816 



ELASTICITY ELBE. 



ELASTIC GUM. See Caoutchouc. 



ELALIA ; a town of Africa, in Tunis, near the 

 eastern coast, in a large extent of ruins on the bor- 

 ders of a fertile plain ; 90 S. S. E. Tunis ; Ion. 11 

 2' E.; lat. 35 6* N. Besides such ruins as it lias 

 in common with other places, there are several cis- 

 terns, with large paved areas built over them, in 

 order to receive the rain water, that, in the rainy 

 season, is to fill and replenish them. Several con- 

 veniences of the like nature are dispersed all over 

 this dry country. Elalia seems to be the Acolla or 

 Acilla of the ancients. 



ELASTICITY ; the peculiar property of bodies, 

 by virtue of which, the particles of which they are 

 composed, when moved out of their positions by an 

 external force, or pressed into a narrower space, 

 tend to return to their former position, as soon as 

 the external force ceases to act. A bow, bent by 

 the tension of the string, recovers its previous form 

 when the tension is relaxed. Let an ivory ball fall 

 upon a plate of marble, it is partially flattened by 

 the impulse, but becomes immediately round again 

 as soon as the force of the blow is destroyed. Here 

 we see the cause of its rebounding from the hard 

 surface. Feathers are in a high degree elastic. 

 This property of elasticity is particularly observable 

 in atmospheric air. If it is enclosed in a vessel, and 

 pressed with a piston, as soon as the force is removed 

 from the piston, the air throws it up violently. This 

 is the principle of the air-gun. There is an impor- 

 tant difference between the elasticity of solids and 

 fluids ; the former tend to recover their previous 

 form; the latter to expand into a greater space, 

 whence the term expansibility is applied to them. 

 For the sake of distinction, the elasticity of solid 

 bodies may be termed attractive, and tliat of fluids, 

 expansive. The degree of it is very different in 

 different bodies, and in many it is increased by art. 

 Those bodies in which it cannot be perceived at all 

 are called unelastic. The elasticity of a solid body 

 is greater the more its particles are expanded. If 

 all the particles of a body are so far expanded that 

 their elasticity is just equal to the expansive power, 

 the expansion can be carried no further without 

 separating the particles. The weights, necessary to 

 produce a given degree of extension, must be pro- 

 portionate to the extension already existing. If three 

 cords, of the same size and substance, stretched in pro- 

 portion to the numbers 1, 2,3, are to receive each a 

 given amount of additional extension, the weights 

 necessary to produce this extension are as 1, 2, 3. 



The laws of elasticity in fluids are different from 

 those in solids. In heavy elastic fluids, the inferior 

 layers support the weight of the superior ; in a cy- 

 lindrical vessel, therefore, the bottom suffers the 

 pressure of the whole mass of elastic fluid, and the 

 lower strata are sensibly denser than the upper. A 

 difference is made, too, between absolute and specific 

 elasticity. By the former is understood the peculiar 

 property of bodies to repel a pressing force, in itself, 

 and without regard to temperature and density. 

 This must be always equal to the pressing force. 

 But as different kinds of matter may press with 

 equal force under unequal densities and tempera- 

 tures, that is called specifically most elastic, which 

 with a less density presses with a force equally 

 strong, and with an equal density stronger. In all 

 elastic fluids, the specific elasticity increases with 

 the temperature ; it is likewise augmented by greater 

 density : if air is confined, and made more dense, its 

 specific elasticity is greater in proportion to its in- 

 crease of density. 



ELATER ; the name of an insect remarkable for 

 a singular apparatus between the thorax and abdo- 

 men, by which it is enabled to throw itself to a con- 



siderable height in the air, when placed on its back. 

 It thus regains its proper position when accidentally 

 overturned. The arrangement by which this is 

 effected is so curious, tliat we cannot suppose it in- 

 tended solely for this purpose, and deem it most 

 probable tliat other and more valuable services are 

 rendered to the insect by it. A spine is produced 

 from the centre of the breast or sternum, ami enters 

 a socket in the ante-pectus or breast. The ion < 

 and elasticity with which the spine enters its appro- 

 priate receptacle, aided by the form of the tlior;ix, 

 produces a jar or concussion sufficient to throw the 

 insect several inches into the air. When alarmed, 

 the elater draws its limbs close to the body, and tall 

 ing to the earth quite motionless, counterfeits death. 

 Flowers, grass, and decaying wood, are the proper ha- 

 bitations of these animals, which are almost always 

 found singly, and not in numbers collected together, 

 as in the case of many other beetles. One sj iecie> is 

 accused of depredations on the roots of wheat 

 the E. striatus of Fabricius, an inhabitant of Europe. 



The elater noctilucus possesses luminous proper- 

 ties, which are unlike those of the glow-worm, &c., 

 being seated near the head. In South America, 

 where they abound, the natives term them cucvyos, 

 and the Spanish residents, cucujo. Colour, dark 

 brown, with an ash-coloured down ; thorax on' each 

 side, with a convex round spot, from which the 

 light is emitted ; elytra with lines of impressed punc- 

 tures. The light emitted by several of these insects, 

 enclosed in a glass vase, is sufficient to read by with- 

 out much difficulty. As ornaments for the hair and 

 evening dresses of the Spanish ladies, they are said 

 to be in great request ; but it is probable that the 

 feeble light which they produce would be entirely 

 eclipsed by the glare of artificial light. It has been 

 asserted, that the luminous quality of the cucujo is 

 not confined to the spots upon the thorax, but that 

 the whole interior of the animal possesses the pro- 

 perty of affording light. This is considered doubtful. 



ELBA (anciently llva); a small island in the 

 Mediterranean, near the coast of Tuscany, to which, 

 at present, it belongs, and from which it is separated 

 by the channel of Piombino. The island is eigh- 

 teen miles in length, and varies in breadth; was 

 known to the Greeks by the name of Aithalai, and 

 to the Romans by that of llva, or Elva. and has been 

 renowned for its mines from a period beyond the 

 reach of history. Pliny gives it a circuit of 100 

 miles ; late geographers allow only sixty to its 

 circuit. The difference might be accounted for by 

 the encroachments of the sea. and by the tumbling 

 in of rocks, which are in many places of a moulder- 

 ing contexture. Being extremely mountainous, 

 Elba affords but scanty room for cultivation, and 

 produces little more than six months' provision of 

 corn for its inhabitants. The climate is much milder 

 than that of the adjacent continent. Elba contains 

 two grand ports Porto Ferraio, with 3000 inhabi- 

 tants, and Porto Longone, with 1500 inhabitants, 

 both defended by fortifications and garrisons. Lon. 

 10 26' E. ; lat. 42 53' N. ; population, 13,750 ; 

 square miles, 153. It produces annually war 

 36,000 cwt. of iron ore, which yield, at least, fifty per 

 cent, of metal. It is rich in silver, marble, and load- 

 stone ; 600,000 bags of salt are annually produced. 

 In 1814, Elba was granted to Napoleon, with all the 

 rights of sovereignty. He took possession of it 

 May 4, and left it February 26, 1815, to undertake 

 lis memorable march to Paris. 



ELBE (anciently Albis) ; one of the largest rivers 

 of Germany. It rises in the Riesengebirge mountains, 

 about 4260 feet above the level of the sea, and takes 

 a southerly course through a part of Bohemia to 

 Pardubitz, where it turns towards the W. and N. W. 



