written much descriptive matter. Pennine Alps. See Alps. Penninerved. Pinnately veined leaves. Pennington, "william Sanford, 1757-1826. Justice N. J. Supreme Court 1804; Gov. of N. J. 1813-15; U. S. District Judge from 1815. N. J. Supreme Court Reports, 1803-16.—His son, "william, 1796-1862, was Gov. of N. J. 1837-43, M.C. 1859-61, and Speaker 1860.

Pennsylvania. One of the e. States; rudely rectangular in form. Its area is 45,215 sq. m.; its surface is very broken. The s.e. part is a rolling plain: the middle portion is made up of narrow parallel ridges running n.e. and s.w., being the continuation n.e. of the great Appalachian valley. This is succeeded on the n.w. by the Allegany plateau, which from an abrupt escarpment slopes gently n.w. toward lower country. This plateau has been so eroded by streams as to present the appearance of a rugged mountain region. The State is well watered, but the only navigable streams are the Delaware, on the e. boundary, as far as Trenton; the Ohio, in the w. part, to Pittsburg, and its n. fork, the Allegany, for a long distance above Pittsburg, while the Monongahela has been rendered navigable by means of slackwater navigation to the boundary of "W. Va. The surface geology of the State is almost as varied as its topography. The Allegany plateau is almost entirely composed of the Carboniferous formation, and this appears in several localities in the valley country s.e., interspersed with outcrops of Devonian and Silurian. The lower and more rolling country is largely Triassic, with occasional outcrops of older rocks. The industries of P., while very varied, are char

acterized by the extensive mining of coal, both anthracite and bituminous, and of iron, and by the extensive manufacture of iron and steel. Practically all the anthracite coal of the U. S. is found in this State, in a few small localities toward its n.e. part. Bituminous coal is mined very widely over the Allegany plateau and in many localities in the valley section. The State has a very extensive railroad system, which had in 1895 a mileage of 9,661. The population in 1890 was 5,258.014, of which ab. i were of foreign birth. Of its total population, 2,152,051 may be classed as urban, living in cities of 8,000 inhabitants or more. The principal cities are Philadelphia, on the Delaware, at the mouth of the Schuylkill; Pittsburg, at the junction of the Allegany and Monongahela, and its suburb. Allegany. P. was a part of the territory of which the Dutch took possession 1623. In 1681 a charter was granted to William Penn. and P. was governed as a proprietary colony by Penn and his family until the Revolution. A dispute between Md. and P. as to the boundary between the two provinces was settled by Mason and Dixon's establishment of the line which bears their name. P. was one of the foremost colonies in the Revolution in resisting the British, and some of the most important battles were fought on her soil. During the Civil "War it was the only northern State invaded: one of the most decisive battles was fought at Gettysburg 1863. In 1890 there were 18,364,370 acres in farms; the value of farm products in 1889 was $121,328,348. The mineral production in 1895 was: bituminous coal, 51.813,112 tons; anthracite coal, 58,126.345 tons; coke. 6.355,813 tons; iron ore. 697,985 long tons; pig iron, 4,701,163 long tons; steel. 3,883.280 tons; cement, 400,998 bbls. (300 lbs.); roofing slate, 376,068 squares (100 sq. ft.); other slate, 2,942,999 sq. ft.; petroleum, 18,231,442 bbls. of 42 gals.; natural gas, value $5,852,000.

Pennsylvania, University Of. At "West Philadelphia; chartered 1779; united 1791 with the College and Academy of Philadelphia, chartered 1755, which had been developed from a charity school started 1740. Its medical department, the oldest in America, dates from 1765; it has offered a 4-years' course since 1893. The law school was founded 1789. In the college departments (1896) are 106 instructors, in all 251: with 871 collegiate students, 172 in philosophy. 313 in law, 873 in medicine, and 2,632 in all. 450 courses are offered. The libraries contain 135.000 vols. "Women are admitted to graduate courses. The work is organized in 13 departments, the college being one (subdivided in 10 courses), and occupies 23 buildings. It was removed to its present location 1872.

Pennsylvania College. Established 1832 at Gettysburg, Pa.; under Lutheran control. It has 15 instructors and ab. 250 students, including the preparatory department.

Pennsylvania Dutch. Dialect developed among immigrants to Pa., on a s. German basis. It has a limited literature, including poems by H. Harbaugh and C. C. Ziegler.

Pennsylvania State College. At State College, Pa.; began as Farmers'High School 1859: named Agricultural College 1862, and State Coll. 1874. It is organized in 7 schools, and offers 12 full and 8 short courses, mainly in the applied sciences. There are 52 instructors. General library, 10,500 vols., with special libraries in the departments.

Penny. English silver coin, first struck ab.700. Its value declined till ab, 1560, and has since been Tv shilling. Copper


Penny of William I. half pence and farthings have been minted from 1672, pennies since 1797. The Scotch coinage was of much lower value.

Penny, Virginia, b. 1826. American writer on employments of women, 1863-68.

Pennycress. Thlaspi arvense. Herb of the Mustard family, native of Europe, introduced as a weed into America: known as Mithridate Mustard. Penny Postage. See Post-office. Pennyroyal. Mentha pulegium. Herb of the Mint family, native of Europe, cultivated for its pungent volatile oil.

Pennyroyal, American. Low odorous herbs of the genus Hedeoma, of the Mint family, natives of America. The common e. species is H. pulegioides, and has nearly the taste and odor of the true Pennyroyal. Mentha canadensis has similar I properties and is also called Pennyroyal. These are used as a | domestic medicine.

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Penny Weddings. Seventeenth century practice in Scotland. The invited guests contributed a small amount of money, seldom more than a shilling, to pay the general expenses of the wedding. It was denounced 1(545 by an Act of the General Assembly.

