ATTRACTION. 



ATTRACTION. 



rs 



mutual n/rrtMH beween bodies," adding that the tide, an caused by a 

 /mrtorM OTKT i /a at. The finrt person in this country who 



revived the doctrine .at attraction WM Dr. OUbart of Colchester, and 

 the next was Lord Bacon. It wai Dr. Hooke, however, who, before 

 Newton 'a time, perfected moat the theory of thia subject, ahowing nut 

 only the fact*, but in a general way the law alao, of attraction (which 

 he called a omolw attxdauH). The true Uw however, that of the 

 inverse iquare of the distaix-n, WM discovered by Newton, who about 

 the year 1M4, being driven from London by the plague, wan led by 

 cirruzustances to diacover it by mathematical investigation. 



AUroetiu* may be divided into two kind* (1), that which act* only 

 at ineenaible or imjimiteumal HitUn"*t ; this kind of attraction existing 

 between atom* manifests itaelf in several way*, (i.) an Chemical Affinity 

 [CHEMICAL Arrmrt] ; (ii) ai CBYSTALLIZATIOX, which may be con- 

 sidered a* a polarity in molecular aggregation, (iii.) ai ADHESION and 

 COHESION, under which may be included the phenomena of CAFII.LA- 

 BITT ; and (2) that which acta at aU distances however great ; thin U 

 manifeated chiefly in the pfr^ireTWft of GRAVITATION, but alao in 

 KLLCTBICAL, MAOXSTIC, and other attractions. In the present article 

 we shall apeak chiefly of AJkuiun and Cvkttivu, referring to other 

 article* for the net. 



All attractive forces, whether exerted between atoma or mane*, 

 obey the general law mentioned above, namely, the attractive force is 

 inversely aa the iguara of the distances between the attracting bodies. 

 T.is ii exemplified by the following table : 



I'. '..:. - 



1 





A 



10 



Ti. 





Wham two smooth surfaces of the lame substance are pressed together, 

 they talurt. Thia eokeam is very evident, if we take plates of lead, or 

 of glass, which will remain so firmly united, that in many cases they 

 may be worked as one single piece. Adlunon, on the contrary, is 

 aid to take place between two surfaces of different substances, such as 

 a solid and a liquid. Thus, when a liquid irrtt a cup, 4c., it is said to 

 udkert, and not to taken to the cup. The force of this adhesion may 

 be measured approximately by the method of Guyton and Quctelet, 

 who, with Gay-Lussac, have studied this subject. They formed discs of 

 various substances, and placed them as counterpoises to one scale-pan 

 of a balance. They then allowed the disc to fall gently on the surface 

 of some liquid, to which it adhered with some force, this force being 

 then measured by placing weights in the opposite scale-pan until the 

 force of adhesion was just neutralised, and the disc flew up from the 

 liquid. By this process they obtained the following results : 



A Glari Duw, 4-6 in. 



in diameter 



adhering to 



Water 



Alcohol of 



(1) Sp. gr. = -8196 

 (Ii) Sp. gr. = -8595 

 (iii) Sp.gr. = -94 U 



( Turpentine ) 

 \ Sp. gr. = -893 ) 



Force of 



Adhesion 



in grunt. 



414-7 



477-4 

 505-1 

 569-8 



623-6 



The force of Cohuion, on the other hand, has been approximately 

 measured by observing the size and weight of the drop which any 

 liquid forms when suspended on the end of a rod. 



When two bodies, both of them capable of motion, attract one 

 another, it is easily demonstrable by mathematical reasoning, that they 

 will mutually move towards one another, and will meet in their centre 

 of gravity ; thia point being, of course, nearer to the larger body, if 

 they are homogeneous. Hence the smaller body will have a greater 

 velocity communicated to it by the attractive force, and we shall find 

 that the law of attraction may be expressed as depending not only on 

 the distance, but also that it varies directly as the product of the 

 mum of the bodies ; so that, if u and m be the masses, and r the 



distance between them, the force of attraction will be = '- 



In order rigidly to tost these laws, various experiments have from 

 time to time been instituted. Thus, it is plain that any large body 

 brought near to a smaller one, must cause, according to this law, a 

 motion in the smaller body much greater than that in the larger one. 

 This U well seen in the celebrated experiment of Cavendish, described 

 in the ' Phil. Trans' for 1788. It was thia : If we balance one ball of 

 la-ad by another on a horizontal lever, no horizontal oscillation takes 

 place ; but any little disturbance makes the lever turn completely 

 round again and again, till friction restores the equilibrium. Cavendish 

 balanced two balls of lead very nicely on a levin-, which he suspended 

 by a thread. A firm stand was provided, and the whole was inclosed 

 in a wooden case, to prevent agitation by the air, inserting only a 

 taleacops aad a lamp on one aide. When the apparatus was firm and 



no motion was perceived in the interior pendulum, other leaden balls 

 of considerable size were suddenly presented, outside the case, to each 

 end of the lever, whereupon horizontal oscillations immediately began 

 in the lever like those of a pendulum upon the earth such osculations 

 as would take place if the balls attracted one another. He observed 

 the duration of these oar-illations ; and thence, knowing the duration 

 of the oscillation which the earth creates in a pendulum, and also 

 knowing the relative densities of lead and water, he ascertained that 

 if the commonly received law of attraction be correct, the earth's 

 average density must be 54 times aa great as that of water. II 

 on reoalfiulating his result, found reason to think the 4 should be ft. 



