PRINCIPLES of the NEWTONIAN PHILOSOPHY, occafionally compared with the Opinions of the Ancients. AS S Sir Ifaac Newton made all his discoveries for explaining the motions of the planets, by reasoning from experiments or known facts, it will be requifite that the learner know the general principles on which he proceeded. Thefe chiefly refpect the properties of matter, and the laws of motion. 1. GENERAL PROPERTIES of MATTER. THE inherent properties of matter, or of body in general, are folidity, inactivity, mobility, and divisibility. 1. SOLIDITY and EXTENSION.-All matter has length, breadth, and thicknefs; hence every body is comprehended under fome shape or figure, and hinders all other bodies from occupying the fame part of space that it poffeffes, which is called impenetrability.If a piece of wood be placed between two plates of metal, it never can be fqueezed fo hard, as to allow the plates to come into contact; and a fmall quantity even of water or air, if fixed between two bodies, can by no force be fo compreffed, as to permit the bodies to meet one another, till the water or air be removed. Thus, if a globe of the hardest metal, with a hollow in the middle full of water, be ftrongly compreffed, the water will ooze through its pores, and appear on the furface. Space void of matter is called a vacuum, the existence of which, in oppofition to Defcartes, Newton maintained. 2. INACTIVITY, paffiveness, or the vis inertia, i. e. the want of power in body to move itself.-Every body endeavours to continue in the flate it is in, whether of reft or motion. Bodies on this earth, when set in motion, foon ftop from the action of gravitation, from the refiftance of the air, or of friction. But if a body were carried to a certain diftance from the earth, and there projected in a particular direction, and with a certain velocity, it would continue for ever to move round the earth, without falling to it; as is the cafe with the moon. 3. MOBILITY, or the property in body, that it may be moved from one place to another. 4. DIVISIBILITY ad infinitum, or without end, that is, no of part matter can be conceived fo fmall but there may still be a fmaller. Certain bodies may be divided into very minute parts A grain of gold may be beaten into a leaf 50 inches fquare, which may be divided into 500,000 parts, visible to the naked eye if viewed with a microscope, that magnifies the object only ten times, the fifty millionth part of a grain may be fuppofed vifible. Mr. Reaumur computes, that a grain of gold may be extended on a filver wire upwards of half a mile in length; and cover a furface of 1400 square inches; fo that the thicknefs of the gold will be no more than the fourteenth millionth part of an inch, that is, about 1200 times the thinnefs of ordinary gold leaf, which gold leaf is about 39 times thinner than thin poft paper. But this is nothing to the fubtilty of parts in odoriferous bodies, and to the minutenefs of certain microfcopic animals, and their parts. Similar notions concerning the infinite divifibility of matter were entertained by fome of the ancients; Cic. Acad. i. 7.; Plutarch. de placit. phil. i. 16. This property, however, exifts only in idea; for infinity in minutenefs, as well as magnitude, is altogether beyond our conception. A body not eafily pierced or broken, or whofe parts cannot be easily divided, is faid to be hard; the contrary, foft. A folid body, easy to be broken in pieces, is faid to be brittle; that which may be bent, pulled, or twisted, without breaking, is faid to be tough. A body whofe parts yield to any impreffion, and are easily moved in respect 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, cohesion, gravitation, magnetism, 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 these are fome of the effects.If a fmall glass tube, open at both ends, be dipped in water, the Mr. Boyle demonftrated the practicability of dividing a grain of gold into $8,000,000 vifible parts; and that a grain of copper, being diffolved in fpirit of fal ammoniac, and mixed with a certain quantity of water, might be divided into 22,788,000,0ɗo fmall vifible parts. water 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 veffel to a fmall 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 afbes. 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 quickfilver 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 clofely together, that it will not be eafy to dif join 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 fphere of attraction ends, a repulfive force begins. Thus water repels moft bodies till they are wet. Hence a fmall needle, if dry, will fwim on water; and flies walk on it without wetting their feet.-The repulfive force between water and oil is fo great, that it is almoft impoffible to mix them fo as not to feparate again; thus water will rife confiderably above the edges of a cup, if they are dry, before it overflow; which is owing both to the cohefion 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 the fenfitive plant, on the flightest touch, fhrinks back, and folds up its leaves, as a fnail retires within its fhell. One of thefe plants, called Dionea, if a fly perch upon any of its leaves, clofes inftantly, and crufhes the infect to death. Many plants expand their flowers and leaves in good weather, or while the fun fhines, and clofe them in dark or cloudy wea ther. Some plants follow the fun, others turn from it; which things were obferved by the ancients; Plin. i. 41. Plants of ten direct their roots to procure food; and when forced from their D 3 1 their natural direction, are endowed with a power to restore themselves. A hop-plant twisting found a pole direct sits 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. Fug. 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 twift 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 wholesome and nutritive, the latter noxious, and ge nerally 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; so 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, fhews the general belief; Plin. ii, 64, 65. The attraction of mountains has lately been proved, by their draw ing the plummet line of philofophical inftruments from the perpendicular. All bodies that we know have gravity or weight. This is demonstrated 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 à feather and a guinea will fall from the top to the bottom in the fame time. So a piece of wood, when immersed in a jar of water, rifes 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 mat ter they contain, that is, to their weight. All bodies are full of pores; even gold itfelf, the heaviest of all known bodies, is fuppofed to contain a greater quantity of open space than of matter. A body A body is faid to have double, triple, &c. the denfity 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 homogeneous, or homogeneal; a body of unequal denfity 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 fpace 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. Filhes which want this bladder, remain at the bottom; as founders, eels, &c. A folid body heavier than water, when immersed 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 lofe only the 19th part of its weight, and the brass counter the 8th part. The inftrument used for weighing metals in water is called the Hydrostatic Balance; faid to have been invented by Archimedes, from an observation which he made while bathing, that the water rofe in proportion to the part of his body immerled; whereupon he is faid to have been so transported with joy, that he ran out crying, (ugnxa, iugna), "I have found it, I have found it." The cause of this exultation is faid to have been owing to the following circumftance; Dionyfius the tyrant of Syracuse had employed an artift to make him a crown of gold, which, although of the weight required, he fufpected to be mixed with alloy, and applied to D 4 Archi |