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tance may be held the same as if the whole of the matter of which they consist were condensed into their respective centres. To what extent in the heavens the power of gravitation ranges it is impossible to determine; there can be little doubt, however, that it extends from one fixed star to another, although its effects are too inconsiderable to be calculated by man. It may possibly influence the progressive motion of several of the stars, and, as I had occasion to observe in a preceding lecture, is the cause to which Dr. Herschel ascribes the origin of the material universe, which he supposed at one time, though he seems afterward to have modified his opinion, as we shall notice in our next study, to have issued from an immense central mass of matter, peculiarly volcanic in its structure, and to have been, consequently, thrown forth in different quantities, and at different times, by enormous explosions; each distinct mass, thus forcibly propelled, assuming, from the common law of projectiles, an orbicular path, and endowed with the common property of the parent body, ejecting in like manner, minuter masses at different periods of time, which have equally assumed the same orbicular motion, and ultimately become planets to the body from which they have immediately issued, and which constitutes their central sun. To produce such an effect, however, and in reality to produce any of the motions which occur to us in the celestial bodies, the Passivity of matter is just as necessary as its gravitation. I have already observed that, owing to its passivity, or vis Inertue, matter has a tendency to persevere in any given state, whether of motion or of rest, till opposed by some exterior power; and that the path it assumes must necessarily be that of a right line, unless the power it encounters shall bend it into a different direction. A projectile, therefore, as a planet, for example, thrown forth from a volcano, would travel in a right line for ever, and with the exact velocity with which it was thrown forth at first, if there were nothing to impede its progress, or to alter the course at first given to it. But the attraction of the volcanic sphere from which it has been launched does impede it, and equally so from every point of its surface: the consequence of which must necessarily be, that every step it advances over the parent orb it must be equally drawn back or reined in, and hence its rectilinear path must be converted into a curve or parabola, and a tendency be given to it to escape in this line, which may be contemplated as a line of perpetual angles, instead of in a direct course; and as soon as the projectile or planet has acquired the exact point in which the two antagonist powers precisely balance each other—the power of flying off from the centre, communicated to it by the volcanic impulsion, and which is denominated its Centrifugal Force, and the power of falling forwards to the centre, communicated by the attractive influence of the aggregate mass of matter, which the parent sphere contains in itself, and which is called its Centripetal Force—it will have reached its proper orbit; and, through the influence of this constant antagonism of the two properties of passivity and gravitation, of a centrifugal and centripetal force, persevere in the same to the end of time. Of the immediate cause of gravitation, or the nature of that power which impels different bodies to a union, we are in a very considerable degree of ignorance; or rather, perhaps, may be said to know nothing at all. It is necessary, however, to notice one very singular phenomenon concerning it, and to give a glance at two out of various theories by which gravitation has been attempted to be accounted for. The phenomenon is, that although owing to this power, all bodies have a tendency to come into contact, they never come into actual contact: some kind of pore or open space being still left between the corpuscles of bodies that approach the nearest to each other. Thus, a plate of heated iron, solid as it appears to be, and altogether destitute of pores, becomes contracted in every direction by cold. So, too, as I have already observed, equal measures of water and alcohol, or of water and sulphuric acid, have their bulk sensibly diminished. In like manner, Newton has remarked, that when two plates of class are within about a ten thousandth part of an inch of each other, using fine metallic plates as a micrometer on this occasion, they support each other's weight as powerfully as if they were in actual contact, and that some additional force is requisite in order to make them approach still nearer. Nor is the force necessary to produce this effect of trivial moment: Professor Robison has calculated it, and has ascertained by experiment that it is equal to a pressure of a thousand pounds for every square inch of glass. Air is not necessary to this resistance, for it is equally manifest in a vacuum; yet it is a very curious fact, that under water it almost entirely disappears. It is, however, might be expected to cohere in such