moved in refpect to each other, is called a fluid; as water, melted metals, &c. There is a fifth property of matter, called ATTRACTION; of which there are feveral kinds, cobefion, gravitation, magne tifm, electricity, &c. 1. The attraction of COHESION is that by which the small particles of matter are made to stick and cohere together. Of this kind of attraction thefe are fome of the effects.-If a fmall glafs tube, open at both ends, be dipped in water, the water will rife up in the tube to a confiderable height above its level in the bafon, owing to the attraction of the tube. Hence water may be emptied from a vessel to a small depth, by means of capillary tubes of about one tenth of an inch bore, or by putting one end of a lift of cloth into the veffel, and letting the other end hang over the fide. So liquids will afcend between contiguous planes, or in a tube filled with ashes. Thus, a piece of loaf-fugar will draw up a fluid, and a fpunge will fuck in water. On the fame principle, fap, according to the opinion of fome naturalifts, afcends in trees. We fee in all liquors that the parts attract one another, from the round figure which the drops always affume.-If two drops of filver are placed near each other, they will run together, and become one large drop.-Two polifhed plates of marble or brafs, when their furfaces are brought into contact, will stick fo closely together, that it will not be easy to disjoin them.-If two pieces of cork, equal in weight, be placed near each other in a bafon of water, they will move equally faft toward each other, with accelerated motion, till they meet; and then if either is moved it will draw the other after it. If the corks are of unequal weight, their motion will be proportionally different. But this kind of attraction does not extend far When the sphere of attraction ends, a repulfive force begins Thus water repels moft bodies till they are wet. Hence a fmall needle, if dry, will swim on water; and flies walk on it without wetting their feet.--The repulfive force between water and oil is fo great, that it is almost impoffible to mix them fo as not to separate again; fo water will rife confiderably above the edges of a cup, if they are dry, before it overflow; which is owing both to the cohesion of the water, and the repulfion of the cup. The power of attraction and repulfion in vegetables is fo ftrong, that in fome inftances it feems to refemble fenfation, the diftinguishing property of animals. Thus One Thus the fenfitive plant, on the flightest touch, fhrinks back, and folds up its leaves, as a fnail retires within its fhell. of thefe plants, called Dionea, if a fly perch upon any of its leaves, clofes inftantly, and crushes the infect to death. Many plants expand their flowers and leaves in good weather, or while the fun fhines, and close them in dark or cloudy weather. Some plants follow the fun, others turn from it; which things were observed by the ancients; Plin. ii. 41. Plants often direct their roots to procure food; and when forced from their natural direction, are endowed with a power to restore themselves. A hop-plant twifting round a pole directs its course from fouth to weft, as the fun does; untwift it, and tie it in the oppofite direction, it dies. Leave it loofe in the wrong direction, it recovers its natural direction in a fingle night. Thus trees, if at freedom, grow upwards; Salluft. Jug. 93. Lay a wet spunge near a root laid open to the air, the root will direct its courfe to the fpunge; change the place of the fpunge, the root varies its direction. Thruft a pole into the ground at a moderate distance from a fcandent plant, the plant directs its growth to the pole, lays hold of it, and rifes on it to its natural height. Of the Planta contorta, or such as twist round other plants, fome in climbing follow the direction of the fun; as the fcarlet kidney bean, &c. others in climbing follow a contrary direction, as the black bryony. The former kind are wholefome and nutritive, the latter noxious, and generally poisonous. 2. GRAVITY, or the attraction of gravitation, is that property or power by which diftant bodies tend towards one another. Thus ftones fall, and bodies are kept to the furface of the earth. All bodies, on whatever fide of the earth, are attracted in lines perpendicular to its furface; fo that on oppofite fides they fall towards its centre in oppofite directions. Hence its rotundity, about which the opinions of the ancients were various but the very name orbis or globus terra, shews the general belief; Plin. ii. 64. 65. The attraction of mountains has lately been proved, by their drawing the plummet line of philofophical inftruments from the perpendicular. All bodies that we know have gravity or weight. This is demonftrated by experiments made with the air-pump, even in fmoke, vapours, and fumes. The fmoke of a candle, which afcends to the top of a tall receiver when full of air, upon the air's being exhaufted, falls to the bottom. In an exhaufted receiver a feather and a guinea will fall from the top to the bottom in the fame time. So a piece of wood, when immerfed immerfed in a jar of water, rises to the top, because it has a lefs degree of weight than its bulk of water has; but if the jar is emptied of water, the wood falls to the bottom. Gravity in all bodies is in proportion to the quantity of matter they contain, that is, to their weight. All bodies are full of pores; even gold itself, the heaviest of all known bodies, is fuppofed to contain a greater quantity of open space than of matter. A body is faid to have double, triple, &c. the density of another body, when, fuppofing their bulks equal, it contains a double or triple quantity of matter. A body every where of the fame denfity is faid to be homogenesus, or homogeneal; a body of unequal density in different parts, or of an oppofite or diffimilar nature, is called heterogeneous. The gravity of a body confidered with relation to its bulk, is called its specific gravity. The comparative specific gravity of bodies is most exactly afcertained by weighing