Pennyweight. In Troy weight, 24 grains. See Weights and Measures.

Pennywort, Water. Low marsh herbs of the genus Hydrocotyle, natural family Umbelliferce, of wide geographical distribution.

Peiiobiieot. Chief river of Me. It heads in the w. part of the State, Bowse, and s.e.. and then s. to P. Bay. It is navigable to Bangor. Length ab. 300 m., drainage area 8,934 sq. m., discharge 2,438 cu. ft. per second.

Penology. See Prison Discipline.

Penry, John (ap Henry), 1559-1593. Welsh Puritan, unjustly executed.

Pensacola. City of Escambia CO., Fla., on P. Bay. It has a good harbor and considerable commerce. It was an old Spanish settlement, held for a time by the British, taken by Jackson 1814, and annexed with Fla. 1819. The Navy-yard near bv was seized by Confederates Jan. 1861, but regained 1862. Pop., i890, 11,750.

Penshurst. Town in Kent, England, at the confluence of the Eden and the Tun, 4i m. w.s.w. of Tunbridge. It is noted

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for its old homes. P. Palace is the birthplace of Sir Philip Sydney 1554. Pop. 2,000.

Pension. Sum paid from time to time by government, or a former employer, in consideration of past services. June 30, 1890, there were 970.678 names on the pension rolls of U. S.: War of 1812. 7 survivors, 3.287 widows; Mexican War, survivors 11,800, widows 8.017; Indian Wars, survivors 2,718, widows 4,237; War for the Union. 1861-65. invalids 749,530, widows, etc., 191,082. The total disbursements for the year ending June 30, 1896, were f 139.280,075; from 1861-96, $1,997,515,154.72. The amount paid for pensions by Great Britain annually for army, navy and civil lists is ab. £7,500,000.

Pensionary, Grand. Sec. of Holland till 1795. His duties were largely financial and diplomatic. Barneveldt made the post eminent.

Pentacarpcllary. Ovary of five carpels.

Pentaele. Five-pointed figure used as a symbol of mystery, perfection, or of the universe by Gnostics, Neoplatonists, and Pythagoreans. In the Middle Ages it was used to keep off evil spirits and witches.

Pentacrtnltes. Group of Crinoids with pentangular stems.

Pentacrinus. Most common of living species of Crinoids (q.v.); stone-lilies.

Pentaetine. Sponge spicule, produced from a hexactine by suppression of one ray.

PentadelphouM. Flowers whose stamens are united into five sets, as in the Basswood.

Pentads. In Chemistry, elements having a valence of V.

Pentagon. Polygon of five sides.

Pcntngynia. Order of plants which have five pistils.

Pciitagynons. Flowers with five pistils.

Pentamera. Coleoptera, with five-jointed tarsus. Here are included most of the Beetles, such as the Spring-beetles

(Elateridiv), LameUicornia, Palpicm-nia, Swimming-beetles (Dytiseidte). Tiger-beetles (Cincindelickr), and the great family of Running-beetles (Carabidve). See these terms.

Pcntaineron. Fifty Neapolitan stories by G. Basile 1637. Thirty-one of them were tr. 1848.

Pcntanierous. Having five similar parts; e.g.. arms of a Starfish, and joints in the tarsus of certain beetles.—In Botany, flowers constructed on the plan of five members of each circle of parts, or the quinary plan.

Pentameter. Verse of five feet; abridged hexameter, common in Greek, Latin, and modern poetry.

Pentaniethyl Pararosaniline. See Methyl Violet.

Pentandria. Linna?an class of plants, comprising all those which have five separate stamens, as in Forget-me-not, Potato, Cowslip, Violet, and Flax.

Pentandrous. Flowers with five stamens.

Pentane. C6H1V Amy] hydride; known in three forms; which are liquids boiling 9°-37° C.; present in crude petroleum, and a considerable constituent of petroleum ether; used in standard burners to furnish a standard of light.

Pentapetalous. Corolla of five petals.

Pcntaphyllous. Calyx or corolla composed of five leaves (sepals or petals).

Pentasepalous. Calyx composed of five sepals, as in the Apple.

Pcntnstichoug. Five-ranked, as the arrangement of the alternate leaves in the Apple, the scales of a Larch cone, and numerous other plants.

Pentastomida. See Linqcatulida.

Pentateuch. "Five books of Moses," first of O. T.

Pcntathionlc Acid. H^tOs. Made by the action of hydrogen sulphide upon a solution of sulphur dioxide in water.

Pentathlon. Five exercises of running, leaping, wrestling, throwing the discus, and throwing the spear, popular in ancient Greece.

Pentecost. In Israel, second of the three great feasts, celebrated fifty days after the Passover, commemorating the harvest.—In the Christian Church, day of the Descent of the Spirit.

Penthesilea. Queen of Amazons; ally of Priam; slain by Achilles.

Penthcns. King of Thebes; driven mad by Bacchus and torn in pieces by his mother and aunts in a Bacchic frenzy.

Pentland Firth. Between Scotland and the Orkneys, connecting North Sea and Atlantic.

Pentland Hills. S.w. of Edinburgh: 16 m. long. Greatest altitude 1,898 ft. Here a body of Covenanters was defeated Nov. 28, 1666.

Pentonville. District of London, containing a model prison, erected 1840-42.

Pcntremltes. See Blastoidea.

Pcntyl Alcohols. CjH,,OH. Seven compounds of this formula are known. Only two are of importance, the AlTYL Alcohols (q.v.).

Penumbra. One of the results of the rectilinear propagation of radiation is the production of shadows cast by opaque objects. If, e.g., a disk be held opposite a candle flame, and straight lines be drawn from the flame tangent to the edge of the disk, the conical space behind the disk which receives no

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