the changes of corporeal things are to be placed only in the various separations, and new associations and motions of these permanent particles: compound bodies being apt to break, not in the midst of solid particles, but where those particles are laid together, and touch only in a few points."

The Epicurean doctrine, moreover, of a flux and reflux of elementary particles exterior to every material system, perpetually feeding and replenishing it, and carrying off its dissolved and rejected rudiments, bears no small resemblance to the ethereal medium of Sir Isaac Newton; and, in its law of action, has been singularly revived within the course of the last six years by Professor Leslie, in his principles of impulsion, as detailed in his "Inquiry into the Nature of Heat." It is a doctrine, also, peculiarly coincident with Dr. Herschel's recent theory of nebulæ, or milky ways in the heavens, which, contrary to his own earlier opinions, and those of former astronomers, who ascribed such appearance to the mixed light thrown forth from clusters of stars too remote to be reached by the best telescopes, he now resolves, as we shall have occasion to show more minutely in due time, into masses of a luminous fluid, existing independently of all stars or planets, though originally, perhaps, emitted from them; aggregated by a variety of causes that tend to give its minute particles unity; sometimes forming new stars by its condensation, and often feeding and regenerating those that are exhausted.

Such is a brief survey of the chief theories of the primitive or elementary substance of matter which have been offered in ancient or modern times; from a combination of the different particles of which, in different modes and proportions, and under the operation of different laws, all sensible bodies are supposed to have proceeded.

Of sensible bodies thus produced, some, however, in direct repugnancy to the Atomic philosophy, whether of ancient or more recent times, have been very generally conceived to have been formed first; to be peculiarly simple in their composition, indecomposable by any known powers in their structure, and to be the basis of all other bodies, or those from which all other bodies proceed, by different unions and modifications: and hence such substances have been denominated constituent principles, or constituent elements; concerning the kind and number of which, however, we have had almost as many opinions offered as concerning the origin and nature of the primitive principles themselves.

Thus, among both the ancients and the moderns, sometimes fire, sometimes air, sometimes earth, and sometimes water, has been considered as the sole constituent element or source of things. Sometimes two of these substances have been thus denominated, and sometimes three; but more generally the whole. Occasionally, indeed, a fifth and even a sixth have been added to the number, as cold and oil, each of these having at times been considered as simple and indecomposable substances: while, under the old Atomic system, and especially as improved by Epicurus, all such principles were completely swept away, and no one sensible substance whatever was conceived to be better entitled to the character of a constituent principle than another; the whole equally flowing from peculiar modifications and combinations of the primitive or elementary principles-the RERUM PRIMORDIA-and equally resolving into them upon decomposition.

Of these different theories, the greater number are scarcely worth examining; and I shall only therefore observe, that for that which supposes the existence of four distinct elements, fire, air, earth, and water, and which for ages has been in almost universal acceptation, and would have been so still but for the recent discoveries of chemistry, we are indebted to Empedocles. This celebrated philosopher, and very excellent poet, flourished about four centuries before the Christain era. His opinions, like those of almost all the earliest sages, were given in metre, in a didactic poem, " ON NATURE," of which only a few fragments have descended to our own times. He was a native of Sicily, and his talents and his country are celebrated by Lucretius, who was, nevertheless, of a very different school of philosophy, in verses so

elegant and so descriptive, that I cannot refrain from presenting you with a literal but very humble translation of them; introduced, more especially, as they are, with observations upon different rival philosophers, who employed one, two, and various other numbers of the commonly esteemed elements, and in various combinations, as the basis of their respective theories.

