admiration of the virtuous and the excellent. That he did not feel the force of any argument offered by nature in proof of the immortality of the soul, and was in this respect considerably below the standard of Socrates and Cicero, must be equally admitted and lamented; and should teach us the high value of that full and satisfactory light which was then so much wanted, and has since been so gloriously shed upon this momentous subject. But let it at the same time be remembered, that, with a far bolder front than either of the philosophers here adverted to, he dared to expose the grossness and the absurdities of the popular religion of his day, and in his life and his doctrines gave a perpetual rebuke to vice and immorality of every kind. And hence, indeed, the main ground of the popular calumny with which his character was attacked, and which has too generally accompanied his memory to the present day.

LECTURE IV.

ON THE PROPERTIES OF MATTER, ESSENTIAL AND PECULIAR.

In our last lecture I endeavoured to render it probable, that all visible or sensible matter is the result of a combination of various solid, impene trable, and exquisitely fine particles or units of the same substance, to minute to be detected by any operation of the senses. Of the shape or magnitude of these particles we know nothing and even their solidity and impenetrability, as I then observed, is rather an assumption, for the purpose of avoiding several striking difficulties and absurdities that follow from a denial of these qualities, than an ascertained and established fact.

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From this unsatisfactory view of it in its elementary and impalpable state, let us now proceed to contemplate it in its manifest and combined forms, and to investigate the more obvious properties they offer, and the general laws by which they are regulated.

The change of distance between one material body and another, or, in other words, their approach to our separation from each other, is called MOTION; and the wide expanse in which motion of any kind is performed is denominated SPACE.

Matter has its ESSENTIAL, and its PECULIAR PROPERTIES. Its essential properties are those which are common to it under every form or mode of combination. Its peculiar properties are those which only appertain to it under definite forms or definite circumstances.

The ESSENTIAL PROPERTIES of matter are usually classed under the six following heads: passivity, extension, density, impenetrability, divisibility, and gravitation; which, however, may easily be reduced to four, since extension, density, and impenetrability may be comprehended under the general term of cohesibility.

PASSIVITY, inertia, or vis inertia, is the tendency in a body to persevere in a given state, whether of rest or motion, till disturbed by a body of superior force. And hence these terms, which are mere synonyms, imply a power of mobility as well as a power of quiescence; although passivity has often been confined to quiescence, while mobility has been made a distinct property. Thus it is from the same power or tendency to passivity, that a cannon ball continues its motion after being projected from a gun, as that by which it remained at rest before it was thrown off; for it is a

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well-known theorem in projectiles, that the action of the powder on a bullet ceases as soon as the bullet is out of the piece. In like manner a billiard-ball at rest will continue so till put into motion by a billiard-ball in motion, for it can never commence motion of its own accord. While a billiard-ball in motion would persevere in motion, and in the same velocity of motion, for ever, if it met with no resistance. But it does meet with resistance from a variety of causes, as the friction of the atmosphere, the friction of the green cloth, and at last a contact with one of the sides of the table, or with the ball against which it is directed.

In this last case either ball will receive conversely the same precise proportion of rest or motion which it communicates. Thus, if the ball in motion strike the ball at rest obliquely, the latter will be put into a certain degree of activity, and the former will, in the very same degree, be impeded in its progress, and receive an equal tendency to a state of rest. If the latter, on the contrary, by what is significantly called a dead stroke, receive the whole charge of motion which belongs to the former, it will give to the former, in like manner, the whole possession of its quiescence, and the state of each will be completely reversed; the ball hitherto at rest proceeding with all the velocity of that hitherto in motion, and the ball hitherto in motion exhibiting the dead stand of that hitherto at rest.