them in water. A folid body of the fame specific gravity with water, when immersed in it, will neither rife nor fink. A body lighter than water will rife to the top, and take up fuch a space below the furface, that the weight of water which that space would contain, will be equal to the weight of the body. Thus a fhip difplaces a bulk of water equal to the weight of the veffel and lading. Fishes have within them a bladder of air, by compreffing or dilating which, and thus diminishing or enlarging their bulk, they are enabled to fink or rife in the water at pleasure. Fishes which want this bladder, remain at the bottom; as flounders, eels, &c. A folid body heavier than water, when immerfed in it, difplaces a quantity of water equal to its own bulk, and lofes as much of its weight as is equal to the weight of that bulk of water. By weighing metals in water, we can difcover their adulterations or mixtures with tolerable exactnefs without injuring them. Thus a real guinea and a counterfeit one or a brass counter, if weighed in air, will appear both of the fame weight. But if weighed in water, the real guinea will lose only the 19th part of its weight, and the brafs counter the 8th part. The inftrument used for weighing metals in water is called the Hydroftatic Balance; faid to have been invented by Archimedes, from an obfervation which he made while ba thing, that the water rofe in proportion to the part of his body immerfed; whereupon he is faid to have been fo transport ed ed with joy, that he ran out crying, (iupnxa, iupnxa), I have found it, I have found it." The caufe of this exultation is said to have been owing to the following circumstance. Dionyfius the tyrant of Syracufe had employed an artist to make him a crown of gold, which, although of the weight required, he fufpected to be mixed with alloy, and applied to Archimedes to afcertain the fact. From the impoffibility of doing this any other way, without injuring the crown, than by the hydroftatic balance, and from the anxiety of Archimedes to folve fo curious and useful a point, we may conceive his joy on the difcovery. He firft found how much the crown foft of its weight when weighed in water; next how much a piece of gold equal in weight to the crown loft of its weight; and fo detected the fallacy. As we can thus difcover the specific gravity of different folids, by plunging them into the fame fluid, fo we can discover the fpecific gravity of different fluids by plunging the same solid bodies in them. For in proportion as the fluid is heavy, fo much will it diminish the weight of the bodies immerfed in it. Thus a folid that fwims in water, will fink in spirit of wine; and a folid will fink in water, which fwims upon fpirit of nitre. The inftrument for measuring the lightnefs or weight of different fluids is called an Hydrometer. Liquids of different gravity may be contained in the fame veffel without mixing. That fcience which teaches the art of conveying water is called Hydraulics; and any machine made for this purpose, an Hydraulic, or hydraulical engine. By the gravity or force of water various kinds of mills and wheel-engines are moved. Fluids may be conveyed in bended pipes, over hills and vallies, to any height not greater than the level of the fpring whence they flow; of which the ancients were not altogether ignorant, as is commonly fuppofed; Plin. xxxi. 6. f. 31. So jets of water, or jets d'eau, would rife to the height of the refervoirs from which the water comes, were it not for the effects of friction on the fides of the machine, and the refiftance of the air. Making allowance for thefe, water from a refervoir 5 feet 1 inch high will rife 5 fect, from 10 feet 4 inches to 10 feet, from 33 fect to 30 feet, from 72 feet to 60 feet, from 117 feet to 90 feet, from 133 feet 4 inches to 100 feet, &c. II. PAR II. PARTICULAR PROPERTIES of MATTER, CERTAIN bodies have properties peculiar to themselves. Thus the Leadstone attracts iron and fteel only, which force (from its Latin name, (magnes, -etis) is called MAGNETISM 3 and, what is remarkable, communicates its properties to a piece of fteel, when rubbed on it, without lofing any of its own force. When fufpended by a thread, it conftantly turns one of its fides to the north, and another to the south; whence the invention of the magnetic needle, of fo much use in navigation. This property of the magnet is found to vary a little in different places, which was first observed by Columbus, in his voyage to discover America, and greatly perplexed him; nor has it yet been properly accounted for. The cause of magne tism, and the laws by which it acts, are equally involved in ob[curity. Amber, glass, jet, fealing wax, and fome precious stones, have a peculiar property of attracting and repelling light bodies, when rubbed; which is called electrical attraction. ELECTRICITY was hardly known to the ancients, although they appear to have been acquainted with the electrical properties of amber, (Adtritu digitorum acceptâ caloris animâ, fc. fuccina vel electra, trahunt in fe paleas ac folia arida, qua levia funt; ut magnes lapis, ferrum, Plin. xxxvii. 3. f. 12.) as early as the days of Thales, who afcribed the attractive power of the magnet, and of amber, to their being animated by a living principle, D. Laert. i. 24.; and the word electri city feems to be derived from the Latin name of amber, electrum. They also knew the electric fhock of the torpedo, Plin. ix. 42.f. 67. xxxii. 1. f. 2.; Cic. de nat. D. ii. 50. although ignorant of the caufe of it, which indeed feems hardly yet fufficiently afcertained, although it is proved to be of the fame nature with electricity. Some fuppofe that the ancients even understood the method of drawing down the electric fire from the clouds; whence, as they suppose, the name of Jupiter Elicius; and that, in attempting to do this, Tullus Hoftilius loft his life, Liv. i. 20. But thefe opinions feem to be founded merely on conjec ture. The first who made any discoveries of importance concerning electricity was Dr W. Gilbert, which he gave an account of in F |