ATTRACTION. 



79 



Attraction tion of electrical and magnetical substances, like those 

 Mimical. Q f t j ie pl aneta ry bodies, seems to follow the law of 



' * ' the inverse ratio of the squares of the distances. This 



has been ascertained by the accurate experiments 

 made with a torsion balance by the celebrated Cou- 

 lomb ; and we are surprised to see it stated in some of 

 the foreign journals, that M. Simon of Berlin, by 

 means of a balance made entirely of glass, has found 

 the law of electrical action to be in the simple inverse 

 ratio of the distance. As the apparatus which he 

 used is said to have been less sensible than the 

 torsion balance used by Coulomb, we are not dis- 

 posed to put any confidence in this new result. 



There is no method of ascertaining the law of the 

 attractive force, by which the rays of light are re- 

 fracted in their passage through transparent media ; 

 and it is almost equally difficult to determine the rate 

 of its variation when the luminous particles are in- 

 flected by passing near opaque bodies. From some 

 experiments, however, we are led to conclude, that 

 the ultimate effects produced by the inflecting body 

 upon a row of particles placed at different distances 

 from it, are in the simple inverse ratio of the distances. 

 It has been much disputed among philosophers, as 

 has been already remarked in the preceding article, 

 whether the attraction of affinity is merely a case of 

 universal gravitation, or depends on some separate 

 cause, and follows a different law. The first of these 

 opinions has been maintained by Buffon, Libes, and 

 we may perhaps add La Place. The last is the opi- 

 nion of Newton, and has been adopted by most of 

 his followers, We shall endeavour to give a short 

 view of the reasoning that has been employed on 

 both sides of the question. 



In the chapter in Physical Astronomy on the Gra- 

 vitation to a Sphere, we have shewn, that when the 

 law of the attracting force varies in the inverse r.tio 

 of the square of the distance, the total attraction of 

 a sphere upon a particle, situated at any distance from 

 rt whatever, is the same as if all the attracting par- 

 ticles had been concentrated in the centre of the 

 spherical body. Considering spheres as simple gra- 

 vitating points, it is obvious, that the attraction of a 

 sphere, upon a point in contact with it, can never be 

 infinitely great, when compared with the attraction 

 which it experiences when out of contact ; for the 

 radius of the sphere, which is in this case a measure of 

 the attraction, must always have a finite ratio to the 

 distance of the particle when out of contact. 



When the attracting force varies as the cube of 

 the distance, or according to any higher ratio, it has 

 been shewn by Newton, (see the following article on 

 the Attraction of Solids,) that the attraction of, 

 the sphere is indefinitely greater when the particle is 

 in contact than, when it is placed at any finite dis- 

 tance. This result, which is conformable to the 

 phenomena of chemical attraction, induced Newton 

 to believe, that the law of the force was in the in- 

 verse ratio of the cube of the distance, or perhaps 

 some higher power of the distance. 



M. Libes ha3 maintained, that when the law of 

 the force is inversely in the duplicate ratio of the dis- 

 tance, the action of a sphere upon a particle in con- 

 tact with it is not proportional to the radius of the 

 sphere. " When the elementary molecule," he ob- 



serves, " is placed on the surface of the sphere, it is Attra-ctiou 

 in contact with one of the molecules of the solid, 4> f Moun- 

 whose action is == go . The molecule of the sphere, taini " 

 situated at the opposite extremity of the same diame- """* 

 ter, exerts upon the molecule attracted a force 



. Whence the two molecules of the sphere, of 



oo c 



which one touches the attracted molecule, and the 

 other is situated at the opposite extremity of the 

 same diameter, do not attract the molecule in the 

 same manner as they would do if they were united in 

 the centre of the sphere ; and, consequently, the ac- 

 tion of a sphere upon an elementary molecule which 

 it touches is not proportional to the radius. If the 

 masses of two finite bodies which attract each other 

 become infinitely small, their mutual action, in so far 

 as their masses are concerned, will suffer an infinite 

 diminution. But if these masses, which have become" 

 infinitely small, are in contact, their centres of action 

 will be infinitely near each other, whence the attrac- 

 tion, when it follows the inverse ratio of .the square 

 of the distance, will be augmented infinitely more by 

 the approximation of the centres of action than it 

 was diminished by the extreme smallness of the 

 masses, consequently the attraction is infinite." Diet. . 

 de Physique, vol. i. p. 100. 



It has been suggested by La Place, that the dis- 

 tances between the molecules of bodies may be in- 

 comparably greater than the diameters of the mole- 

 cules themselves, so that the density of each molecule 

 is much greater than the density of the body in 

 which it exists, or the density which it would have 

 if all the matter of the molecule were uniformly dis- 

 tributed within the body. The attraction of a par- 

 ticle touching a sphere composed of these dense mo- 

 lecules would thus be very great, compared with the 

 attraction which it would experience at a finite dis- 

 tance, even when the law of action was the same as 

 that of gravity ; so that if matter is thus constitu- 

 ted, the attraction of affinity may, in all probability, 

 be only a case of universal gravitation. See Cohe- 

 sion, Capillary Attraction, Electricity, Mag- 

 netism, and Optics, (o) 



ATTRACTION of Mountains. If every por- 

 tion of matter is attracted by every other portion of 

 matter, with a force directly proportional to the 

 number of gravitating particles, and inversely as the 

 square of the distance, it might naturally be ex- 

 pected, that the attractive force of a large and solid 

 mountain might be determined by direct experi- 

 ments. Though the clouds and vapours which crown 

 the summits of lofty mountains, or hover along their . 

 sides, evidently indicate the exertion of an attractive 

 force, yet astronomers have sought for a more une- 

 quivocal proof of its existence, by measuring the de- 

 flection which it produced in attracting a plumb-line 

 from its perpendicular position. 



The earliest hint of this method was suggested by 

 Sir Isaac Newton ; and it was first put in execution 

 by the French academicians, who were sent to mea- 

 sure a degree of the meridian in Peru. The celebra- 

 ted Bouguer selected the mountain Chin.boraco as the 

 most proper for this purpose ; and from a rough calcu- ' 

 lation he concluded, that its attraction would be equal 

 to the 1000th part of that of the whole earth, and 



