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The philosopher Anaxagoras, who lived about 500 years before the Christian era, is generally considered as the first who noticed this principle, as subsisting between the heavenly bodies and the earth, which he considered as the centre of their motions. The doctrines of Epicurus and of Democritus are founded on the same opinion.
Nicholas Copernicus appear to have been one of the first among the moderns, who had just notions of this doctrine.
After him,. Kepler brought it still nearer perfection; having determined that bodies tended to the centres of the larger round bodies of which they formed a part, and the smaller celestial bodies to the great ones nearest to them, instead of to the centre of the universe: he also accounted for the general motion of the tides on the same principle, by the attraction of the moon; and expressly called it virtus tractoria quæ in luna est; besides this, he refuted the old doctrine of the schools, "that some bodies were naturally light, and for that reason ascended, while others were by their nature heavy and so fell to the ground;" declaring that no bodies whatsoever are absolutely light, but only relatively so, and that all matter is subject to the law of gravitation.
Dr. Gilbert, a physician at London, was the first in this country who adopted the doctrine of attraction; in the year 1600, he published a work entitled, "De Magnete Magneticisque Corporibus;["} which contains a number of curious things; but he did not sufficiently distinguish between attraction and magnetism.
The next after him was Lord Bacon, who, though not a convert to the Copernican system, yet acknowledged an attractive power in matter.
In France also, we find Ferinat and Roberval, mathematicians of great eminence, maintaining the same opinion. The latter, in particular, made it the fundamental principle of his system of physical astronomy, which he published in 1644, under the title of "Arist. Samii de Mundi Systema."
Dr. Hooke, however, was the person who conceived the most just and clear notions of the doctrine of gravitation, of any before Newton; in his work called "An Attempt to prove the Motion of the Earth:" 1674. He observes that the hypothesis on which he explains the system of the world, is in many respects different from all others; and that it is founded on the following principles: 1. That all the heavenly bodies have not only an attraction or gravitation toward their own centres, but that they mutually attract each other within the sphere of their activity. 2. That all bodies which have a simple or direct motion, continue to move in a right line, if some force operating without incessantly does not constrain them to describe a circle, an ellipse, or some other more complicated curve. 3. That attraction is so much the more powerful, as the attracting bodies are nearer to each other.
But the precise determination of the laws and limits of the doctrine of attraction, was reserved for the genius of Newton: in the year 1666, he first began to turn his attention to this subject, when, to avoid the plague, he had retired from London into the country; but, on account of the incorrectness of the measures of the terrestrial meridian, made before this period, he was unable to bring his calculations on the subject to perfection at first.
Some years afterwards his attention was again called to attraction by a letter of Dr. Hooke's; and Picard, having about this time measured a degree of the earth, in France, with great exactness, he employed this measure in his calculations, instead of the one he had before used, and found, by that means, that the moon is retained in her orbit by the sole power of gravity, supposed to be reciprocally proportional to the squares of the distances.
According to this law, he also found, that the line described by bodies in their descent is an ellipse, of which the centre of the earth occupies one of the foci; and considering afterwards, that the orbits of the planets are in like manner ellipses, having the centre of the sun in one of their foci, he had the satisfaction to perceive, that the solution which he had undertaken only from curiosity, was applicable to some of the most sublime objects in nature. These discoveries gave birth to his celebrated work, which has justly immortalizeed his name, entitled "Philosophicæ Naturalis Principia Mathematica."
In generalising these researches, he shewed that a projectile may describe any conic section whatsoever, by virtue of a force directed towards its focus, and acting in proportion to the reciprocal squares of the distances. He also developed the various properties of motion in these kinds of curves, and determined the necessary conditions, so that the section should be a circle, an ellipse, or an hyperbola, which depend only upon the velocity and primitive position of the body, assigning in each case the conic section which the body would describe.
He also applied these researches to the motion of the satellites and comets, shewing that the former move round their primaries, and the latter round the sun, according to the same law; and he pointed out the means of determining by observation the elements of these ellipses.
He also discovered the gravitation of the satellites towards the sun, as well as towards the planets; and that the sun gravitates towards the planets and satellites, as well as that these gravitate towards each other: and afterwards extending, by analogy, this property to all bodies, he established the principle, that every molecule of matter attracts every body in proportion to its mass, and reciprocally as the square of the distance from the body attracted.