a manner as to become an individual mass. It is hence clear that matter, from some cause or other, is possessed of a Bepulsive as well as of an Attractive force; and that, like the latter, although its law has not been hitherto exactly ascertained, it increases in a regular proportion to its decrease of distance, or, in other words, as bodies approximate each other. It has hence been said, and this is the common theory of those who regard gravitation as an essential property of matter, that matter is universally endowed with two opposite powers; by the one of which material substances attract each other, and induce a perfect union; and by the other of which they repel each other when they are on the point of union, and prevent a perfect contact. It is admitted, however, on all hands, and is indeed perfectly clear in itself, that the repulsive power is of an almost infmitely less range than the attractive. I have supposed the attractive power, or that of gravitation, to operate from world to world; yet the repulsive power can never be exerted, except "between such particles as are actually, or very nearly, in contact with each other; since it requires no greater pressure, when acting on a given surface, to retain a gallon of air in the space of half a gallon, than to retain a pint in the space of half a pint, which could not possibly be, if the particles exercised a mutual repulsion at all possible distances."* This idea, however, of double and opposite powers co-existing in the same substance, and in every corpuscle of the same substance, has been uniformly felt difficult of admission by the best and gravest philosophers; and hence Sir Isaac Newton, while allowing the repulsive power of matter, which in truth is far more obvious to our senses in consequence of its very limited range, has felt a strong propensity to question gravity as forming an essential property of matter itself, and to account for it from another source. "To show," says he," that I do not take gravity for an essential property of bodies, I have added one question concerning its cause, choosing to propose it by way of question, because I am not yet satisfied about it, for want of experiments."f In this question he suggests the existence of an ethereal and elastic medium pervading all space; and supports his supposition by strong arguments, and consequently with much apparent confidence, deduced from the mediums, or gases, as they are now called, of light and heat, and magnetism, respecting all which, from their extreme subtlety, we can only reason concerning their properties. This elastic medium he conceives to be much rarer within the dense bodies of the sun, the stars, the planets, and the comets, than in the more empty celestial spaces between them, and to grow more and more dense as it recedes from the celestial bodies to still greater distances: by which means all of them, in his opinion, are forced towards each other by the excess of an elastic pressure. It is possible, undoubtedly, to account for the effects of gravitation by an ethereal medium thus constituted; provided, as it is also necessary to suppose, that the corpuscles of such a medium are repelled by bodies of common matter with a force decreasing, like other repulsive forces, simply as the distances increase. Its density, under these circumstances, would be every where such as to produce the semblance of an attraction, varying like the attraction of gravitation. The hypothesis in connexion with the existence
highly probable that the contact
* Dr. Yotuif'B Lett. vol. i. p. 612. t Optica, pref. to the Bocond edition.
of a repulsive force in common matter has a great advantage in point of simplicity, and may perhaps hereafter be capable of proof, though at present it can only be regarded, and was at first only offered, as an hypothesis. M. la Place, equally dissatisfied as Sir Isaac Newton with the idea of gravitation being an essential property of matter, passes away from the inquiry with suitable modesty, to practical subjects of lar higher importance, and which equally grow out of it, in whatever light it is contemplated. "Is this principle," says he, "a primordial law of nature? or is it a general effect of an unknown cause? Here we are arrested by our ignorance of the nature of the essential properties of matter, and deprived of all hope of answering the question in a satisfactory manner. Instead, then, of forming hypotheses on the subject, let us content ourselves with examining more particularly the manner in which philosophers have made use of this most extraordinary power."*
