268 



CITRIC ACID 



CITY 



combining with the citric acid, falls to the bottom 

 as citrate of calcium. The supernatant liquid 

 being drawn off, sulphuric acid is added to the 

 precipitate, decomposing it, with the formation of 

 citric acid and sulphate of lime, CaSO 4 . By 

 crystallisation it may be obtained pure in the form 

 of colourless, odourless prisms, which effloresce in 

 dry air, and possess an agreeable acid taste and an 

 acid reaction. It is readily soluble in water and 

 alcohol, but almost insoluble in ether and chloro- 

 form. A solution of it in water cannot be kept 

 owing to its tendency to ferment. Dissolved in 

 syrup it keeps much longer, and is used largely in 

 the manufacture of lemonade and other aerated 

 beverages, communicating an acid, fruity taste. 

 When heated, the crystals melt, then decompose, 

 and are finally reduced to a combustible form of 

 charcoal. In addition to its employment as a 

 flavouring agent it is largely used in the manu- 

 factures. Calico-printers employ it for discharging 

 the mordant from the cloth in patterns, and it is 

 used in dyeing silk with safflower, &c. 



Citric acid may also be prepared artificially, but 

 the process is too complex ever to come into prac- 

 tical use. Chemically it is of great interest as 

 exemplifying that as soon as the chemical constitu- 

 tion is known there is a possibility of producing 

 the substance artificially. There is a popular idea 

 that some day quinine, morphia, and other natural 

 products will be formed by the random mixing 

 together of chemicals in the chemist's beaker, but 

 the first step towards this result must be the 

 knowledge or their constitution, to be followed by 

 a series of careful experiments. 



Citric acid is a powerful tribasic acid (see AciDS), 

 and the solution in water readily dissolves zinc and 

 iron. It forms a class of salts called Citrates, 

 many of which are employed in medicine. 



The so-called Citrate of Magnesia, a granular 

 substance, which effervesces on the addition of 

 water, and is very popular as a gentle aperient, is 

 not really a citrate at all, but consists of a mixture 

 of tartaric and citric acids, bicarbonate of soda, and 

 sugar, with perhaps a trace of some magnesium 

 salt to justify the name. The granulating is effected 

 by mixing the powders and placing them in a pan 

 heated by steam, when, in proportion to the citric 

 acid present, the powders run together into a pasty 

 mass. This is forced through a coarse riddle, and 

 the granules are dried by a gentle heat. 



Citrate of Potash, KaC s tl,O 7 ,H. z O,is prepared by 

 neutralising citric acia with bicarbonate of potash. 

 It forms a white, granular, deliquescent powder, 

 which acts as a cooling diaphoretic in cases of 

 fever. Dissolved in lemon-juice it is of much value 

 in rheumatism. It is given in doses of 20 to 30 gr. 



Citrate of Ammonia can only be obtained in 

 solution, as when this is evaporated decomposition 

 takes place and ammonia escapes. It is employed, 

 like the potash salt, in febrile diseases. 



Citrate of Iron and Citrate of Iron and Quinine 

 are but examples of a large number of compounds 

 obtained in the form of brown or greenish-brown 

 scales which are largely used in medicine. In these 

 the medicinal value lies not in the citric acid, but 

 in the iron or quinine with which it is associated, 

 although the acid has doubtless a subsidiary action. 

 A peculiarity about these ' scale preparations,' as 

 they are called, is that the inky taste, so character- 

 istic of iron in the form of steel drops, is in great 

 measure removed. Citric acid also forms a series 

 of organic salts called citric ethers, of which 

 citrate of ethyl is an example. Lemon-juice, in 

 which citric acid is the most active ingredient, is a 

 most valuable medicine in scurvy, and when it 

 cannot be obtained the acid may be used as a sub- 

 stitute. Citric acid in solution does not, however, 

 appear to be nearly so effective as the juice itself. 



Citron (Citrus medica), a tree cultivated in the 

 south of Europe and other warm temperate or 

 subtropical countries for its fruit ; a native of 

 northern India. By many botanists the lemon 

 (C. Limonium), the lime (C. Limetta), and the 

 bergamot ( C. Bergamea ) are reckoned varieties ; 

 but it at anyrate avoids confusion to describe these 

 separately. The fruit is large, warty, and fur- 

 rowed. The pulp is acid and cooling, and is used in 

 the preparation of syrups, lemonade, &c. ; but the 

 part chiefly valued is the thick and tender rind, 

 which has a delicious odour and flavour, and is 

 preserved or candied. From this also the fragrant 

 Oil of Citron, or Oil of Cedrate, used by perfumers, 

 is procured. In Germany the name cedrate is 



Citron ( Citrus medica) : 

 a, fruit ; 6, transverse section of fruit. 



extended to all kinds of citron, and the name citron 

 is usually given to the lemon. The fruit of the 

 largest kinds ( var. macrocarpa ) sometimes weighs 

 5 Ib. In the United States the name citron is 

 applied to different varieties of the melon, espe- 

 cially one resembling the water-melon, but edible 

 only when made into preserves or pickles. It is 

 probable that the citron is meant in some passages 

 of the Old Testament where the word apple is used 

 in the English version. 



Citromvood, or CITRUS-WOOD, the most costly 

 furniture-wood of Roman antiquity, is usually re- 

 garded as derived from Biota (Thuja) orientalis, 

 or possibly from Callitris mtadrivalvis, allied coni- 

 ferous trees, both popularly known as Arbor Vitce 

 ( oriental and African ). Cicero is said to have paid 

 an enormous sum for a table of this wood. 



CitFllS, a genus of Aurantiacese (q.v. ), trees and 

 shrubs of tropical, subtropical, and warm temper- 

 ate Asia, but many of them now cultivated in all 

 similar climates for their fruit. To it belong the 

 Orange, Citron, Lemon, Lime, Bergamot, Shaddock, 

 Forbidden Fruit, &c. 



Citta di Castello, a town of Central Italy, on 

 the Tiber, 25 miles NNW. of Perugia. It has a 

 castle, a cathedral, and several palaces rich in 

 works of art. In the church of Santa Trinitk is 

 preserved Raphael's first commission, a guild- 

 banner. Pop. 5433. 



Cittavec'chia. See MALTA. 



City (Fr. cite, Lat. civitas). In the sense in 

 which it was first used in the Romanic languages 

 of modern Europe, the word city, like its Latin 

 original, was probably equivalent to state (res- 

 publica) rather than to town or borough (urbs, 

 municipium). This usage may be contrasted with 