Having ascertained this principle, he from it determined, that the attractive force of a body on a point placed without it is the same as if the whole mass were united at the centre. He also proved that the rotation of the earth upon its axis must occasion a flattening of it about the poles; which has since been verified by actual measurement: and determined the law of the variation of the degress in different latitudes, upon the supposition that the matter of the earth was homogeneous.
But, with the exception of what concerms the elliptical motions of the planets and comets, and the attraction of the heavenly bodies, these discoveries were not wholly completed by Newton. This theory of the figures of the planets is limited by the supposition of their homogeneity; and his solution of the problem of the precession of the equinoxes is defective in several respects. He has perfectly established the principle which be had discovered; but left the complete development of its consequences to the geometers that should succeed him.
The profound analysis also, of which be was the inventor, had not been sufficiently perfected, to enable him to give complete solutions to all the difficult problems which arise, in considering the theory of the system of the world; so that he was oftentimes obliged to give only imperfect sketches or approximations, and leave them to be verified by a more rigorous calculation.
Attraction may be divided, with respect to the law it observes, into two kinds. 1. That which extends to sensible distances, such is the attraction of gravity, of which we have been treating, which is found in all bodies, and the attraction of magnetism and of electricity found in some particular bodies; 2. That which extends to very small, or insensible distances.
The attractions belonging to the first class must be as numerous as there are bodies situated at sensible distances. It has been proved that their intensity varies with the mass and the distance of the attracting bodies; it increases with the mass of those bodies, but diminishes as the distance between them increases. The rate of variation has been demonstrated to be inversely as the square of the distance in all cases of attraction belonging to the first class.
The nature of the attraction of gravity has been already discussed. It is, as far as the experience of man can extend, universal in all matter. The attraction of magnetism and of electricity are partial, being confined to certain sets of bodies, while the rest of matter is destitute of them; for it is well known that all bodies are not electric, and that scarcely any bodies are magnetic, except iron, cobalt, nickel, and chromium; and there is good reason to suspect that the magnetism of the three latter substances is calmed by their containing some iron united to them.
The intensity of these three attractions increases as the mass of the attracting bodies, and diminishes as the square of the distance.
The first extends to the greatest distance at which bodies are known to be separated from each other. How far electricity extends has not been ascertained; but magnetism extends at least so far as the semi-diameter of the earth. All bodies possess gravity; but it has been supposed that the other two attractions are confined to two or three subtile fluids, which constitute a part of all those bodies that exhibit the attractions of magnetism or of electricity.
If we compare the different bodies acted on by gravitation, we shall find that the absolute force of their gravitation is in all cases the same, provided their distances from each other, and their mass be the same; but this is by no means the case with electrical and magnetic bodies: in them the forces by which they are attracted to each other, called electricity and magnetism, are exceedingly various, even when the mass and the distance are the same. Sometimes these forces disappear almost entirely; at other times they are exceedingly intense.
Gravity, therefore, is a force inherent in bodies, electricitv and magnetism not so; a circumstance which renders the opinion of their depending on peculiar fluids extremely probable. If we compare the absolute force of these three powers with each other, it would appear that the intensity of the two last, every thing else being equal, is greater than that of the first; but their relative intensity cannot be compared, and is therefore unknown. Hence it follows that these different attractions, though they follow the same laws of variation, are not the same in kind.
The attractions between bodies at insensible distances, have been distinguished by the name of affinity, while the term attraction has been more commonly confined to cases of sensible distance.
Affinity may be considered as operating on homogeneous or heterogeneous substances. Homogeneous affinity urges substances of the same nature together as iron to iron, soda to soda. Heterogeneous affinity draws substances of different nature into union, as acid and alkalis.
Homogeneous affinity is usually denominated cohesion, and sometimes adhesion when the surfaces of bodies are only referred to; it is nearly universal; as far as is known, caloric and light alone are destitute of it.
Heterogeneous affinity is the cause of the formation of compound substances; thus muriatic acid unites with sodas and forms sea-salt; and sea-salt in saturated solution is united into masses by homogeneous affinity. Heterogeneous affinity is universal as far as is known; that is to say, there is no substance which is not attracted by some other substance. It is generally taken for granted that every substance has more or less affinity for all others, though it is certainly assuming more than even analogy can warrant, and is a point which we have no means of ascertaining.