It U evident that if matter attract matter, a mountain contiguous to 

 a plumb-line or a spirit-level will, in a slight degree, alter the position 

 of the former, or the surface of the latter. We van hardly expect to 

 measure the trifling displacement by direct means; but since the 

 instruments alluded to are the rcyulaton of some astronomical inxtru- 

 inente, it is plain that a false plumb-line or level may show itself l-y 

 giving false positions to the stars. And it is well known that the 

 mean of a number of observations detects very small instrumental 

 errors. Bouguer, in Peru, suspected that the proximity . .f i lumUiraco 

 affected his plumb-line ; and even detected an error of a few seconds 

 which he could in no other way explain : but his results remain 

 unverified. In 1772, Haskelyne (one of the best observer.- 

 proceeded to Scotland, to try the effect of Schehallien. He made a 

 great number of observations both north and south of the mountain ; 

 for he argued that since the plumb-line, if disturbed, miust 

 towards the mountain in both cases, the discordance he sought v. -uli 1 

 be doubled, and more easily perceptible. He found in this way, that 

 the north plumb-line and the south plumb-line made an angle of 114" 

 more than could be explained by the difference of latitud- ..f his two 

 stations. Huttun, on calculating the mean density of the earth from 

 thia result, found it five times as great as water : a result \ <-ry m :n K 

 that afterwards produced by Cavendish, when it is considered that 

 both the mean density and form of such a mass as Schehallien could 

 not be very accurately determined. 



In 1810, Baron Zach undertook a similar labour, in which he 

 employed a different instrument, and a different methoil of \,-rii, 

 He was carrying on a trigonometrical survey in the neighbourhood of 

 Marseille, and he had three small observatories near Mount Mimet, 

 north of that town. He measured on the earth their position with 

 respect to other stations too distant from the mountain to be senniolv 

 affected, and he then obtained the latitude of his observatories by 

 astronomical observation on the spot All three, without exception, 

 gave a difference of 2" between the geodesical and astronomical lati- 

 tudes, and in all the observed latitude was greater than the measured, 

 being the sort of effect which would be produced by attraction in the 

 mountain. M. Zach published the fullest detail of his method, and all 

 the observations, in his 'Attraction des Montagues,' Avignon, 1814. 

 For details of Magkelyne's measurements, see Hutton's ' Tracts/ vol. ii 

 and 'Phil. Trans. 1 1778. 



According to the common notion entertained of matter, it is solid, 

 or at least composed of particles which come into absolute contact If 

 we could substantiate the first supposition, it would still be apparent 

 that the term force of tolidijicatiun must take the place of fvrce of 

 cohetion, and give rise to inquiries into its quantity and mode of action : 

 and even if we could imagine absolute contact of particles, we should 

 find it necessary to appeud a notion of some force by which partii-l.- 

 in contact remain in contact when some of them are put in motion, so 

 aa to draw the rest after them. But the balance of probabilities is very 

 strong indeed against the supposition that matter is composed of 

 particles in contact ; so much so, that we are almost entitled to conclude 

 it to be composed of particles separated by interstices of much greater 

 dimensions than the particles themselves. If any one should assert 

 the particles of the densest matter to be as far apart in proportion to 

 their bulk as the bodies of the solar system, it would be impossible to 

 bring any direct evidence in contradiction. 



Such being the case, we may ask 1. What is the force of cohesion ? 

 for such a force there certainly is. 2. What is that law of action by 

 which the particles of bodies are not drawn into absolute contact, but 

 compelled to remain separate, and yet prevented from separating 

 indefinitely ? 



With regard to the first question, it is most probable that as two 

 bodies approach each other, a strong r.-pnl.-h .- force is the cause of tin- 

 first phenomenon which is perceived. When one billiard ball strikes 

 another, we have no evidence, except that of our senses, of absolute 

 contact taking place ; that is to say, we only know that the first visible 

 action takes place when the distance of particles is ton small for the 

 eye to perceive. All the evidence which is at all conclusive, is against 

 the supposition of such contact being produced : and we ore obliged to 

 admit that our explanation must end in the statement that, arise from 

 whence it may, there is a power in matter by which other m.v 

 repelled, and which begins to act before contact has taken place. But 

 if two pieces of solid matter be pronsed together with great force, it 

 would seem as if the ]>articles would thereby be brought within a 

 degree of nearness at which an attractive force begins to act. I'.- 

 bits of lead pressed together remain in coherence even in a vacuum ; 

 and metal plates can be hammered together until the cohesion is as 

 strong as if they bad been naturally united. 