There is, indeed, one very striking objection to Sir Isaac Newton's suggestion, and which it seems very difficult to repel. It is, that though it may account for the attraction of gravitation, as a phenomenon common to matter in general, it by no means accounts for a variety of particular attractions which are found to take place between particular bodies, or bodies particularly circumstanced; and which, excepting in one or two instances, ought, perhaps, to be contemplated as modifications of gravitation. Upon these particular attractions, or modes of attraction, including homogeneous attraction, or the attraction of aggregation, heterogeneous attraction, or the attraction of capillary bodies, elective attraction, and those of magnetism and electricity, each of which is replete with phenomena of a most interesting and curious nature, I intended to have touched in the present lecture, but our limited hour is so nearly expired, that we must postpone the consideration of them as a study for our next meeting. Yet it is not possible to close the observations which have now been submitted, without testifying our gratitude to the memory of that transcendent genius whom the providence of the adorable Architect of the universe at length gave to mankind six thousand years after its creation, to unravel its regular confusion, and reduce the apparent intricacy of its laws to that sublime and comprehensive simplicity which is the peerless proof of its divine original. It has been said, that the discovery of the universal law which binds the pebble to the earth, and the planets to the sun, which connects stars with stars, and operates through infinity, was the result of accident. Nothing can be more untrue, or derogatory to the great discoverer himself. The earliest studies of Newton were the harbinger of his future fame: his mighty mind, that comprehended every thing, was alive to every thing; the little and the great were equally the subjects of his restless researches: and his attention to the fall of the apple was a mere link in the boundless chain of thought, with which he had already been long labouring to measure the phenomena of the universe. Grounded, beyond all his contemporaries, in the sure principles of mathematics, it was at the age of twenty-two that he first applied the sterling treasure he had collected to a solution of the system of the world. The descent of heavy bodies, which he perceived nearly the same on the summit of the loftiest mountains and on the loweft surface of the earth, suggested to him the idea that gravity might possibly extend to the moon; and that, combined with some projectile motion, it might be the cause of the moon's elliptic orbit round the earth: a suggestion in which he was instantly confirmed by observing that all bodies in their fall describe curves of some modification or other. And he further conceived, that if the moon were retained in her orbit by her gravity towards the earth, the planets must also in all probability be retained in their several orbits by their gravity towards the sun. To verify this sublime conjecture, it was necessary to ascertain two new and elaborate positions: to determine the law of the progressive diminution
* Exposition du Systiime du Monde, liv. iv. oh. x».
of gravity, and to develope the cause of the curves or ellipses of falling bodies. Both these desiderata he accomplished by a series of reasonings and calculations equally ingenious in their origin and demonstrative in their result and ascertained the truth of his principles by applying them, practically and alternately,tothephenomenaof the heavens, and to a variety of terrestrial bodies. The bold and beautiful theorem being at length arrived at, and unequivocally established—a theorem equally applicable to the minutest corpuscles, and the hugest aggregations of matter—that all the particles of matter attract each other directly as their mass, and inversely as the square of their distance, he at once beheld the cause of those perturbations of motion to which the heavenly bodies are necessarily and so perpetually subject: it became manifest, that the planets and comets, reciprocally acting and acted upon, must deviate a little from the laws of that perfect ellipse which they would precisely follow if they had only to obey the action of the sun: it was manifest, that the satellites of the different planets, exposed to the complicated action of the sun, and of each other, must evince a similar disturbance: that the corpuscles which composed the different heavenly bodies in their formation, perpetually pressing towards one common centre, must necessarily have produced, in every instance, a spherical mass: that their rotatory motion must at the same time have rendered this spherical figure in some degree imperfect, and have flattened these masses at their poles; and, finally, that the particles of immense beds of water, as the ocean, easily separable as they are from each other, and unequally operated upon by the sun and the moon, must evince such oscillations as the ebbing and flowing of the tides. The origin, progress, and perfection of these splendid conjectures, verifications, and established principles, were communicated in two distinct books, known to every one under the titles of his "Principia" and his "Optics ;"—books which, though not actually inspired, fall but little short of inspiration, and have more contributed to exalt the intellect of man, and to display the perfections of the Diety, thau 8".v thing upon which inspiration has not placed its direct and awful stamp. LECTURE V. ON THE PROPERTIES OF MATTER, ESSENTIAL AND PECULIAR. (The subject continued.)