Affinity, like sensible attraction, varies with the mass and the distance of the attracting bodies That cohesion varies with the mass cannot indeed be ascertained, because we have no means of varying the mass without at the same time altering the distance. But in cases of the adhesion of the surfaces of homogeneous bodies, which is undoubtedly an instance of homogeneous affinity, it has been demonstrated that the force of adhesion increases with the surface, which in some respect is the same as with the mass.
That heterogeneous affinity increases with the mass has been observed long ago in particular instances, and has been lately demonstrated by Berthollet to take place in every case. Thus a given portion of water is retained more obstinately by a large quantity of sulphuric acid, than by a small quantity. Oxygen is more easily abstracted from oxides which are oxydised to a maximum, than from those which are oxyded [oxydised?] to a minimum. Lime only takes off the greatest part of the carbonic acid from potash, which still retains a portion of it; and sulphuric acid does not totally displace phosphoric acid from the lime united to it in phosphate of lime, a part of it remains undisturbed. In these and many other cases, a small portion of one substance is retained by a given quantity of another more strongly than a large portion; and Berthollet has shewn, that in all cases a large quantity of one substance is capable of abstracting a portion of another from a small portion of a third, how weak soever the affinity between the first and second is, and how strong soever that between the second and third.
That the force of affinity increases as the distance diminishes, and the contrary, is obvious; for it becomes insensible, whenever the distance is sensible, and, on the other hand, it becomes exceedingly great, when the distance is exceedingly diminished. But the particular rate which this variation follows is still unknown; some have supposed the rate to be the same as that of sensible attraction, and that its intensity varies inversely, as the square of the distance; no sufficient argument has ever been advanced, to prove this law to be incompatible with the phenomena of affinity; but, on the other hand, no proof has ever appeared in support of this opinion.
Affinity agrees with sensible attraction in every determinable point: like sensible attraction, it increases with the mass, and diminishes as the distance augments; consequently it is just to conclude, that attraction, whether it be sensible or insensible, is in all cases, the same kind of force, and regulated precisely by the same general laws.
The forces of affinity, though the same in kind, and possessing the same rate of variation with regard to distances, and also in respect to the mass, are vastly more numerous than those of sensible attraction; for, instead of three, they amount to as many as there are heterogeneous bodies. But even when the distance and the mass are the same, as far as can be judged, the affinity of two bodies for a third is not the same. Thus barytes has a stronger affinity for sulphuric acid than potash has; for, on equal portions of them being mixed with a small quantity of the acid, the barytes seizes a much larger proportion of the acid than the potash does The difference of intensity extends to all substances, for there are scarcely any two bodies whose particles have precisely the same affinity for a third, and scarcely any two bodies whose component parts adhere together with exactly the same force.
Because these affinities do not vary in common circumstances, like magnetism and electricity, but are always the same when other circumstances are equal, it has been argued that they do not, like them, depend on peculiar fluids, the quantity of which may vary; but that they are permanent forces, inherent in every part of the attracting bodies.
But after the extraordinary discoveries that have been lately made of the powerful effects which electricity, as excited by the galvanic apparatus, has in chemical attractions: and when the great force of the affinity of the bases of potash and of soda to oxygen have been overcome by it, we must hesitate at least in continuing the above opinion; if we do not totally reject it, to adopt its reverse, and consider electric fire in future as the great agent of elective affinities. There is no reason why electric fire may not be subject to the same laws of attraction as other substances, and why it may not remain united to bodies in a latent or inactive state, as well as caloric; we have already shewn, that the mass of any substance has a powerful effect on its degree of affinity; many of the effects of electric fire on affinity might be explained by this increased power of it when acting in a mass, or at farthest by supposing, that its power increased with its mass in a greater ratio than that of other subtances.
It has been judiciously remarked, by a respectable chemical writer, that the variation of intensity, which forms so remarkable a distinction between affinity and gravitation, may be only apparent, and not real, and may only arise from the much nearer approach which the parts of one substance may be capable of, to those of a second, than to those of a third; and that thus it may be that barytes attracts sulphuric acid with greater intensity than potash, because the particles of barytes, when they act upon the acid, are at a smaller distance from it than the particles of the potash; to which we shall add, that it is possible that the degree of insensible distance to which the parts of substances can approach, depends on the quantity of latent electric fire combined with them, or in other words, on the degree of their relative attractions to electric fire.
