factured with, care to avoid an excess of chalk or lime, which would precipitate constituents of the juice that cause the fermentation of the citrate and the production of calcium
acetate and butyrate.Citric acid has an agreeable sour taste. It is soluble in fths of its weight of cold, and in half its weight of boiling water, and dissolves in alcohol, but not in ether. At 150 C. it melts, and on the continued application of heat boils, giving off its water of crystallization. At 175 C., it is resolved into water and aconitic acid, C 6 H 6 6 , a substance found in Equiietum fluviatile, monkshood, and other plants. A higher temperature decomposes this body into carbonic anhydride and itaconic acid, C 5 H 6 4 , which, again, by the expulsion of a molecule of water, yields citraconic anhydride, C 5 H 4 O 3 . Citric acid digested at a temperature below 40 C. with concentrated sulphuric acid gives off carbonic oxide. With fused potash it forms potassium oxalate and acetate. It is a strong acid, and dissolved in water decomposes the carbonates and attacks iron and zinc. Citric acid, in common with other tribasic acids, evolves about three times the amount of heat disengaged by acetic acid when quantities of these two bodies in the proportion of their molecular weights are saturated with soda, one molecule of the tribasic being equivalent to three of the monobasic acid.
The citrates are a numerous class of salts, the most soluble of which are those with alkaline bases ; the alkaline earthy citrates are insoluble. Citric acid, being tribasic, forms either acid monometallic, acid di metallic, or neutral trimetallic salts ; thus, mono-, di-, and tri-potassic and sodic citrates are known. In a few salts a fourth atom of hydrogen is replaced by a metal, and citric acid is therefore considered by some chemists to be tetrabasic. Citric acid gives with excess of lime-water a slight precipitate of calcium citrate ; a further precipitate is produced by boiling, but it is redissolved as the liquid cools. Solution of citric acid may be approximately titrated by means of baryta-water and litmus.
The impurities occasionally present in citric acid are salts of potassium and sodium, traces of iron, lead, and copper derived from the vessels used for its evaporation and crystallization, and free sulphuric, tartaric, and even oxalic acid. Tartaric acid, which is sometimes present in large quantities as an adulterant in commercial citric acid, may be detected in the presence of the latter, by the production of a precipitate of acid potassium tartrate when potassium acetate is added to a cold solution of the sample of acid to be tested. Another mode of separating the two acids is to convert them into calcium salts, which are then treated with a perfectly neutral solution of cupric chloride, soluble cupric citrate and calcium chloride being formed, while cupric tartrate remains undissolved. Citric is furthermore distinguished from tartaric acid by the fact that an ammonia solution of silver tartrate produces a brilliant silver mirror when boiled, whereas citric of silver is reduced only after prolonged ebullition.
Citric acid is used in calico- printing, also in the preparation of effervescing draughts, and occasionally as a refrigerant and antiscorbutic, instead of fresh lemon juice, to which, however, it is therapeutically inferior. In the form of lime juice it has long been known as an antidote for scurvy, and several of the citrates are much employed as medicines.
CITRON, a species of Citrus (C. medica, Risso), belong ing to the Natural Order Aurantiacere, which furnishes also the orange, lime, and shaddock. The citron-tree is an evergreen growing to a height of about 8 feet ; it has long, pendent, and, in the wild varieties, spiny branches, pale-green, oblong, and sub-serrate leaves, and flowers purpje without and white within. The fruit is ovate or oblong, protuberant at the ti^. and from 5 to 6 inches long, with a rough, furrowed, adherent rind, the inner portion of which is thick, white, and fleshy, the outer, thin, greenish-yellow, and very fragrant. The pulp is sub-acid and edible, and its seeds are bitter. There are many varieties of the fruit, some of them of great weight and size. The Madras citron has the form of an oblate sphere ; and in the " fingered citron " of China, the lobes are separated into finger-like divisions.
Gallesio and De Candolle consider citrons and lemons to be distinct species ; but the former authority states that seed produced by lemon-trees growing amongst citron-trees gave varieties which were intermediate between the two species ; and by some botanists citrons, lemons, and oranges are held to be all varieties of the wild Citrus medica. According to Dr G. Birdwood, however, the orange and lemon are both natives of Upper India, the former being derived from the wild Citrus Aurantiumoi Gurhwal, Sikkim, and Khasia, and the lemon, lime, and probably citron also, from the wild Citrus Limonum of the valleys of Sikkim and Kumaon, of which the Citrus medica was the firsfc cultivated variety (Athenaeum, No. 2544, July 29, 1876., p. 151).
The citron-tree thrives in the open air in China, Persia, the West Indies, Madeira, Sicily, Corsica, and the warmer parts of Spain and Italy ; and in conservatories it is often to be seen in more northerly regions. It was described by Theophrastus as growing in Media; it appears, however, not to be indigenous to Persia, but to have been introduced into that country and other lands from North India, where it was found growing wild by Dr Royle. It was early known to the ancients, and the fruit was held in great esteem by them ; but they seem to have been acquainted with no other member of the Aurantiacecc, the introduction of orangesand lemons into the countries of the Mediterranean being due to the Arabs, between the 10th and 15th centu ries. Josephus tells us that " the law of the Jews required, that at the feast of tabernacles every one should have branches of palm-tree and citron-tree" (Antiq., xiii. 13, 5); and the Hebrew word tappuach, rendered "apples" and "apple-tree" in Cant. ii. 3, 5, Prov. xxv. 11, c., probably signifies the citron-tree and its fruit. Oribasius in the 4th century describes the fruit, accurately distin guishing the three parts of it. About the 3d century the tree was introduced into Italy; and, as Gallesio informs us, it was much grown at Salerno in the llth century. In China, citrons are placed in apartments to make them fragrant. The rind of the citron yields two perfumes, oil of ccdra and oil of citron, isomeric with oil of turpentine ; and when preserved it is much esteemed as a sweetmeat.
Oribasii Sardiani Collcdorum Medicinal ium Libri XVII., 1. i. c. 64 (De citrio) ; Gallesio, Traite du Citrus (1811); Danvin, Ani mals ami Plants under Domestication, vol. i. pp. 334-6 (1868); IJrandis, Forest Flora of North- West and Central India, p. 51 (1874).
CITTÀ DELLA PIEVE, a town of Italy, in the province of Umbria and district of Orvieto, about six miles from the station of Chiusi on the railway between Siena and Rome. It was the birthplace of Pietro Perugino, and still preserves some of his finest works. Of these several are to be found in the cathedral, and his fresco of the adoration of the Magi adorns the oratory of the Disciplinati. Population, 6500.
situated on the left bank of the Tiber, in the province of Perugia, 25 miles N. by W. from the town of that name. It has a cathedral, dedicated to St Floridus and dating from 1503, a large number of interesting churches svith valuable paintings, a communal palace of the 13th century, an epis copal palace remodelled since 1789. and no fewer than four mansions belonging to the Yitelli family, who governed the
town in the 1 5th century, and were among the first to