Tolstoy, Leo, Count, Rus. Lev Nikolayevich Tolstoi (lyĕf), 1828-1910, Russian novelist and philosopher, considered one of the world's greatest writers.

Early Life

Of a noble family, Tolstoy was born at Yasnaya Polyana, his parents' estate near Tula. Orphaned at nine, he was brought up by his aunts and privately tutored. At 16 he was sent to the Univ. of Kazan, where he studied languages and law. His classes bored him, and he left without a degree. He returned to his estate in 1849 and made several abortive attempts to aid and educate the serfs there. Tolstoy then began a profligate life in Moscow and St. Petersburg.

Early Works

In 1851 Tolstoy followed his brother into army service in the Caucasus, where he wrote Childhood (1852). This became the first part of an autobiographical trilogy, which includes Boyhood (1854) and Youth (1857). In 1854 he took part in the defense of Sevastopol, descriptions of which were published in Nekrasov's journal The Contemporary, attracting considerable attention for their unvarnished picture of war. He left army service in 1855 and for several years divided his time between his estate and the literary circles of St. Petersburg. His diary of the period reveals his intense dissatisfaction with his libertine existence. He set up a school for peasant children on his estate, emphasizing a spontaneous approach to learning. When his school proved impractical, he visited Western Europe and there began to question the bases of modern civilization.

In 1862 Tolstoy married Sophia Andreyevna Bers, a young, well-educated woman who bore him 13 children. His candor concerning his infidelities and his harsh conception of her wifely duties contributed to the instability of their marriage. During this time he wrote The Cossacks (1863) and his masterpieces War and Peace (1862-69) and Anna Karenina (1873-76). War and Peace is a vast prose epic of the Napoleonic invasion of 1812. It illustrates Tolstoy's view of history as proceeding inexorably to its own ends, a view in which mankind appears as an accidental instrument. This thesis is conveyed by a stream of brilliantly conceived characters and incidents. Anna Karenina, his most popular work, concerns the tragedy of a woman's faith in romantic love.

Later Life and Works

About 1876 the doubts that had beset Tolstoy since youth, fed by his puritan temperament in conflict with his sensuality, gathered force. The result of his painful self-examination was his conversion to the doctrine of Christian love and acceptance of the principle of nonresistance to evil. The steps in his conversion are set forth in his Confession (1879). For the rest of his life Tolstoy dedicated himself to the practice and propagation of his new faith, which he expounded in a series of works, among them A Short Exposition of the Gospels (1881), What I Believe In (1882), What Then Must We Do? (1886), and The Law of Love and the Law of Violence (1908).

Tolstoy preached nonviolence and a Rousseauistic simplicity of life. He was an anarchist to the extent that he considered wrong all organizations based on the premise of force, including both the government and the church. A Tolstoy cult grew up in Russia and abroad, and his estate became a place of pilgrimage. Because of his prestige the government did not interfere with his activities, although the Russian Church excommunicated him in 1901.

Moral questions are central to Tolstoy's later works, which include the story "The Death of Ivan Ilyich" (1884), the drama The Power of Darkness (1886), and the novel The Kreutzer Sonata (1889). To his last period belongs the essay What Is Art? (1897-98), in which he argued for the moral responsibility of the artist to make his work understandable to most people; he denounced acknowledged masterpieces, including his own earlier works. His last works also include the novels Hadji Murad (1896-1904) and Resurrection (1899-1900) and the drama The Living Corpse (pub. 1911).

Tolstoy's insistence on putting his beliefs into practice and abandoning all earthly goods led to a permanent breach between himself and his wife. His children, with the exception of the youngest daughter, Alexandra, sided with their mother. In 1910, at 83, Tolstoy left home with Alexandra without a specific destination. He caught a chill and died at the railroad stationmaster's house at Astapovo.

Szilard, Leo, 1898-1964, American nuclear physicist and biophysicist, born in Hungary. He was educated at the Budapest Institute of Technology and the Univ. of Berlin, receiving a doctorate from the latter in 1922. Working at the Univ. of Chicago with Enrico Fermi, he developed the first self-sustained nuclear reactor based on uranium fission. Szilard was one of the first to realize that nuclear chain reactions could be used in bombs and was instrumental in urging the U.S. government to prepare the first atomic bomb, but he later actively protested nuclear warfare and supported the use of nuclear energy for peaceful purposes.