This conjecture of the agency of electric fire, in elective attractions, has, at least, the advantage of the atomic theory, which has been advanced to account for the same phenomena, that it relates to matters which we know really exist, and which are not beyond the bounds of hope, indeterminable by experiment. With all due deference to the respectable characters who have used the atomic theory as an universal explainer, we beg leave to remind its admirers, that it is totally inconsistent with the laws of sound philosophy, to assume a fact as the basis of argument, which itself has never had the shadow of proof to support it, and which in its nature is incapable of experiment. It is idle, in the present respectable state of science, to talk any more of atoms: as well may we again revive the dreams of the ancients, about the materia subtilis; or those of Des Cartes, relative to vortices as to reason of the shape, form, nature, and properties of atoms, which, from their very definition, are merely visionary, and which, the moment we conceive them as having shape, lose their essential quality of indivisibility; if the existence of atoms cannot be disproved, that is no argument in favour of their existence, in the way usually supposed; and the atomic theory has only this property in common with every other which lies beyond the reach of our senses.
Judicial astrology, magic, and many other chimeras, cannot be disproved; but, at least since the great law of truth has been adopted for philosophy, that no argument was to be admitted in it that was not demonstrable by experiment, or by proof equaly satisfactory, mankind has ceased to be led astray by them.
It is now high time either to banish the atomic theory into the same regimes of oblivion as the others above mentioned, or to prove the existence of the atoms on which it is founded, but as this is in its nature an impossibility, it is to be hoped that the time is not far distant, when philosophers will cease to confound imaginary beings with real existences, and when all that has been written of atoms, will be in no more esteem than the voluminous treatises de Pygmeis et Salamandris, which are to be found among the folios of some of our great academical libraries.
It is true, that the atomic theory accounts plausibly for many things we otherwise must be content to own are as yet beyond our knowledge; this may be a convenience to those who wish to impose on the ignorant; but all true lovers of science will despise so paltry a resource, especially when so much is now known, that we need no longer blush to own those points which are still involved in obscurity, and shew the boundaries on the map of science between the regions of knowledge and the terra incognita of visionary theory.
In the above respect of accounting for matters unknown, the ideal system of Bishop Berkley is equally powerful as, if not superior to, the atomic theory, and has the advantage over it, of turning our thoughts incessantly to the Almighty Author of all things; for which reason, if we must have recourse to improved [unproved?] theories, Berkley's very much deserve the preference.
As to the more minute nature of bodies, we know that all mineral substances are resolvable into small laminæ or spicula, of determinate shapes, which by their mulitifarious combinations produce the variously formed chrystals, which all mineral bodies may be resolved into by art, which most may be made to exhibit by skilful dissection, and which so many shew naturally. Vegetable substances are resolvable into small fibres, as are likewise animal substances for the most part: and from the laws of sound philosophy we must consider the laminæ or spicula, which form the basis of crystallization, as the primary parts of mineral bodies and fibres as those of organized bodies until something further can be proved on the subject. These primary parts of bodies adhere together, it is most probable, by the attraction of cohesion, (as do also their combinations into crystals and other forms), modified in some degree by that attraction caused by electric fire.
The attraction which takes place among substances in solution is not so easily comprehended; as we know nothing as yet of the exact state in which a substance, capable of solidity, exists, when dissolved in a fluid. In our present state of knowledge, we can only consider it as a fluid itself capable of reassuming a solid form in certain circumstances.
The attraction which takes place between bodies in a state of vapour, is similar to that in a fluid state; their precise and minute state in that condition is unknown; but the combinations which ensue from the attractions of many in both states, are familiar to all chemists, and from them have proceeded many of the most useful substances which we possess. It is very fortunate for us, however, that if the knowledge of the minute and primary state of bodies is, as it were, concealed from our view by an impenetrable veil, it is not of any very great importance to us; as the effects which bodies produce on each other can be known to us without it, and it is this latter species of knowledge that affords us the dominion over nature, supplies our wants, and forms the basis of worldly happiness.
The characteristic marks of affinity may be reduced to the three following: