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 attack Athens to avenge the murder of his son Androgeus, for which Aegeus was directly or indirectly responsible, he laid siege to Megara. He finally gained possession of the city through the treachery of the king’s daughter Scylla, who, enamoured of Minos, pulled out the golden (or purple) lock from her father’s head, on which his life and the safety of the city depended (for similar stories, see Frazer, Golden Bough, iii. 1900, p. 358). Megara was captured, and Nisus, who died fighting (or slew himself), was changed into a sea-eagle. Minos, disgusted at Scylla’s treachery, tied her to the rudder of his ship, and afterwards cast her body ashore on the promontory called after her Scyllaeum; or she threw herself into the sea and swam after Minos, constantly pursued by her father, until at last she was changed into a ciris (a bird or a fish). In Virgil, Scylla, the daughter of Nisus, is confused with the sea-monster, the daughter of Phorcys. Nisus was the eponymous hero of the harbour of Nisaea, and local tradition makes no mention of his betrayal by his daughter. According to Roscher (in his Lexikon der Mythologie), who identifies the ciris with the heron, the story of Nisus and Scylla (like these of Aëdon, Procne, Philomela and Tereus) was invented to give an aetiological explanation of the characteristics of certain birds. The birds were regarded as originally human beings, whose acts and characters were supposed to account for certain habits of the birds into which they had been changed. E. Siecke, De Niso et Scylla in aves mutatis (progr. Berlin, 1884), holds that the purple or golden hair of Nisus is the sun, and Scylla the moon, and that the origin of the legend is to be looked for in a very ancient myth of the relations between the two, which he endeavours to explain with the aid of Indian and German parallels.

NITHARD (d. 844), Frankish historian, was the illegitimate son of Angilbert, the friend of Charlemagne, by Bertha, a daughter of the great emperor. He was educated at the imperial court and became abbot of St Riquier in commendam, never taking the vows. Little else is known about his life, but he appears to have served his cousin, Charles the Bald, on peaceful errands and also on the field of battle. He fought for Charles at Fontenoy in June 841, and died as the result of wounds received whilst fighting for him against the Northmen near Angouléme. The date of his death was probably the 14th of June 844. In the 11th century his body, with the fatal wound still visible, was found in the grave of his father, Angilbert. Nithard’s historical work consists of four books on the history of the Carolingian empire under the turbulent sons of the emperor Louis I., especially during the troubled period between 840 and 843. This Historiae or De dissensionibus filiorum Ludovici pii is valuable for the light which it throws upon the causes which led to the disintegration of the Carolingian empire. Although rough in style, partisan in character and sometimes incorrect in detail, the books are the work of a man who had an intimate knowledge of the events which he relates, who possessed a clear and virile mind, and who above all was not a recluse but a man of action. They are dedicated to Charles the Bald, at whose request they were written.

The Historiae has been printed several times. Perhaps the best edition is in Band ii. of the ''Monumenta Germaniae historica. Scriptores''; it has also been edited by A. Holder (Freiburg, 1882). It has been translated into German by J. von Jasmund (Berlin, 1851; new edition by W. Wattenbach, Leipzig, 1889); and into French in tome iii. of Guizot’s Collection des mémoires (Paris, 1824).

See O. Kuntzemüller, Nithafd und sein Geschichtswerk (Jena, 1873); G. Meyer von Knonau, Über Nithards vier Bücher Geschichten (Leipzig, 1866); and W. Wattenbach, Deutschlands Geschichtsquellen, Band i. (Berlin, 1904).

NITHSDALE, WILLIAM MAXWELL, (1676–1744), Jacobite leader, was a member of the family of (q.v.), being a son of Robert the 4th earl (d. 1696) and a collateral relation of Robert Maxwell (d. 1646) who was created earl of Nithsdale in 1620. He became famous by his loyalty to the royalist tradition of his family, and by the heroism of his wife Winifred, daughter of William Herbert, 1st marquess of Powis. After becoming earl in 1698 he served the exiled house of Stuart in secret, was suspected as a Jacobite conspirator, and was much molested on that account. In 1712 he resigned his estate to his son William (d. 1776), reserving a life rent to himself. When the Jacobite rising took place in 1715 he joined his friends in the north of England and was taken prisoner at Preston, being sent to London for trial. The countess of Nithsdale, who was at Terregles when she heard of the capture of her husband, followed him to London, making part of the journey on horseback in bitter winter weather. The earl and the other Jacobites were brought to trial in Westminster Hall on the 19th of January 1716, and condemned to death on the 9th of February. The execution was fixed for the 24th. The countess presented a petition to George I. which he refused to receive, and when she knelt before him and took hold of the skirts of his coat he dragged her half across the room before he could break away. Finding that no pardon could be obtained the countess laid a plan to rescue her husband from the Tower of London. With the help of two Jacobite ladies, Mrs Morgan and Mrs Mills, she very cleverly extricated her husband from his cell on the night before the day fixed for the execution by disguising him as a woman. The earl escaped from England and was followed by the countess, but not till she had gone back to Scotland to rescue important legal papers which proved the transfer of the estate to their son. The earl and countess went to Rome after a short stay in France. In Rome they were attached to the court of the Pretender and lived in poverty and obscurity. The earl died on the 20th of March 1744, and the countess in 1749. Their son, William Maxwell, regained the possession of the family property after his father’s death in 1744, since the government could only confiscate his father’s life-interest; but the title was forfeited, and he died childless.

NITRE, the name given to naturally occurring potassium nitrate; “cubic nitre” is sodium nitrate. The word is adapted from Lat. nitrum, which is itself adapted from Gr. . These words were originally applied to the naturally occurring sodium carbonate; the connexion with potassium nitrate (sal petrae or sal petrosum) may be traced to Raimon Lull’s name sal nitri, which substance, however, he distinguished from nitrum. In the 16th century the ancient nitrum became altered to natron, a term still used for native sodium carbonate, while nitrum, and its adaptation nitre, were retained for potassium nitrate or (q.v.).

NITRIC ACID (aqua fortis), HNO3, an important mineral acid. It is mentioned in the De inventione veritatis ascribed to Geber, wherein it is obtained by calcining a mixture of nitre, alum and blue vitriol. It was again described by Albert le Grand in the 13th century and by Raimon Lull, who prepared it by heating nitre and clay and called it “eau forte.” Glauber devised the process in common use to-day, viz. by heating nitre with strong sulphuric acid. Its true nature was not determined until the 18th century, when A. L. Lavoisier (1776) showed that it contained oxygen, whilst in 1785 H. Cavendish determined its constitution and showed that it could be synthesized by passing a stream of electric sparks through moist air. The acid is found to exist to a slight extent in the free condition in some waters but chiefly occurs in combination with various metals, as nitrates, principally as nitre or saltpetre, KNO3, and Chile saltpetre, NaNO3. It is formed when a stream of electric sparks is passed through moist air, and in the oxidation of nitrogenous matter in the presence of water.

For experimental purposes it is usually obtained by distilling potassium or sodium nitrate with concentrated sulphuric acid. The acid so obtained usually contains more or less water and some dissolved nitrogen peroxide which gives it a yellowish red colour. It may be purified by redistillation over barium and silver nitrates, followed by treatment of the distillate with a stream of ozonized air. The product so obtained is then redistilled under diminished pressure and finally distilled again from a sealed and evacuated apparatus (V. Veley and Manley, Phil. Trans., 1898, A. 291, p. 365). On the large scale it is obtained by distilling Chile saltpetre with concentrated sulphuric acid in horizontal cast iron stills, the vapours being condensed in a series of