Strauss, Leo, 1899-1973, American philosopher, b. Hesse, Germany. Strauss fled the Nazis and came to the United States, where he taught at the Univ. of Chicago (1949-68). Strauss is known for his controversial interpretations of political philosophers, including Xenophon and Plato. Strauss wrote an influential critique of modern political philosophy, i.e., philosophy since Machiavelli, arguing that it suffers from an inability to make value judgments about political regimes, even about obviously odious ones. As a model for how political philosophy should proceed, Strauss held up the work of the Ancients, i.e., Xenephon and Plato. He defended the antihistoricist position that it is possible for a person to grasp the thought of philosophers of different eras on their own terms, i.e., unencumbered by presuppositions inherent in his own historical context. Strauss's works include Natural Right and History (1952), Thoughts on Machiavelli (1958), and The City and Man (1964).

Sowerby, Leo, 1895-1968, American composer and organist, b. Grand Rapids, Mich. Sowerby studied at the American Conservatory, Chicago, and with Percy Grainger. In 1921 an American Prix de Rome was created to enable him to study in Rome. In 1925 he became teacher of composition at the American Conservatory and in 1927 organist and choirmaster of St. James Episcopal Church, Chicago. A prolific composer, he wrote such important works as A Set of Four (1917), Symphony in B Minor (1927), and Symphony in F Sharp Minor (1940), for orchestra; a concerto (1938), for organ and orchestra; the oratorios The Vision of Sir Launfal (1925) and The Canticle of the Sun (1944); and Symphony in G (1932), for organ.

Slezak, Leo, 1873-1946, Czech tenor, pupil of Jean de Reszke. After his debut as Lohengrin at Brno in 1896, he sang in Vienna, Berlin, and later at the Metropolitan Opera, New York City (1909-12). He was famous for his robust voice and physique, and he had a flamboyant sense of humor.

Leo XIII, 1810-1903, pope (1878-1903), an Italian (b. Carpineto, E of Rome) named Gioacchino Pecci; successor of Pius IX. Ordained in 1837, he earned an excellent reputation as archbishop of Perugia (1846-77), and was created cardinal in 1853. Leo's election brought a turn in the course of the papacy; he was abreast of the times and tried, especially by preaching to the whole church, in encyclical letters, to form Roman Catholic attitudes appropriate to living in the modern world. His influence was increased by the length of his reign; thus he was able to furnish the college of cardinals with an unusual number of excellent men (including John Henry Newman in 1879 and James Gibbons in 1886). By a combination of vigor and tact he ended the Kulturkampf (1887). He tried repeatedly to bring French Roman Catholics to support the republic. In 1885 his encyclical Immortale Dei charted the course of Catholics as responsible citizens in modern secular, democratic states; he thus refuted both the French royalists' claim that they were especially good Catholics and the contention of French anti-Catholics that the church was committed to political reaction. The letter was a great vindication of Catholic democrats. With the anti-Catholic government of Italy there was no conciliation. Leo's program for society appeared in Rerum novarum (1891), an arraignment of capitalism that also showed the insufficiencies of Marxian socialism; it set up Catholic aims and ideals. (It was supplemented in Quadragesimo Anno [1931] of Pius XI and in Mater et Magistra [1961] of John XXIII.) Leo met the intellectual attack on Christianity by advancing Thomism, with its insistence that there can be no conflict between science and faith; to this end he wrote Aeterni Patris (1879), declaring the philosophy of St. Thomas Aquinas official and requiring its study; he also founded the institute of Thomistic philosophy at the Univ. of Louvain. He was profoundly interested in the advancement of learning. He opened the Vatican secret archives to all scholars, and he reminded Catholic historians that nothing but the whole truth must be found in their work. He encouraged Bible study and set up (1902) the permanent Biblical Commission. He sponsored a number of faculties and universities, including the Catholic Univ. at Washington, D.C. For sheer productivity Leo surpassed all his predecessors in modern times. He was succeeded by Pius X.