So, if it were possible to place an orb quietly in some particular part of space, where it would be equally free from the attractive influence of every one of the celestial systems, it would, from the same tendency to inertitude, remain quiescent, and at rest for ever. While, on the contrary, if a body were to be thrown from any one of the planets by the projectile force of a volcano, or of any other agency, beyond the range of the attractive or centripetal power of such planet, it would continue the same velocity of motion for ever which it possessed at the moment of quitting the extreme limit of the planet's influence; unless in its progress it should encounter the influence of some other planet; and in this last case it would be either drawn directly into contact with the planet it thus casually approached, or would have its path inflected into a circle, and revolve around it as a satellite, according to its velocity, and the relative direction of its course at the moment the planetary influence began to take effect. Thus a body projected horizontally to the distance of about 4.35 miles from the earth's surface, provided there were no resistance in the atmosphere, would not fall back again, but become a satellite to the earth, and perpetually revolve around it at this distance. The moon is supposed to have no atmosphere, or, at the utmost, one rarer than we can produce with our best air-pumps : she is also supposed to possess larger and more active volcanos than any which are known to exist on the earth. And hence it requires no great stretch of imagination to conceive that bodies may occasionally be thrown from the moon, by the projectile power of such volcanos, to such a distance as that they should never return to her surface for if the momentum be only sufficient to cause the mass ejected to proceed at the rate of about 8,200 feet in the first second of time,* and in a line passing through the moon and the earth, such effect would necessarily be produced; since, in this case, the propelled mass would quit the centripetal power of the former, and be drawn into that of the latter, and would either become a satellite to the earth, or be precipitated to its surface, according as the rectilinear

Laplace, Exposition du Systeme du Monde.

force of the projectile was equal or inferior to the attractive force of the earth at their first meeting together.

Yet this is, perhaps, but little more than the velocity with which a twenty-four pound cannon-ball would travel from the moon's surface : since its velocity on the earth's surface may be calculated at about 2,000 feet for the first second; and it would rush nearly four times as rapidly if not impeded by the resistance of the atmosphere. And hence it is to this cause that M. Olbers first, and M. Laplace has since, ascribed the origin of those wonderful aerolites, or stones, that are now known to have fallen from the air at some period or other in every quarter of the globe; believing them to be in every instance volcanic productions of the moon, thrown by the impulse of the explosion beyond the range of her centripetal influence.

COHESIBILITY is the tendency which one part of matter evinces to unite with another part of matter, so as to form out of different bodies one common mass. It includes the three modes, which have often been regarded as three distinct properties, of extension, density, and impenetrability. EXTENSION is a term as applicable to space as to matter: "The extension of body," observes Mr. Locke, “being nothing but the cohesion or continuity of solid, separable, moveable parts; and the extension of space the continuity of unsolid, inseparable, and immoveable parts." Hence extension applies to all directions of matter, for its continuity may take place in all directions; but in common language the longest extension of a body is called its length, the next its breadth, and the shortest its thickness.

DENSITY is a property in matter to cohere with a closer degree of approximation between the different particles of which it consists; so that the same body, when in the exercise of this property, occupies a smaller portion of space than before it was called into act. Hence density cannot be a property of space, the parts of which, as I have just observed, are immoveable, and cannot, therefore, either approach or recede.

IMPENETRABILITY is the result of density, as density is of extension. It is that property in matter which prevents two bodies from occupying the same place at the same time. They are all branches of the common property of cohesibility. A wedge of iron, indeed, may force its way through the solid fibres of the trunk of a tree; but it can only do this by separting them from each other: it cannot penetrate the matter of which those fibres consist. In like manner, when a ship is launched, her hulk cannot sink into the water without displacing the exact bulk of water which existed in the space that the hulk below the surface now occupies. To a cursory survey, however, there are some phænomena that seem to show that certain bodies are penetrable by others. Thus, if a cubic inch of water be mixed with a cubic inch of spirit of wine or sulphuric acid, the bulk of the compound will be something less than two cubic inches. But in this case, one of the fluids appears to admit a part of the other fluid into its pores; a fact of which there can be little doubt, since, if no evaporation be allowed to take place, though the bulk of the mixture is somewhat diminished, its weight is precisely equal to what it ought to be. The combination of different metals affords, not unfrequently, similar instances of equal introsusception.

