Page:The New International Encyclopædia 1st ed. v. 02.djvu/381

The principal arguments in favor of the rule may be briefly summed up as follows:

(1) Avogadro's rule furnishes the chief method of ascertaining the atomic weights of the chemical elements, and there are verv strong reasons for believing that the atomic weights thus obtained represent the true numbers characterizing the elements. See Periodic Law.

(2) The modern theories of the chemical con- stitution of compounds could not have come into existence and could not be applied in individual cases, if the molecular and atomic weights to which the rule leads were unknown. Avogadro's rule is at the very foundation of those theories; and hence, conversely, the correctness of the theories, as indicated by the numerous triumphs achieved by them for both pure and applied chemistry, is to be considered as strong proof in favor of the fundamental rule.

(3) The strongest argument in favor of Avogadro's rule in its hypothetical form, lies in the fact that it follows, by mathematical deduction, from the kinetic theory of gases. (See Gases, Properties of.) The correctness of this theory, on the other hand, is strongly indicated by the fact that it also leads to those laws of gases which have long been established experimentally.

Avogadro's rule can evidently hold good only in those cases in which the gases or vapors in question behave very nearly in accordance with the laws characteristic of matter in the per- fectly gaseous state; i,e. if at constant tempera- ture the volume varies inversely as the pressure, and if under constant pressure the volume varies as the absolute temperature. For if, under equal pressures and temperatures different gases did not always occupy equal volumes, then equal volumes could not always contain equal numbers of molecules.

The rule is sometimes spoken of as the law of Avogadro and Gerhardt, the latter chemist hav- ing done much toward establishing it on a firm scientific basis. Gerhardt's pupil, Cannizzaro, continued his researches, and finally succeeded in demonstrating to the scientific world its great importance. In 1886 Van't Hoff showed the rule to be applicable not only to gases and vapors, but also to substances in solution and as a con- sequence, the rule is also often referred to, espe- cially by German authors, as the rule of Avo- gadro-Van't Hoff. Consult: Avogadro's original memoir on the molecular theory, German trans., Ostwald's Klassiker der exakten Wissenschaften No. 8; and Cannizzaro, "Abriss eines Lehrganges der theoretischen Chemie," Ostwald's Klassiker, No. 30. See also Atomic Weights; Molecules — Molecular Weights; Solution; Chemistry.

AVOID'ANCE. See Benefice.

AVOIRDUPOIS, av'er-du-poiz', or AVERDUPOIS' (ME. aver de peis, OF. aveir de peis, equivalent to ML. averia ponderis, goods of weight). The name given to the system of weights and measures applied in Great Britain and America to all goods except the precious metals and precious stones.

The grain is the foundation of the avoirdupois system, as well as of the troy. A cubic inch of water weighs 252,458 grains. Of the grains so determined, 7000 make a pound avoirdupois, and 5760 a pound troy. (See Weights and Measures.) The avoirdupois pound is divided into

16 ounces and the ounce into 16 drams. A dram, therefore, contains 27 11-32 grains, and an ounce 437½ grains.

Table ok .-UomDUPOis Weight.

ij-^tamfor drachms... " 1 ounce =UTA grains. 16 ounces " l pound =26i) drams=7000 grains. m poinds, aohundredw't... " 1 toni =80 q'rs=2240 pounds,

A cubic foot of water weighs 997.14 ounces avoirdupois, or nearly 1000 ounces, which gives an easy rule for determining the weight of a cubic foot of any substance from its specific gravity. Avoirdupois is the weight used in the United States, where, however, in most places, the hundredweight contains only 100 pounds, and the ton, 2000 pounds.

AVOLA, a'vola. A city in Sicily, 20 miles southwest of Syracuse. The surrounding country produces almonds and raw sugar, and the chief industries of the town are sugar-refin- ing and the manufacture of straw matting. Population, in 1881 (commune), 12,478; in 1901, 16,264.

AVON, a'von (Celt, avon, Welsh afon, Gæl. abhain, abhuinn, water, river, probably akin to Lat. aqua, water). A name borne by several rivers in England and Scotland. The more important of them are: the Upper Avon, which rises in Northamptonshire, and flows southwest through Warwickshire, entering the Severn at the northern boundary of Gloucestershire, after a course of 96 miles, passing Stratford, the birth- place of Shakespeare (Map: England, D 4); the East Avon, which rises in Wiltshire and flows in a southerly direction, entering the English Chan- nel at Christchurch (Map: England, E 6); the Lower Avon, which rises in Gloucestershire, flows through Wiltshire and Somersetshire, past Bris- tol, and enters the Bristol Channel, after a course of 62 miles.

AVON. A village in Livingston County, N. Y., 18 miles south by west of Rochester, on the Erie Railroad and the Genesee River. It is noted for its mineral springs, which have me- dicinal properties. Population, in 1900, 1601.

AVOSET, or AVOCET (Fr. avocette, Sp. avoceta, of unknown origin). A limicoline bird, nearly related to the stilts and snipes, about 17 inches in length, and having long wading-legs, webbed feet, and a remarkably long, slender, and upcurved bill. It frequents marshes and the edges of pools, and searches the shallow water with its sensitive beak for crustaceans, snails, and similar small prey creeping over the mud.

beak of an avoset.

Various species dwell in summer in the temperate parts of both the Old and the New World, migrating to the tropics in winter. They form a poor nest upon the ground in marshy places, and lay four olive or buff eggs, thickly spotted with dark brown. The principal American species,