Leo X, 1475-1521, pope (1513-21), a Florentine named Giovanni de' Medici; successor of Julius II. He was the son of Lorenzo de' Medici, was made a cardinal in his boyhood, and was head of his family before he was 30 (see Medici). Leo was not a competent ruler; he was a good, pious man, a dilettante of letters and art, but not greatly interested in the advancement of the church. His chief fame rests on his patronage of Raphael, on the continuation of St. Peter's by Bramante, and on his literary circle, including Cardinals Bembo and Bibbiena and many others. The Fifth Lateran Council, called with the hope that it would effect reforms, achieved little. The Protestant Reformation began when Martin Luther posted (1517) his famous theses against the sale of indulgences, an activity practiced by Leo to provide income for his building program. Leo excommunicated the reformers, notably with the bull Exsurge Domine (1520), but he failed to deal effectively with the trouble. In politics he brought the papacy temporary hegemony in Italy by dexterity in diplomatic maneuvers. Leo granted Henry VIII of England the title Defender of the Faith (Defensor Fidei). He was succeeded by Adrian VI.

Leo VI (Leo the Wise or Leo the Philosopher), 862?-912, Byzantine emperor (886-912), son and successor of Basil I. He added to the work of his father by the publication (887-93) of the Basilica, a modernization of the law of Justinian I and of canon law. Leo attempted to end the schism which had been provoked by the patriarch Photius, but the quarrel was renewed (906), partly over the issue of Leo's fourth marriage. During his reign, Leo was forced to pay tribute to the Bulgars after his defeat in 896. The Arabs completed the conquest of Sicily by taking Taormina in 902. They then sacked Salonica (906), and advanced in Asia Minor. Among Leo's edicts are the Tactics, for the army and navy, and the Book of the Prefect, on the duties of that officer, including his jurisdiction over the guilds of Constantinople. Leo was succeeded by his brother Alexander (reigned 912-13) and by his son Constantine VII.

Leo V (Leo the Armenian), d. 820, Byzantine emperor (813-20), successor of Michael I. A former general, Leo successfully defended (813) Constantinople against the Bulgars and concluded a 30-year truce with them. Reviving iconoclasm, he deposed the patriarch Nicephorus in the synod of 815 and persecuted the orthodox party led by Theodore of Studium. Leo was murdered by the supporters of his successor, Michael II.

Leo IX, Saint, 1002-54, pope (1049-54), a German named Bruno of Toul, b. Alsace; successor of Damasus II. A relative of Holy Roman Emperor Henry III, he was educated at Toul and was made bishop there in 1027. Leo traveled widely, vigorously combating clerical incontinence and simony; his pontificate marks the beginning of papal reform in the 11th cent. The heresy of Berengar of Tours concerning the Real Presence also occupied the attention of the pope. St. Leo mediated questions presented by England, France, and Hungary. He added to the papal lands in Italy through an exchange with Emperor Henry III. He fought the Normans of S Italy, but was defeated (1053) at Civitella. The bitter feeling between East and West brought an attack (1053) on the pope by Michael Cerularius, the patriarch of Constantinople. This culminated in the excommunication of Michael and those in his communion by the papal legates (1054). He was succeeded by Victor II. Feast: Apr. 19.

Leo IV, Saint, d. 855, pope (847-55), a Roman; successor of Sergius II. He had seen the Saracen attack on Rome (846), and to prevent its recurrence he fortified the city and its suburbs. He built a wall around the Vatican, established and fortified the part of Rome still called the Leonine City, and rebuilt churches. He crowned Louis II Holy Roman emperor. Leo was succeeded by Benedict III. Feast: July 17.

Leo IV (Leo the Khazar), d. 780, Byzantine emperor (775-80), son and successor of Constantine V. He owed his nickname to his mother, a Khazar princess. Leo tempered the iconoclastic excesses of his father's reign. On his death his Athenian wife, Irene, became regent for their son, Constantine VI.