Wi closed our last lecture with remarks on the universal operation of the common principle of gravity over matter in all its visible forms, from the minutest shapes developed by the microscope, to the mightiest suns and constellations in the heavens. But we observed, also, that, independently of this universal and essential power of attraction, matter possesses a variety of peculiar attractions dependent upon circumstances of limited influence, and which consequently render such attractions themselves of local extent. These I will now proceed to notice to you in the following order:—1st, The attraction of homogeneous bodies towards each other, which is denominated, in chemical technology, the attraction of aggregation: 2dly, The attraction of heterogeneous bodies towards each other, under particular circumstances, which in its more obvious cases is denominated capillary attraction: 3dly, The attraction of bodies exhibiting a peculiar degree of affinity to each other, and which is denominated electrive attraction: 4thly, The attraction of the electric fluid; and, 5thly, That of the magnetic. I. The law of physics, which has rendered every material substance capable of attracting and being attracted by every other material substance, seems at the same time to have produced this power in a much stronger degree between Substances Of Like NATURES. Thus, drops of water placed upon a plate of dry glass have a tendency to unite, not only when they touch, but when in a state of vicinity to each other; and globules of quicksilver still more so: and it is this kind of attraction which is called the attraction of aggregation. And in both these cases the attraction in question evinces a considerable superiority of force to the general attraction of gravitation; since the particles of the drops or globules ascend from the surface of the glass, except those that form their narrow base, and are drawn towards their proper centres, instead of being drawn towards the centre of the earth. If, however, the convex shape of the drop of water be destroyed by pressing it over the glass into a thin extended film, the general attraction of gravitation, acting with increased effect upon an increased space, will overpower the individual attraction of aggregation, and the particles of water will be restrained from attempting a spherical figure as before. In the quicksilver, nevertheless, the attraction of aggregation being much stronger than in the water, it will still continue to prevail; and it is only by a very minute and elaborate division of the particles of this material that we can give to the attraction of gravitation a predominancy. The same result occurs in the homogeneous particles of oil. And hence, if we divide its particles by shaking a certain portion of it in water, we find, upon giving the mixture rest, that the water will first sink to the bottom, or, which is the same thing, the particles of the oil will rise to the surface; and then that these particles, as soon as they have reached the range of each other's attraction, will unite into one common body. Now, in all these cases it is obvious that the particles of matter thus obeying the law of homogeneous attraction assume or attempt to assume a spherical figure; and we not unfrequently perceive a similar attempt, even where the breadth of the surface, and the consequent potency of the attraction of gravitation, would hardly induce us to expect that there could be the least effort towards it: as, for example, in a glass brim-full, or somewhat more than brim-full of wine, or any other liquid. We behold the same figure in the drops of rain as they descend from the clouds; a figure which, in fact, is the sole cause of the vaulted form of the rainbow, as I may possibly take leave to explain more particularly on some future occasion. We behold it in reality throughout all nature, in every substance whose particles are capable of uniting and separating with ease; and, consequently, of readily obeying the laws of cohesibility and divisibility, as those of liquids; and we should see it equally in solids, but that the particles of these last are incapable of doing readily either the one or the other. What, then, is the general cause that produces so general an effect? Clearly this: a cause to which I have already in some degree adverted, in speaking of the general attraction of gravitation: that, there being an equal tendency in every particle of homogeneous bodies to press together, they must press equally towards one common centre, and strive to be as little remote from that centre as possible. Such a strife, however, must necessarily produce a globular or spherical form; for it is in such a form only that the extreme particles, or those constituting its surface, and which are prevented from a closer approach by those that lie within, are equally near and equally remote in every direction. Hence, then, the cause of the globular figure of drops of quicksilver, drops of water, drops of rain, and drops of dew, collected and suspended from the fresh leaves of plants in the balmy air of the morning: and hence ori&reason, though there is also another that concurs with it, and which I shall explain presently, for the convex shape assumed by a wine-glass of liquid of any kind, on its surface, when brim-full, or somewhat more. The same reasoning may be applied to account for the spherical figure of the heavenly bodies; each of which, though probably composed of many different or heterogenous substances in itself, may be fairly contemplated as a homogeneous mass when compared with those by which it is surrounded: and hence, too, we see the necessity for their having at first existed, from some cause or other, in a fluid state; since, otherwise, the different corpuscles which enter into their make could not have assumed that symmetrical arrangement which alone gives sphericity to the total bulk. We have equal proofs of the same peculiar attraction existing between