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Rh Next in chronological order stands the letter “To the Children of Jason” (vi.). Jason, tyrant of Pherae, had been assassinated in 370 ; and no fewer than three of his successors had shared the same fate. Isocrates now urges Thebe, the daughter of Jason, and her half-brothers to set up a popular government. The date is 359 The letter to Archidamus III. (ix.)—the same person who is the imaginary speaker of Oration vi.—urges him to execute the writer’s favourite idea,—“to deliver the Greeks from their feuds, and to crush barbarian insolence.” It is remarkable for a vivid picture of the state of Greece; the date is about 356 The letter to Timotheus (vii., 345 ), ruler of Heraclea on the Euxine, introduces an Athenian friend who is going thither, and at the same time offers some good counsels to the benevolent despot. The letter “to the government of Mytilene” (viii., 350 ) is a petition to a newly established oligarchy, begging them to permit the return of a democratic exile, a distinguished musician named Agenor. The first of the two letters to Philip of Macedon (ii.) remonstrates with him on the personal danger to which he had recklessly exposed himself, and alludes to his beneficent intervention in the affairs of Thessaly; the date is probably the end of 342 The letter to Alexander (v.), then a boy of fourteen, is a brief greeting sent along with the last, and congratulates him on preferring “practical” to “eristic” studies—a distinction which is explained by the sketch of the author’s , and of his essay “Against the Sophists,” given above. It was just at this time, probably, that Alexander was beginning to receive the lessons of Aristotle (342 ). The letter to Antipater (iv.) introduces a friend who wished to enter the military service of Philip. Antipater was then acting as regent in Macedonia during Philip’s absence in Thrace (340-339 ). The later of the two letters to Philip (iii.) appears to be written shortly after the battle of Chaeronea in 338 The questions raised by it have already been discussed.

No lost work of Isocrates is known from a definite quotation, except an “Art of Rhetoric,” from which some scattered precepts are cited. Quintilian, indeed, and Photius, who had seen this “Art,” felt a doubt as to whether it was genuine. Only twenty-five discourses—out of an ascriptive total of some sixty—were admitted as authentic by Dionysius; Photius (circ. 850) knew only the number now extant—twenty-one.

With the exception of defects at the end of Or. xiii., at the beginning of Or. xvi., and probably at the end of Letters i., vi., ix., the existing text is free from serious mutilations. It is also unusually pure. The smooth and clear style of Isocrates gave few opportunities for the mistakes of copyists. On the other hand, he was a favourite author of the schools. Numerous glosses crept into his text through the comments or conjectures of rhetoricians. This was already the case before the 6th century, as is attested by the citations of Priscian and Stobaeus. Jerome Wolf and Koraes successively accomplished much for the text. But a more decided advance was made by Immanuel Bekker. He used five MSS., viz. (1) Codex Urbinas III., (this, the best, was his principal guide); (2) Vaticanus 936, ; (3) Laurentianus 87, 14, (13th century); (4) Vaticanus 65, ; and (5) Marcianus 415,. The first three, of the same family, have Or. xv. entire; the last two are from the same original, and have Or. xv. incomplete.

J. G. Baiter and H. Sauppe in their edition (1850) follow “even more constantly than Bekker.” Their apparatus is enriched, however, by a MS. to which he had not access—Ambrosianus O. 144,, which in some cases, as they recognize, has alone preserved the true reading. The readings of this MS. were given in full by G. E. Benseler in his second edition (1854–1855). The distinctive characteristic of Benseler’s textual criticism was a tendency to correct the text against even the best MS., where the MS. conflicted with the usage of Isocrates as inferred from his recorded precepts or from the statements of ancient writers. Thus, on the strength of the rule ascribed to Isocrates— —Benseler would remove from the text every example of hiatus (on the MSS. of Isocrates, see H. Bürmann, Die handschriftliche Überlieferung des Isocrates, Berlin, 1885–1886, and E. Drerup, in Leipziger Studien, xvii., 1895).

