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 began to retire with their booty, let loose his light horse upon them. No connected resistance was offered, and the Turks slaughtered the fugitives until checked by the fresh troops of the Christian right wing. Into this fight Guy’s reserve, charged with holding back the Saracens in Acre, was also drawn, and, thus freed, 5000 men sallied out from the town to the northward; uniting with the Saracen right wing, they fell upon the Templars, who suffered severely in their retreat. In the end the crusaders repulsed the relieving army, but only at the cost of 7000 men.

ACRE, a land measure used by English-speaking races. Derived from the Old Eng. acer and cognate with the Lat. ager, Gr. , Sans. ajras, it has retained its original meaning “open country,” in such phrases as “God’s acre,” or a churchyard, “broad acres,” &c. As a measure of land, it was first defined as the amount a yoke of oxen could plough in a day; statutory values were enacted in England by acts of Edward I., Edward III., Henry VIII. and George IV., and the Weights and Measures Act 1878 now defines it as containing 4840 sq. yds. In addition to this “statute” or “imperial acre,” other “acres” are still, though rarely, used in Scotland, Ireland, Wales and certain English counties. The Scottish acre contains 6150·4 sq. yds.; the Irish acre 7840 sq. yds.; in Wales, the land measures erw (4320 sq. yds.), stang (3240 sq. yds.) and paladr are called “acres”; the Leicestershire acre (2308 sq. yds.), Westmoreland acre (6760 sq. yds.) and Cheshire acre (10,240 sq. yds.) are examples of local values.

ACRIDINE, C13H9N, in chemistry, a heterocyclic ring compound found in crude coal-tar anthracene. It may be separated by shaking out with dilute sulphuric acid, and then precipitating the sulphuric acid solution with potassium bichromate, the resulting acridine bichromate being decomposed by ammonia. It was first isolated in 1890 by C. Graebe and H. Caro (Ann., 1871, 158, p. 265). Many synthetic processes are known for the production of acridine and its derivatives. A. Bernthsen (Ann., 1884, 224, p. 1) condensed diphenylamine with fatty acids, in the presence of zinc chloride. Formic acid yields acridine, and the higher homologues give derivatives substituted at the meso carbon atom, Acridine may also be obtained by passing the vapour of phenylortho-toluidine through a red-hot tube (C. Graebe, Ber., 1884, 17, p. 1370); by condensing diphenylamine with chloroform, in presence of aluminium chloride (O. Fischer, Ber., 1884, 17, p. 102); by passing the vapours of orthoaminodiphenylmethane over heated litharge (O. Fischer); by heating salicylic aldehyde with aniline and zinc chloride to 260° C. (R. Mohlau, Ber., 1886, 19, p. 2452); and by distilling acridone over zinc dust (C. Graebe, Ber., 1892, 25, p. 1735).

Acridine and its homologues are very stable compounds of feebly basic character. They combine readily with the alkyl iodides to form alkyl acridinium iodides, which are readily transformed by the action of alkaline potassium ferricyanide to N-alkyl acridones. Acridine crystallizes in needles which melt at 110° C. It is characterized by its irritating action on the skin, and by the blue fluorescence shown by solutions of its salts. On oxidation with potassium permanganate it yields acridinic acid (quinolone -dicarboxylic acid) C9H5N(COOH)2. Numerous derivatives of acridine are known and may be prepared by methods analogous to those used for the formation of the parent base. For the preparation of the naphthacridines, see F. Ullmann, German patents 117472, 118439, 127586, 128754, and also Ber., 1902, 35, pp. 316, 2670. Phenyl-acridine is the parent base of chrysaniline, which is the chief constituent of the dyestuff phosphine (a bye-product in the manufacture of rosaniline). Chrysaniline (diamino-phenylacridine) forms red-coloured salts, which dye silk and wool a fine yellow; and the solutions of the salts are characterized by their fine yellowish-green fluorescence. It was synthesized by O. Fischer and G. Koerner (Ber., 1884, 17, p. 203) by condensing ortho-nitrobenzaldehyde with aniline, the resulting ortho-nitro-para-diamino-triphenylmethane being reduced to the corresponding orthoamino compound, which on oxidation yields chrysaniline. Benzoflavin, an isomer of chrysaniline, is also a dye-stuff, and has been prepared by K. Oehler (English Patent 9614) from meta-phenylenediamine and benzaldehyde. These substances condense to form tetra-aminotriphenylmethane, which, on heating with acids, loses ammonia and yields diaminodihydrophenylacridine, from which benzoflavin is obtained by oxidation. It is a yellow powder, soluble in hot water. The formulae of these substances are:— ACRO (or ), HELENIUS, Roman grammarian and commentator, probably flourished at the end of the 2nd century He wrote commentaries on Terence and perhaps Persius. A collection of scholia on Horace, originally anonymous in the earlier MSS., and on the whole not of great value, was wrongly attributed to him at a much later date, probably during the 15th century. It has been published by Pauly (1861) and Hauthal (1866), together with the other Horace scholia.

ACROBAT (Gr. , to walk on tiptoe), originally a rope-dancer; the word is now used generally to cover professional performers on the trapeze, &c., contortionists, balancers and tumblers. Evidence exists that there were very skilful performers on the tight-rope (funambuli) among the ancient Romans. Modern rope-walkers (e.g. Blondin) or wire-dancers generally use a pole, loaded at the ends, or some such assistance in balancing, and by shifting this are enabled to maintain, or readily to recover, their equilibrium.

ACROGENAE (“growing at the apex”), an obsolete botanical term, originally applied to the higher Cryptogams (mosses and ferns), which were erroneously distinguished from the lower (Algae and Fungi) by apical growth of the stem. The lower Cryptogams were contrasted as Amphigenae (“growing all over”), a misnomer, as apical growth is common among them.

ACROLITHS (Gr. , i.e. ending in stone), statues of a transition period in the history of plastic art, in which the trunk of the figure was of wood, and the head, hands and feet of marble. The wood was concealed either by gilding or, more commonly, by drapery, and the marble parts alone were exposed. Acroliths are frequently mentioned by Pausanias, the best known specimen being the Athene Areia of the Plataeans.

ACROMEGALY, the name given to a disease characterized by a true hypertrophy (an overgrowth involving both bony and soft parts) of the terminal parts of the body, especially of the face and extremities (Gr. , point, and  , large). It is more frequent in the female sex, between the ages of 25 and 40. Its causation is generally associated with disturbances in the pituitary gland, and an extract of this body has been tried in the treatment, as one of the recent developments in organo-therapeutics; thyroid extract has also been used, but without marked success, on the apparent analogy of acromegaly with myxoedema.

ACRON, a Greek physician, born at Agrigentum in Sicily, was contemporary with Empedocles, and must therefore have lived in the 5th century before Christ. The successful measure of lighting large fires, and purifying the air with perfumes, to put a stop to the plague in Athens (430 ), is said to have originated with him; but this has been questioned on chronological