Leo III, Saint, pope (795-816), a Roman; successor of Adrian I. He was attacked about the face and eyes by members of Adrian's family, who hoped to render him unfit for the papacy. Leo recovered and fled (799) to Charlemagne's protection at Paderborn. In 800, Charlemagne went to Rome and conducted a trial during which Leo successfully defended himself against charges of misconduct made by his enemies. On Christmas day, 800, Leo crowned Charlemagne emperor, the event that traditionally marks the beginning of the Holy Roman Empire. Leo's successor, Stephen IV, crowned Charlemagne's son, Louis the Pious, and thus was established the papal claim to the right to consecrate the emperor. In the East-West controversy over the Procession of the Holy Spirit, Leo declared that the Filioque of the creed was dogmatically necessary but liturgically dispensable, and he recommended its omission in the name of East-West unity. Leo did much to beautify Christian Rome. He was canonized in 1673. Feast: June 12.

Leo III (Leo the Isaurian or Leo the Syrian), c.680-741, Byzantine emperor (717-41). He was probably born in N Syria (rather than in Isauria, as once thought). He held diplomatic and military posts before he deposed and succeeded Theodosius III. His accession ended the anarchy into which the empire had fallen since the reign of Justinian II. Leo defended Constantinople against the last Arab siege (717-18), and although he had to contend with Arab attacks in Asia Minor, he succeeded in ending serious Arab threats for nearly two centuries and reorganized the military provinces (themes) of the empire for greater efficiency. His civil code, the Ecloga, written in Greek rather than in Latin, was a practical handbook that had considerable influence in Byzantium. He is also credited with issuing military, maritime, and rural codes. Leo's attack (726) on devotion to holy images began the long struggle over iconoclasm. Riots and rebellions broke out in Greece, while Byzantine rule in Italy (the exarchate of Ravenna and the Pentapolis of Rimini, Ancona, Fano, Pesaro, and Senigallia) began to crumble. The popes Gregory II and Gregory III opposed Leo's iconoclasm and successfully defied his armed expeditions, thus virtually ending Byzantine suzerainty over Rome. Nevertheless, Leo left a revitalized empire to his son, Constantine V. The Isaurian, or Syrian, dynasty, which he founded, ruled the Byzantine Empire until 802.

Leo I, Saint (Saint Leo the Great), c.400-461, pope (440-61), an Italian; successor of St. Sixtus III. A Doctor of the Church, he was one of the greatest pontiffs of the early years of the church. He waged a firm campaign against schism and heresy. With the aid of Valentinian III, the Roman emperor of the West, he campaigned to eliminate Manichaeism from Italy. Later, asserting his authority over St. Hilary of Arles, he obtained an imperial rescript that effectively confirmed the authority of the pope over all his bishops. In the Nestorian-Monophysite controversy Leo was the leader in defending Catholic teaching. He wrote the celebrated Tome of Leo, a doctrinal letter defining the two natures and one person of Christ that was later adopted as ecumenical at Chalcedon (see Chalcedon, Council of), when the heresiarch Eutyches was condemned. He was also effective as a statesman and met (452) Attila the Hun to persuade him not to invade Rome. In 455 he similarly urged Gaiseric the Vandal to spare the lives of the Romans. St. Leo's letters and sermons reflect the many aspects of his career and personality, including his great personal influence for good, and are invaluable historical sources. His rhythmic prose style, called cursus leonicus, influenced ecclesiastical language for centuries. The celebrated Leonian Sacramentary, the oldest form of the Roman Missal, is probably not his work. He was succeeded by St. Hilary. Feast: Apr. 11.

Leo I, d. 474, Byzantine or East Roman emperor (457-74). Chosen by the senate to succeed Marcian, he sought to counteract the preponderance of Germans in the Roman army by enlisting Isaurians. A naval expedition (468) against the Vandals of Africa failed through the incompetence or treachery of the commander, Basiliscus, who delayed his attack until Gaiseric was able to attack him with fire ships. Leo I was succeeded by his grandson, Leo II, a child of six, who died in the year of his accession; Leo I's son-in-law, Zeno, then became emperor.

Leo Africanus, c.1465-1550, Moorish traveler in Africa and the Middle East. His Arabic name was Al-Hasan ibn Muhammad. Captured by pirates, he was sent as a slave to Pope Leo X. He became a Christian, adopting the name Johannes Leo, and taught Arabic in Rome. There he wrote in Arabic a description of his journeys in Africa (issued in Italian in 1526), which was for many years the only known source on the Sudan. An English translation (1600) was reissued by the Hakluyt Society as The History and Description of Africa (3 vol., 1896, repr. 1963).

