X the twenty-fourth letter of the English alphabet. Its
position and form are derived from the Latin alphabet,
which received them from the Western Greek alphabet.
The alphabet of the Western Greeks differed from the
Ionic, which is the Greek alphabet now in general use, by the
shape and position of X and of some other consonants. The
Ionic alphabet placed x (ξ) immediately after N and, in the
oldest records, in the form 
, from which the ordinary Greek
capital Ξ was developed. The position and shape of this
symbol show clearly that it was taken from the Semitic Samekh,
which on the Moabite stone appears as 𐤎. Why the Greeks
attached this value to the symbol is not clear; in Semitic the
symbol indicates the ordinary s. Still less clear is the origin
of the form X, which in the Ionic alphabet stands for χ (k
followed by a breath). In a very ancient alphabet on a small
vase found in 1882 at Formello near the ancient Veii in Etruria,
a symbol appears after N consisting of three horizontal and
three vertical lines, 
. From this it has been suggested that
both forms of the Greek x are derived, Ξ by removing the
vertical lines, X in its earliest form 
by removing the four
marginal lines. The Ionic symbol, however, corresponds closely
to the earliest Phoenician, so that this theory is not very plausible
for Ξ, and there are various other possibilities for the development
of X (see Alphabet). This symbol appears in the very
early Latin inscriptions found in the Roman Forum in 1899
as 
. In its usual value as ks it is superfluous. In the Ionic
alphabet it was useful, because there it represented a single
sound, which before the invention of the symbol had to be
represented by kh. In the alphabet in use officially at Athens
before 403 B.C. x was written by χσ (khs). In English there is
an interesting variation of pronunciation in many words according
to the position of the accent: if the accent precedes, x is
pronounced ks; if it follows, x is pronounced gz: compare éxit
(eksit) with exáct (egzact).
The symbol X was used both by the Romans and the Etruscans for the numeral 10. Which borrowed from the other is uncertain, but the Etruscans did not use X as part of their alphabet. X with a horizontal line over it was used for 10,000, and when a line on each side was added, |X|, for a million. (P. Gi.)
XANTHI (Turkish Eskije), a town of European Turkey in the vilayet of Adrianople; situated on the right bank of the river Eskije and at the S. foot of the Rhodope Mountains, 29 m. W. of Gumuljina by the Constantinople-Salonica railway. Pop. (1905) about 14,000, of whom the bulk are Turks and Greeks in about equal proportions, and the remainder (about 4000) Armenians, Roman Catholics or Jews. There are remains of a medieval citadel, and on the plain to the S. the ruins of an ancient Greek town. Xanthi is built in the form of an amphitheatre and possesses several mosques, churches and monasteries, a theatre with a public garden, and a municipal garden. A preparatory school for boys and girls was founded and endowed by Mazzini. The town is chiefly notable for the famous Yénidjé tobacco.
XANTHIC ACID (xanthogenic acid), C2H5O⋅CS⋅SH, an organic acid named from the Greek ξανθός, yellow, in allusion to the bright yellow colour of its copper salt. The salts of this acid are formed by the action of carbon bisulphide on the alcoholates, or on alcoholic solutions of the caustic alkalis. They react with the alkyl iodides to form dialkyl esters of the dithio-carbonic acid, which readily decompose into mercaptans and thiocarbamic esters on treatment with ammonia: C2H5O⋅CS⋅SR₁+NH3 = C2H5O⋅CS⋅NH2+R₁⋅SH; with the alkali alcoholates they give salts of the alkyl thiocarbonic acids: C2H5O⋅CS⋅SR+CH3OK+H2O = CH3O⋅CO⋅SK+C2H5OH+R⋅SH. Ethyl xanthic acid, C2H5O⋅CS⋅SH, is obtained by the action of dilute sulphuric acid on the potassium salt at 0° C. (Zeise, Berz. Jahresb., 3, p. 83). It is a colourless oil which is very unstable, decomposing at 25° C. into carbon bisulphide and alcohol. The potassium salt crystallizes in colourless needles and is formed by shaking carbon bisulphide with a solution of caustic potash in absolute alcohol. On the addition of cupric sulphate to its aqueous solution it yields a yellow precipitate of cupric xanthate. Potassium xanthate is used in indigo printing and also as an antidote for phylloxera. Tschugaeff (Ber., 1899, 32, p. 3332) has used the xanthic ester formation for the preparation of various terpenes, the methyl ester when distilled under slightly diminished pressure decomposing, in the sense of the equation, CnH2n-1⋅O.CS⋅SCH3 = CnH2n-2+COS+CH3SH. According to the author molecular change in the hydrocarbon is prevented, since no acid agent is used.