—In Oratores Attici, ed. Imm. Bekker (1823, 1828); W. S. Dobson (1828); J. G. Baiter and Hermann Sauppe (1850). Separately Ausgewählte Reden, Panegyrikos und Areopagitikos, by Rudolf Rauchenstein, 6th ed., Karl Münscher (1908); in Teubner’s series, by G. E. Benseler (new ed., by F. Blass, 1886–1895) and by E. Drerup (1906–&emsp;&emsp;); Ad Demonicum et Panegyricus, ed. J. E. Sandys (1868); Evagoras, ed. H. Clarke (1885). Extracts from Orations iii., iv., vi., vii., viii., ix., xiii., xiv., xv., xix., and Letters iii., v., edited with revised text and commentary, in Selections from the Attic Orators, by R. C. Jebb (1880); vol. i. of an English prose translation, with introduction and notes by J. H. Freese, has been published in Bohn’s Classical Library (1894). See generally Jebb’s Attic Orators (where a list of authorities is given) and F. Blass, Die attische Beredsamkeit (2nd ed., 1887–1898), and the latter’s Die Rhythmen der attischen Kunstprosa (1901). There is a special lexicon by S. Preuss (1904). On the philosophy of Isocrates and his relation to the Socratic schools, see Thompson’s ed. of Plato’s Phaedrus, Appendix 2.

 ISODYNAMIC LINES (Gr. , equal in power), lines connecting those parts of the earth’s surface where the magnetic force has the same intensity (see ).

 ISOGONIC LINES (Gr. , equiangular), lines connecting those parts of the earth’s surface where the magnetic declination is the same in amount (see ).  ISOLA DEL LIRI, a town of Campania, in the province of Caserta, Italy, 15 m. by rail N.N.W. of Roccasecca, which is on the main line from Rome to Naples, 10 m. N.W. of Cassino. Pop. (1901), town, 2384; commune, 8244. The town consists of two parts, Isola Superiore and Isola Inferiore; as its name implies it is situated between two arms of the Liri. The many waterfalls of this river and of the Fibreno afford motive power for several important paper-mills. Two of the falls, 80 ft. in height, are especially fine. About 1 m. to the N. is the church of San Domenico, erected in the 12th century, which probably marks the site of the villa of Cicero (see ).

 ISOMERISM, in chemistry. When Wöhler, in 1825, analysed his cyanic acid, and Liebig his quite different fulminic acid in 1824, the composition of both compounds proved to be absolutely the same, containing each in round numbers 28% of carbon, 33% of nitrogen, 37% of oxygen and 2% of hydrogen. This fact, inconsistent with the then dominating conception that difference in qualities was due to difference in chemical composition, was soon corroborated by others of analogous nature, and so Berzelius introduced the term isomerism (Gr. , composed of equal parts) to denominate the existence of the property of substances having different qualities, in chemical behaviour as well as physical, notwithstanding identity in chemical composition. These phenomena were quite in accordance with the atomic conception of matter, since a compound containing the same number of atoms of carbon, nitrogen, oxygen and hydrogen as another in the same weight might differ in internal structure by different arrangements of those atoms. Even in the time of Berzelius the newly introduced conception proved to include two different groups of facts. The one group included those isomers where the identity in composition was accompanied by identity in molecular weight, i.e. the vapour densities of the isomers were the same, as in butylene and isobutylene, to take the most simple case; here the molecular conception admits that the isolated groups in which the atoms are united, i.e. the molecules, are identical, and so the molecule of both butylene and isobutylene is indicated by the same chemical symbol C4H8, expressing that each molecule contains, in both cases, four atoms of carbon (C) and eight of hydrogen (H). This group of isomers was denominated metamers by Berzelius, and now often “isomers” (in the restricted sense), whereas the term polymerism (Gr. , many) was chosen for compounds like butylene, C4H8, and ethylene, C2H4, corresponding to the same composition in weight but differing in molecular formula, and having different densities in gas or vapour, a litre of butylene and isobutylene weighing, for instance, under ordinary temperature and pressure, about 2.5 gr., ethylene only one-half as much, since density is proportional to molecular weight.

A further distinction is necessary to a survey of the subdivisions of isomerism regarded in its widest sense. There are subtle and more subtle differences causing isomerism. In the case of metamerism we can imagine that the atoms are differently linked, say in the case of butylene that the atoms of carbon are joined together as a continuous chain, expressed by –C–C–C–C–, normally as it is called, whereas in isobutylene the fourth atom of carbon is not attached to the third but to the second carbon atom, i.e. –C–C. Now there are cases in which analogy of internal structure goes so far as to exclude even that difference in linking, the only remaining possibility <section end="Isomerism" />