Leo [Lat.,=the lion], northern constellation lying S of Ursa Major and on the ecliptic (apparent path of the sun through the heavens) between Cancer and Virgo; it is one of the constellations of the zodiac. The Egyptians, Babylonians, Arabs, and Greeks all represented this constellation as a lion; it may be the first constellation to be pictorially represented. The most famous star in Leo is Regulus (Alpha Leonis). The western part of the constellation is a curved line known as the Sickle; it represents the lion's head. The main constellation terminates in Denebola (Beta Leonis), the Lion's Tail. The meteor showers known as the Leonids appear to come from this constellation. Leo reaches its highest point in the evening sky in April.

Klenze, Leo von, 1784-1864, German architect and landscape and portrait painter. He was court architect to Jérôme Bonaparte of Westphalia and to Louis I of Bavaria, for whom he built many structures in the Italian Renaissance and neo-classical styles. His chief works in Munich were the Glyptothek (1816-30), the Pinakothek, and the Odeon (1828). In 1839 he began additions to the Hermitage in St. Petersburg.

Frobenius, Leo, 1873-1938, German archaeologist and anthropologist. An authority on prehistoric art and culture, especially of Africa, he organized 12 expeditions to Africa between 1904 and 1935. In 1922 he founded the Institute for Cultural Morphology, Frankfurt, where he established a noted collection of facsimiles of prehistoric paintings and engravings. He also dealt with living African cultures and their folklore. He wrote The Voice of Africa (tr. 1913) and was coauthor (in English) of Prehistoric Rock Pictures in Europe and Africa (1937).

Friedlander, Leo, 1890-1966, American sculptor, b. New York City, studied in New York, Paris, Brussels, and at the American Academy in Rome. His many decorative works include sculptures on Washington Memorial Arch, Valley Forge, Pa.; reliefs for the National Chamber of Commerce, Washington, D.C.; the main central pediment of the Museum of the City of New York; facades for the Jefferson County (Ala.) Courthouse; groups for the RCA (now GE) Building, Rockefeller Center, New York City; and statues for the Oregon state capitol.

Caprivi, Leo, Graf von, 1831-99, German chancellor, whose full name was Georg Leo, Graf von Caprivi de Caprara de Montecuculi. A former army officer and head of the admiralty, he succeeded (1890) Bismarck as chancellor. Under him the antisocialist law was abrogated and military service was shortened from three to two years. Favoring industrial over agrarian interests, he negotiated (1892-94) a series of reciprocal trade agreements to stimulate industrial exports. The agreements reduced duties on agricultural products and aroused agrarian opposition to Caprivi, which contributed to his dismissal (1894). Prince Hohenlohe-Schillingsfürst succeeded him as chancellor.

Burnett, Leo, 1891-1971, American advertising executive, b. St. Johns, Mich., grad. Univ. of Michigan (1914). He was a newspaper reporter and worked in advertising before moving to Chicago and opening (1935) his own ad agency, Leo Burnett Co., which he headed until 1967. Crafting ad campaigns that emphasized brand image, focused on the highly visual, stressed the essential quality of a product, and established strong emotional ties with the consumer, he was a prime developer of advertising's "Chicago school." Burnett's company was especially known for the creation, in the 1950s and 60s, of commercial icons that were particularly suited to symbolizing various products on the new medium of television. These included such characters as the Marlboro Man, the Jolly Green Giant, the Pillsbury Doughboy, Morris the Cat, and Tony the Tiger.

Baekeland, Leo Hendrik, 1863-1944, American chemist, b. Belgium, grad. Univ. of Ghent, 1882. In 1889 he emigrated to the United States. He founded (1893) and conducted, until 1899, when he sold the rights to Eastman, a company for producing a photographic paper of his own invention. In 1909 he announced his invention of Bakelite, the first important synthetic plastic, and from 1910 to 1939 he served as president of the Bakelite Corp. He wrote Some Aspects of Industrial Chemistry (1914).