XANTHIPPE, the wife of Socrates (q.v.). Her name has become proverbial in the sense of a nagging, quarrelsome woman. Attempts have been made to show that she has been maligned, notably by E. Zeller (“Zur Ehrenrettung der Xanthippe,” in his Vorträge und Abhandlungen, i., 1875).
XANTHONE (dibenzo-γ-pyrone, or diphenylene ketone oxide), C13H8O2, in organic chemistry, a heterocyclic compound containing the ring system shown below. It is obtained by the oxidation of xanthene (methylene diphenylene oxide) with chromic acid; by the action of phosphorus oxychloride on disodium salicylate; by heating 2⋅2′-dioxybenzophenone with concentrated sulphuric acid; by distilling fluoran with lime; by the oxidation of xanthydrol (R. Meyer, Ber., 1893, 26, p. 1277); by boiling diazotized 2⋅2′-diaminobenzophenone with water (Heyl., Ber., 1898, 31, p. 3034); by heating salol with concentrated sulphuric acid (C. Graebe, Ann., 1889, 254, p. 280), and by heating potassium-ortho-chlorobenzoate with sodium phenolate and a small quantity of copper powder to 180-190° C. (F. Ullmann, Ber., 1905, 38, pp. 729, 2120, 2211). It crystallizes in needles which melt at 173-174° and boil at 349-350° C., and are volatile in steam. Its solution in concentrated sulphuric acid is of a yellow colour and shows a marked blue fluorescence. The carbonyl group is not ketonic in character since it yields neither an oxime nor hydrazone. When fused with caustic potash it yields phenol and salicylic acid. Mild reducing agents convert it into xanthydrol, the group >CO becoming >CH⋅OH, whilst a strong reducing agent like hydriodic acid converts it into xanthene, the group >CO becoming >CH2. Phosphorus pentasulphide at 140-150° C. converts it into xanthion by transformation of >CO to >CS (R. Meyer, Ber., 1900, 33, p. 2580), and this latter compound condenses with hydroxylamine to form xanthone oxime.
All four mono-hydroxyxanthones are known, and are prepared by heating salicylic acid with either resorcin, pyrocatechin or hydroquinone; they are yellow crystalline solids, which act as dyestuffs. The 1⋅7-dihydroxyxanthone, known as euxanthone, is prepared by heating euxanthic acid with hydrochloric acid or by heating hydroquinone carboxylic acid with β-resorcylic acid and acetic anhydride (S. Kostanecki, Ber., 1891, 24, p. 3983; C. Graebe, Ann., 1889, 254, p. 298). It is also obtained from Indian yellow (Graebe, ibid.), formed in the urine of cows fed on mango leaves. It crystallizes in yellow needles which sublime readily. On fusion with caustic potash it decomposes with formation of tetrahydroxy-benzophenone, which then breaks up into resorcin and hydroquinone. The isomeric 1⋅6-dihydroxyxanthone, isoeuxanthone, is formed when β-resorcylic acid is heated with acetic anhydride. Gentisein, or 1⋅3⋅7-trihydroxyxanthone, is found in the form of its methyl ether (gentisin) in gentian root; it is obtained synthetically by condensing phloroglucin with hydroquinone carboxylic acid.
Xanthene, C13H10O, may be synthesized by condensing phenol with ortho-cresol in the presence of aluminium chloride. Its tetramethyl-diamino derivative, which is formed by condensing formaldehyde with dimethyl-meta-aminophenol and subsequent elimination of water from the resulting diphenyl methane derivative, is the leuco base of pyronine, into which it passes by oxidation.
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