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These alone, he affirmed, deserve the name of PLEASURE, and can alone raise the mind above the grovelling and misnamed pleasures of self-indulgence, debauchery, and excess.
There is something gratifying to an enlarged and liberal spirit in being thus able to rescue from popular, but unfounded obloquy, a sage of transcendent genius and almost unrivalled intellect, and in restoring him to the 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 age.
ON THE PROPERTIES OF MATTER, ESSENTIAL AND
IN our last lecture I endeavoured to render it bable, that all visible or sensible matter is the result of a combination of various solid, impenetrable, and exquisitely fine particles or units of the same substance, too 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.
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 or separation from each other, is called MOTION; and the wide expanse in which motion of 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 cohesibility.
PASSIVITY, inertia, or vis inertiæ, 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 well-known fact in projectiles, that the action of the powder on a bullet ceases when the bullet has quitted the piece. In like manner, a billiard-ball at rest will continue so till put in motion by the rod, or by another billiard-ball into 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 force or momentum 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 ap
proached, or would have its path inflected into a circle, or an ellipse, 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 from the top of a hill of 1000 feet high, with the velocity of 8440 feet, or 13 miles, per second, provided it experienced no resistance from the air, would become a satellite to the earth, and revolve round it perpetually in a period of 2 hours, 23 minutes, and 22 seconds. And a body projected from the earth, in any direction, with a velocity of 7 miles per second, would never return. 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 volcanoes 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 volcanoes, 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. In that case the propelled mass would pass the point of equal attraction of the moon and the earth, and would, in consequence, be drawn by that of the latter, and would either become a satellite to the earth, or be precipitated to its surface, according as the recti
*Laplace, Exposition du Système du Monde.