Baeck, Leo, 1873-1956, German rabbi and scholar. He studied at the conservative Jewish Theological Seminary of Breslau and then at the liberal Hochschule für die Wissenschaft des Judentums in Berlin, also attending the universities of Breslau and Berlin; at Berlin he studied philosophy under Wilhelm Dilthey. He held positions as rabbi in Oppeln (1897-1907), Düsseldorf (1907-12), and Berlin (1912-43). In 1943 he was sent to the Theresienstadt concentration camp. After being liberated in 1945, he moved to London, becoming president of the World Union for Progressive Judaism; he also taught occasionally at the Hebrew Union College in Cincinnati. Baeck's works in English translation include The Essence of Judaism (1905, tr. 1936), The Pharisees and Other Essays (1947), and Judaism and Christianity (1958). In This People Israel (1955, tr. 1965), he propounded his belief in the eternal dialectical polarity between "mystery" and "command," the latter being the divine instructions that give concrete expression to the "mystery" in terms of man's obligations to others, which he defined as piety.

(born Feb. 11, 1898, Budapest, Hung., Austria-Hungary—died May 30, 1964, La Jolla, Calif., U.S.) Hungarian-born U.S. physicist. He taught at the University of Berlin (1922–33), then fled to England (1934–37) and the U.S., where he worked at the University of Chicago from 1942. In 1929 he established the relation between entropy and transfer of information, and in 1934 he helped develop the first method of separating isotopes of artificial radioactive elements. He helped Enrico Fermi conduct the first sustained nuclear chain reaction and construct the first nuclear reactor. In 1939 he was instrumental in establishing the Manhattan Project, in which he helped develop the atomic bomb. After the first use of the bomb, he promoted the peaceful uses of atomic energy and the control of nuclear weapons, founding the Council for a Livable World. In 1959 he received the Atoms for Peace Award.

Learn more about Szilard, Leo with a free trial on Britannica.com.

orig. Bruno, count von Egisheim und Dagsburg

(born 1002, Egisheim, Alsace, Upper Lorraine—died April 19, 1054, Rome; feast day April 19) Pope (1049–54). He was consecrated bishop of Toul in 1027. He was named pope by Emperor Henry III but insisted on election by the clergy and people of Rome. His efforts to strengthen the papacy and eradicate clerical marriage and simony laid the foundation for the Gregorian reform movement. His assertion of papal primacy and his military campaign against the Normans in Sicily (1053) alienated the Eastern church. His representatives excommunicated the patriarch of Constantinople. Though Leo had already died, their act triggered the Schism of 1054.

Learn more about Leo IX, Saint with a free trial on Britannica.com.

known as Leo the Great

(born 4th century, Tuscany?—died Nov. 10, 461, Rome; Western feast day November 10, Eastern feast day February 18) Pope (440–461). He was a champion of orthodoxy and a Doctor of the Church. When the monk Eutyches of Constantinople asserted that Jesus Christ had only a single divine nature, Leo wrote the Tome, which established the coexistence of Christ's human and divine natures. Leo's teachings were embraced by the Council of Chalcedon (451), which also accepted his teaching as the “voice of Peter.” Leo dealt capably with the invasions of barbaric tribes, persuading the Huns not to attack Rome (452) and the Vandals not to sack the city (455). Leo was also an exponent of the precept of papal primacy, and his personal example and letters and sermons contributed greatly to the growth of papal authority.

Learn more about Leo I, Saint with a free trial on Britannica.com.

orig. Vincenzo Gioacchino Pecci

(born March 2, 1810, Carpineto Romano, Papal States—died July 20, 1903, Rome) Pope (1878–1903). Born into the Italian nobility, he was ordained a priest in 1837 and entered the diplomatic service of the Papal States. He was appointed bishop of Perugia in 1846 and was named a cardinal in 1853. He was elected pope in 1878, and, despite his advanced age and frail health, he directed the church for a quarter of a century. Like his predecessor, Pius IX, he opposed Freemasonry and secular liberalism, but he brought a new spirit to the papacy by adopting a conciliatory attitude toward civil governments and taking a more positive view of scientific progress.

