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 ordinary light consists of electrical vibrations in an all-pervading ether which possesses the properties of an insulator and of a magnetic medium. Hertz himself gave an admirable account of the significance of his discoveries in a lecture on the relations between light and electricity, delivered before the German Society for the Advancement of Natural Science and Medicine at Heidelberg in September 1889. Since the time of these early experiments, various other modes of detecting the existence of electric waves have been found out in addition to the spark-gap which he first employed, and the results of his observations, the earliest interest of which was simply that they afforded a confirmation of an abstruse mathematical theory, have been applied to the practical purposes of signalling over considerable distances (see ). In 1889 Hertz was appointed to succeed R. J. E. Clausius as ordinary professor of physics in the university of Bonn. There he continued his researches on the discharge of electricity in rarefied gases, only just missing the discovery of the X-rays described by W. C. Röntgen a few years later, and produced his treatise on the Principles of Mechanics. This was his last work, for after a long illness he died at Bonn on the 1st of January 1894. By his premature death science lost one of her most promising disciples. Helmholtz thought him the one of all his pupils who had penetrated farthest into his own circle of scientific thought, and looked to him with the greatest confidence for the further extension and development of his work.

 HERTZ, HENRIK (1797–1870), Danish poet, was born of Jewish parents in Copenhagen on the 25th of August 1798. In 1817 he was sent to the university. His father died in his infancy, and the family property was destroyed in the bombardment of 1807. The boy was brought up by his relative, M. L. Nathanson, a well-known newspaper editor. Young Hertz passed his examination in law in 1825. But his taste was all for polite literature, and in 1826–1827 two plays of his were produced, Mr Burchardt and his Family and Love and Policy; in 1828 followed the comedy of Flyttedagen. In 1830 he brought out what was a complete novelty in Danish literature, a comedy in rhymed verse, Amor’s Strokes of Genius. In the same year Hertz published anonymously Gengangerbrevene, or Letters from a Ghost, which he pretended were written by Baggesen, who had died in 1826. The book was written in defence of J. L. Heiberg, and was full of satirical humour and fine critical insight. Its success was overwhelming; but Hertz preserved his anonymity, and the secret was not known until many years later. In 1832 he published a didactic poem, Nature and Art, and Four Poetical Epistles. A Day on the Island of Als was his next comedy, followed in 1835 by The Only Fault. Hertz passed through Germany and Switzerland into Italy in 1833; he spent the winter there, and returned the following autumn through France to Denmark. In 1836 his comedy of The Savings Bank enjoyed a great success. But it was not till 1837 that he gave the full measure of his genius in the romantic national drama of Svend Dyrings Hus, a beautiful and original piece. His historical tragedy Valdemar Atterdag was not so well received in 1839; but in 1845 he achieved an immense success with his lyrical drama Kong René’s Datter (King René’s Daughter), which has been translated into almost every European language. To this succeeded the tragedy of Ninon in 1848, the romantic comedy of Tonietta in 1849, A Sacrifice in 1853, The Youngest in 1854. His lyrical poems appeared in successive collections, dated 1832, 1840 and 1844. From 1858 to 1859 he edited a literary journal entitled Weekly Leaves. His last drama, Three Days in Padua, was produced in 1869, and he died on the 25th of February of the next year.

Hertz is one of the first of Danish lyrical poets. His poems are full of colour and passion, his versification has more witchcraft in it than any other poet’s of his age, and his style is grace itself. He has all the sensuous fire of Keats without his proclivity to the antique. As a romantic dramatist he is scarcely less original. He has bequeathed to the Danish theatre, in Svend Dyrings Hus and King René’s Daughter, two pieces which have become classic. He is a troubadour by instinct; he has little or nothing of Scandinavian local colouring, and succeeds best when he is describing the scenery or the emotions of the glowing south.

 HERTZBERG, EWALD FRIEDRICH, (1725–1795), Prussian statesman, who came of a noble family which had been settled in Pomerania since the 13th century, was born at Lottin, in that province, on the 2nd of September 1725. After 1739 he studied, chiefly classics and history at the gymnasium at Stettin, and in 1742 entered the university of Halle as a student of jurisprudence, becoming in due course a doctor of laws in 1745. In addition to this principal study, he was also interested while at the university in historical and philosophical (Christian Wolff) studies. A first thesis for his doctorate, entitled Jus publicum Brandenburgicum, was not printed, because it contained a criticism of the existing condition of the state. Shortly afterwards Hertzberg entered the government service, in which he was first employed in the department of the state archives (of which he became director in 1750), soon after in the foreign office, and finally in 1763 as chief minister (Cabinetsminister). In 1752 he married Baroness Marie von Knyphausen, a marriage which was happy, but childless.

For more than forty years Hertzberg played an active part in the Prussian foreign office. In this capacity he had a decisive influence on Prussian policy, both under Frederick the Great and Frederick William II. At the beginning of the Seven Years’ War (1756) he took part as a political writer in the Hohenzollern-Habsburg quarrel, both in his Ursachen, die S.K.M. in Preussen bewogen haben, sich wider die Absichten des Wienerischen Hofes zu setzen und deren Ausführung zuvorzukommen (“Motives which have induced the king of Prussia to oppose the intentions of the court of Vienna, and to prevent them from being carried into effect”), and in his Mémoire raisonné sur la conduite des cours de Vienne et de Saxe, based on the secret papers taken by Frederick the Great from the archives of Dresden. After the defeat at Kolin (1757) he hastened to Pomerania in order to organize the national defence there and collect the necessary troops for the protection of the fortresses of Stettin and Colberg. In the same year he conducted the peace negotiations with Sweden, and was of great service in bringing about the peace of Hubertsburg (1763), on the conclusion of which the king received him with the words, “I congratulate you. You have made peace as I made war, one against many.”

In the later years, too, of Frederick the Great’s reign, Hertzberg played a considerable part in foreign policy. In 1772, in a memoir based upon comprehensive historical studies, he defended the Prussian claims to certain provinces of Poland. He also took part successfully as a publicist in the negotiations concerning the question of the Bavarian succession (1778) and those of the peace of Teschen (1779). But in 1780 he failed to uphold Prussian interests at the election of the bishop of Münster. In 1784 appeared Hertzberg’s memoir containing a thorough study of the Fürstenbund. He championed this latest creation of Frederick the Great’s mainly with a view to an energetic reform of the empire, though the idea of German unity was naturally still far from his mind. In 1785 followed “An explanation of the motives which have led the king of Prussia to propose to the other high estates of the empire an association for the maintenance of the system of the empire” (Erklärung der Ursachen, welche S.M. in Preussen bewogen haben, ihren hohen Mitständen des Reichs eine Association zur Erhaltung des Reichssystems anzutragen). By upholding the Fürstenbund Hertzberg made many enemies, prominent among whom was the king’s brother, Prince Henry. Though the Fürstenbund failed to effect a reform of the empire, it at any rate prevented the fulfilment of Joseph II.’s old desire for the incorporation of Bavaria with Austria. The last act of state in which Hertzberg took part under Frederick the Great