ZISKA 829 covered with crystalline laminaa, which may be separated by dissolving the aluminum in hydrochloric acid. Thus obtained, zirconium is very hard, and resembles antimony in color, lustre, and brittleness. It is readily dissolved in nitromuriatic and hydrofluoric acid ; other acids have little effect. Its specific gravity is 4'15, symbol Zr, and atomic weight 89'6. There is one oxide, ZrO 2 , which acts both as a base and as an acid. Its hydrate gelatinizes, and dissolves readily in acids. The salts of zirconia have an astringent taste ; they are precipitated by the caustic alkalies, and not re- dissolved in excess of these. Infusion of galls gives a yellow precipitate with them, phos- phate of soda a white one. The mineral zir- con ranks among precious stones, its varieties in appearance being brown, red, yellow, gray, white, adamantine, and translucent. A variety of zircon, called jargon, exhibits in its natural state and when fused with borax a remarkable spectrum, which was supposed by Sorby to in- dicate the presence of a new element, for which he proposed the name of jargonium. Subse- quent research convinced him that the lines in the spectrum were due to the presence of uranic oxide, whence he concludes that the sup- posed jargonium has no existence. Zirconia has been employed as a substitute for lime or magnesia in oxy hydrogen illumination. It is very refractory, and possesses great radiating power. To avoid expense, only the tip of the cylinder is made of zirconia. ZISKA, or Zizka, John, the military leader of the Hussites, born near the castle of Trocz- now, in the circle of Budweis in Bohemia, about 1360, died at Przibislaw, Oct. 12, 1424. He was of a noble Bohemian family, and in his boyhood lost an eye. At the age of 12 he became a page at the court of Prague, and subsequently chamberlain. He accompanied the body of volunteers which went from Bo- hemia and Hungary to join the knights of the Teutonic order in the war against the Lithu- anians and the Poles, and was at the battle of Tannenberg on July 15, 1410, where the knights suffered a terrible defeat. He afterward served against the Turks in Hungary, then entered the English army, and took part in the battle of Agincourt in 1415. After this he returned to the Bohemian court. He had early em- braced the doctrines of the Hussites, and en- tered deeply into the feelings of resentment which the execution of Huss and Jerome of Prague excited throughout Bohemia. King Wenceslas, observing his perturbation and in- quiring its cause, is reported to have said to him : "If thou canst devise any means, go and avenge thy countrymen ; thou hast our free permission." His hatred of the Roman Catho- lic clergy was intensified by a private wrong ; for his favorite sister had been seduced by a monk. He soon gained the confidence of the people, and joined the party under Nicholas of Hussinetz, who took up arms ostensibly to protect the person of their king. The weak and perplexed Wenceslas demanded that they should give up their weapons ; but he was too imbecile to protect the followers of the new religion, and the disputes between the Roman Catholics and the Hussites waxed fiercer and fiercer. On July 30, 1419, as a procession of priests of the latter was marching to St. Stephen's church, one of them was struck by a stone which came from the town house, where the magistrates, who were Roman Catholics, were assembled. Ziska and his ad- herents immediately stormed the building, and threw 13 of the city council into the court- yard below, where they were instantly mas- sacred by the mob. This was the signal for the breaking out of the Hussite war. Wen- ceslas, a fortnight later, died in a tumult of passion, leaving Bohemia without a ruler and sundered by intestine war. For a time every- thing was in commotion. Churches were pillaged by the enraged Hussites, convents burned, and altars overturned ; and Prague was besieged by the Hussite leaders, Ziska and Nicholas of Hussinetz. A suspension of arms was finally concluded, general liberty of conscience was granted, and Ziska retired with his followers and fortified himself in Pilsen. Sigismund, emperor of Germany and king of Hungary, brother of Wenceslas, was the next heir to the Bohemian throne, and toward the close of 1419 held a meeting of the nobles of Bohemia and Moravia, and of the deputies of the cities, in which he ordered the deputies from Prague to destroy the barricades in the streets and the fortifications against the castle. He also removed all Hussites from of- fice, and repairing to Breslau proclaimed a cru- sade against them. The spirit of the members of the new religion was now roused to fanati- cism. A solemn league was formed at Pilsen, in which the confederates, in expectation of the speedy coming of Christ, bound themselves to reject Sigismund as king, and to oppose any sovereign who did not admit the claims of the laity to the participation of the cup in the holy sacrament. Ziska formed his headquarters on the top of a steep mountain in southern Bo- hemia, which received the name of Mount Tabor. As vast numbers fled thither for safe- ty, he erected walls for the defence of the place, and this fortress has been considered the first essay in the modern style of fortification. From the name of this mountain his followers were called Taborites, while the more moder- ate Hussites were known as Calixtines. The first exploit of Ziska was the conquest of Prague, with the exception of the castle ; and in order to defend the city against Sigismund, who was approaching at the head of a vast army, he intrenched himself on the hill of Witkow, and there on July 14, 1420, with only 4,000 men, repeatedly drove back the enemy with great loss. The place is still called Ziska's hill. The emperor was finally obliged to conclude a tem- porary armistice with the citizens of Prague, and to grant general liberty of conscience. In