Learn more about Leo XIII with a free trial on Britannica.com.

orig. Giovanni de' Medici

(born Dec. 1, 1475, Florence—died Dec. 1, 1521, Rome) Pope (1513–21), one of the most extravagant of the Renaissance pontiffs. The second son of Lorenzo de' Medici, he was educated at his father's court in Florence and at the University of Pisa. He was named a cardinal in 1492, and in 1494 he was exiled from Florence by the revolt of Girolamo Savonarola. He returned in 1500 and soon consolidated Medici control of the city. As pope, he became a patron of the arts, accelerating construction of St. Peter's Basilica. He strengthened the papacy's political power in Europe, but his lavish spending depleted his treasury. He discouraged reforms at the fifth Lateran Council, and he responded inadequately to the Reformation, excommunicating Martin Luther in 1521 and failing to address the need for change, a lapse that signaled the end of the unified Western church.

Learn more about Leo X with a free trial on Britannica.com.

(born Feb. 11, 1898, Budapest, Hung., Austria-Hungary—died May 30, 1964, La Jolla, Calif., U.S.) Hungarian-born U.S. physicist. He taught at the University of Berlin (1922–33), then fled to England (1934–37) and the U.S., where he worked at the University of Chicago from 1942. In 1929 he established the relation between entropy and transfer of information, and in 1934 he helped develop the first method of separating isotopes of artificial radioactive elements. He helped Enrico Fermi conduct the first sustained nuclear chain reaction and construct the first nuclear reactor. In 1939 he was instrumental in establishing the Manhattan Project, in which he helped develop the atomic bomb. After the first use of the bomb, he promoted the peaceful uses of atomic energy and the control of nuclear weapons, founding the Council for a Livable World. In 1959 he received the Atoms for Peace Award.

Learn more about Szilard, Leo with a free trial on Britannica.com.

orig. Bruno, count von Egisheim und Dagsburg

(born 1002, Egisheim, Alsace, Upper Lorraine—died April 19, 1054, Rome; feast day April 19) Pope (1049–54). He was consecrated bishop of Toul in 1027. He was named pope by Emperor Henry III but insisted on election by the clergy and people of Rome. His efforts to strengthen the papacy and eradicate clerical marriage and simony laid the foundation for the Gregorian reform movement. His assertion of papal primacy and his military campaign against the Normans in Sicily (1053) alienated the Eastern church. His representatives excommunicated the patriarch of Constantinople. Though Leo had already died, their act triggered the Schism of 1054.

Learn more about Leo IX, Saint with a free trial on Britannica.com.

known as Leo the Isaurian

(born circa 675, Germanicia, Commagene, Syria—died June 18, 742, Constantinople) Byzantine emperor (717–41), founder of the Isaurian dynasty. A high-ranking military commander, he seized the throne with the help of Arab armies who hoped to subjugate the Byzantine Empire. He then successfully defended Constantinople against the Arabs (717–718). Having crowned his son Constantine V coemperor (720), Leo used his son's marriage to cement an alliance with the Khazars. Victory over the Arabs at Akroïnos (740) was crucial in preventing their conquest of Asia Minor. Leo issued an important legal code, the Ecloga (726). His policy of iconoclasm (730), which banned the use of sacred images in churches, engendered a century of conflict within the empire and further strained relations with the pope in Rome.

Learn more about Leo III with a free trial on Britannica.com.

known as Leo the Great

(born 4th century, Tuscany?—died Nov. 10, 461, Rome; Western feast day November 10, Eastern feast day February 18) Pope (440–461). He was a champion of orthodoxy and a Doctor of the Church. When the monk Eutyches of Constantinople asserted that Jesus Christ had only a single divine nature, Leo wrote the Tome, which established the coexistence of Christ's human and divine natures. Leo's teachings were embraced by the Council of Chalcedon (451), which also accepted his teaching as the “voice of Peter.” Leo dealt capably with the invasions of barbaric tribes, persuading the Huns not to attack Rome (452) and the Vandals not to sack the city (455). Leo was also an exponent of the precept of papal primacy, and his personal example and letters and sermons contributed greatly to the growth of papal authority.

Learn more about Leo I, Saint with a free trial on Britannica.com.