DIVISIBILITY is a power in matter directly opposed to its cohesibility. It is that property of a body by which it is capacified for separating into parts, the union or continuity of which constituted its extension.

Divisibility, however, does not destroy cohesion in every instance

equally; though the farther it proceeds the farther it loosens it. We are told by Mr. Boyle, that two grains and a half of silk, were, on one occasion, spun into a thread not less than three hundred yards long, which is, notwithstanding, a much shorter length than the spider is capable of spinning his web of the same weight. Muschenbroek mentions an artist of Nuremburg, who drew gold wire so fine that 500 inches of it only weighed one grain; and Dr. Wollaston has obtained platinum wire as fine as th of an inch.* The thickness of tin-foil is about a thousandth part of an inch; that of gold-leaf is less than a two hundred thousandth part of an inch; and the gilding of lace is still thinner, probably in some cases not more than a millionth part of an inch; and there are living beings, visible to the microscope, of which a million million would not make up the bulk of a common grain of sand. Yet it is highly probable, from what has actually been ascertained of the anatomy of minute and microscopic animals, that many of these are as complicated in their structure as the elephant or the whale.

GRAVITATION is the common basis upon which all the preceding properties are built, except passivity; the great principle into which all the rest resolve themselves. Gravitation is the attraction by which bodies of all kinds act upon each other, with a force regulated by the aggregate proportion of their respective quantities of matter, and decreasing as the squares of the distances increase. It is a law impressed on matter universally, and hence operates alike on the minutest and on the largest masses; produces what we call weight on earth, or the tendency of heavy bodies to fall towards the earth's centre; and governs the revolutions of the planets. The five principles which regulate its mode of action, and constitute its magnificent code of laws, are thus summed up by M. La Place.‡ 1.- Gravitation takes place between the most minute particles of bodies. 2. It is proportional to their masses.

3. It is inversely as the squares of the distances.

4. It is transmitted instantaneously from one body to another.

5. It acts equally on bodies in a state of rest, and upon those which, moving within its range, seem to be flying off from its power.

To a casual observer there are many substances that seem to fly away from the earth, and consequently to oppose this general law. Thus smoke, when extricated from burning bodies, and vapour, when separated from liquids, ascend into the atmosphere; and a piece of cork, plunged to the bottom of a vessel of water, rises rapidly to the surface. But, in all these phænomena, the bodies that seem to move upwards merely give way to bodies of a heavier kind, or, in other words, which have a stronger tendency towards the earth. Thus smoke and vapour only ascend, because the surrounding air, which is heavier than these, presses downwards and takes their place; and the cork rises because lighter than the water into which it has been plunged: but empty the vessel, and the cork will remain at the bottom, because heavier than the surrounding air: and let the smoke or the vapour he received into a vacuum, and it will remain as much at the bottom as the cork.

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It was first systematically demonstrated by Sir Isaac Newton, that all the

* Wollaston in Phil. Trans. for 1813, p. 114. Thomson's Annals of Philos. No. III.

224.

Davy's Elem. vol. i. p. 379.

Exposition du Systeme du Monde.

motions of all the heavenly bodies depend upon the same power; and the principle thus struck out has of later years been still more extensively and even more accurately applied to a solution of the most complicated phænomena. This principle in astronomy is denominated the centripetal force, and the term is sufficiently precise for all common purposes; since, although, speaking with perfect strictness, the central point of no solid substance is the actual spot in which its attractive power is chiefly lodged, yet it has been abundantly proved by Sir Isaac, that all the matter of a spherical body, or a spherical surface, may, in generally estimating its attractive force on other matter, be considered as collected in the centre of such sphere. And hence, as all the celestial bodies are nearly spherical, their action on bodies at a distance 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 afterwards 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 INERTIÆ, 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 forward to the centre, communicated by the attractive influence of the aggregate mass of matter which the