(born Nov. 14, 1863, Ghent, Belg.—died Feb. 23, 1944, Beacon, N.Y., U.S.) Belgian-born U.S. industrial chemist. A teacher of chemistry in Belgium, he immigrated to the U.S. in 1889. He invented Velox, the first commercially successful photographic paper, which could be developed under artificial light, and sold the rights to George Eastman for $1 million in 1899. His search for a substitute for shellac led to the discovery in 1909 of a method of forming a hard thermosetting plastic, which he named Bakelite, produced from formaldehyde and phenol. His discovery helped found the modern plastics industry.

Learn more about Baekeland, Leo (Hendrik) with a free trial on Britannica.com.

(born May 23, 1873, Lissa, Posen, Prussia—died Nov. 2, 1956, London, Eng.) Prussian-Polish rabbi, spiritual leader of German Jewry during the Nazi period. After earning his Ph.D. in philosophy at the University of Berlin, he served as a rabbi in Silesia, Düsseldorf, and Berlin, becoming the leading liberal Jewish religious thinker of his time. He synthesized Neo-Kantianism and rabbinic ethics in The Essence of Judaism (1905) and considered the Christian gospels as rabbinic literature in The Gospel as a Document of Jewish Religious History (1938). He negotiated with the Nazis to buy time for the German Jews; finally arrested, he was sent to the Theresienstadt concentration camp, where he wrote and lectured on Plato and Immanuel Kant. Liberated in 1945 on the day before he was to be executed, he settled in England.

Learn more about Baeck, Leo with a free trial on Britannica.com.

(Latin: “Lion”) In astronomy, the constellation lying between Cancer and Virgo; in astrology, the fifth sign of the zodiac, governing approximately the period July 23–August 22. Its symbol, a lion, has been associated with the Nemean lion slain by Heracles. The Nemean lion was considered invulnerable because its skin was impervious to arrows, but Heracles battered it to death with a club. Zeus put the lion in the sky as a constellation.

Learn more about Leo with a free trial on Britannica.com.

(born Feb. 24, 1831, Berlin-Charlottenburg, Prussia—died Feb. 6, 1899, near Crossen-an-der-Oder, Ger.) German soldier and politician. A distinguished soldier, he served as chief of the admiralty (1883–88). He succeeded Otto von Bismarck as Germany's imperial chancellor (1890–94) and Prussian minister president (1890–92). His achievements included an Anglo-German agreement concerning spheres of influence in Africa, commercial treaties with Austria, Romania, and other states, and the reorganization of the German army.

Learn more about Caprivi, (Georg) Leo, count von with a free trial on Britannica.com.

(born Nov. 14, 1863, Ghent, Belg.—died Feb. 23, 1944, Beacon, N.Y., U.S.) Belgian-born U.S. industrial chemist. A teacher of chemistry in Belgium, he immigrated to the U.S. in 1889. He invented Velox, the first commercially successful photographic paper, which could be developed under artificial light, and sold the rights to George Eastman for $1 million in 1899. His search for a substitute for shellac led to the discovery in 1909 of a method of forming a hard thermosetting plastic, which he named Bakelite, produced from formaldehyde and phenol. His discovery helped found the modern plastics industry.

Learn more about Baekeland, Leo (Hendrik) with a free trial on Britannica.com.

(born May 23, 1873, Lissa, Posen, Prussia—died Nov. 2, 1956, London, Eng.) Prussian-Polish rabbi, spiritual leader of German Jewry during the Nazi period. After earning his Ph.D. in philosophy at the University of Berlin, he served as a rabbi in Silesia, Düsseldorf, and Berlin, becoming the leading liberal Jewish religious thinker of his time. He synthesized Neo-Kantianism and rabbinic ethics in The Essence of Judaism (1905) and considered the Christian gospels as rabbinic literature in The Gospel as a Document of Jewish Religious History (1938). He negotiated with the Nazis to buy time for the German Jews; finally arrested, he was sent to the Theresienstadt concentration camp, where he wrote and lectured on Plato and Immanuel Kant. Liberated in 1945 on the day before he was to be executed, he settled in England.

Learn more about Baeck, Leo with a free trial on Britannica.com.