Nor wanders less the sage who AIR with FIRE

Would fain commix, or limpid STREAM with EARTH;
Or those the whole who join, FIRE, ETHER, EARTH,
And pregnant SHOWERS, and thence the world deduce.
Thus sung EMPEDOCLES, in honest fame
First of his sect; whom AGRIGENTUM bore
In cloud-capp'd SICILY. Its sinuous shores
Th' IONIAN main, with hoarse unwearied wave,
Surrounds, and sprinkles with its briny dew;
And, from the fair ÆOLIAN fields, divides
With narrow frith that spurus th' impetuous surge.
Here vast CHARYBDIS raves; here ArsA rears
His infant thunders, his dread jaws unlocks,
And heaven and earth with fiery ruin threats.
Here many a wonder, many a scene sublime,
As on he journeys, checks the traveller's steps;
And shows, at once, a land in harvests rich,
And rich in sages of illustrious fame.
But naught so wond'rous, so illustrious naught,

So fair, so pure, so lovely can it boast,

EMPEDOCLES, as thou! whose song divine,

By all rebears'd, so clears each mystic lore,

That scarce mankind believ'd thee born of man.

Yet e'en EPEDOCLES, and those above

Already sung, of far inferior fame,

Though doctrines frequent from their bosoms flow'd

Like inspiration, sager and more true

Than e'er the PYTHIAN maid, with laurels crown'd,

Spoke from the tripod at APOLLO's shrine;

E'en those mistook the principles of things,

And greatly wander'd in attempt so great.

Let our controvertists of the present day learn a lesson of liberality from this correct and polished reasoner, whose own theory is well known to have been that of Epicurus, to which I have just adverted, namely, that one substance is just as much entitled to the character of a constituent element as another, and that every thing equally proceeds from, and in turn is resolved into, the primitive and invisible atoms or principles of matter.

It is to this theory alone that all the experiments of modern chemistry are giving countenance. Air, water, and earth, suspected to be compounds in the time of Epicurus, have been proved to be such in our own day; while of the actual nature of heat or fire, mankind are just as uninformed now as they were then.

In the process, however, of destroying these supposed elements, chemistry has occasionally seemed to detect others; and hence, instead of air, fire, earth, and water, as simple or indecomposable substances, we have had phlogiston, acids, and alkalies; sulphur and phosphorus; oxygen, hydrogen, nitrogen, and carbon, progressively arising before us, and laying claim to an imperishable existence. All of them, however, have fallen, or are falling in their turn, without having lived long enough to reach the common age of man; all of them have been proved, or reasonably suspected, to be compounds of other substances, that may yet, perhaps, be detected to be compounds of something beyond. Even oxygen, the most brilliant of the whole, the boasted discovery of Lavoisier, and out of which he was supposed to have built to his own memory “a monument more durable than brass," has had its throne shaken to its foundation by Sir Humphry Davy, and is at this moment, like the Roman empire in its decline, obliged to divide its sway with a new and popular power, which this last celebrated chemist has denominated ch orine; while of the more subtle and active agents, light, caloric, the magnetic and electric fluids, we know nothing but from their effects, and can only say of each-stat nominis umbra.

Is physical science, then, a vain show?-a mere house of cards, built up for the sole purpose of being pulled down again?-Assuredly not. The firm

footing we have actually obtained upon many essential points-a footing not to be disturbed by any future change of system, or novelty of discoveryand the ascertainment of a multitude of recondite facts, and their application to some of our most extensive and valuable arts, sufficiently prove that philosophy has neither lived nor laboured in vain. Although we have not been able to break through the spell completely-to follow up the Proteus-form of matter into its deepest recesses, and fix it in its last shape and characterwe have succeeded in developing many of its most important laws, as it will be the object of the ensuing lecture to point out, and to apply them to a solution of many of its most important phenomena. Whatever is sure and trusty has remained to us, and whatever has given way has been mere chimera and shadow: we have chiefly, perhaps only, failed where we have either been too curious, or have suffered imagination to become our charioteer in the slow and sober journey of analysis.

Before we quit this subject, let us, in the candid spirit of genuine philosophy, do the same justice to Epicurus as we attempted in our last lecture to Pythagoras and Plato. It has been very generally said and very generally believed, principally because it has been very generally said, that the great and mighty cause of this beautiful and harmonious formation of worlds, and systems of worlds, in the opinion of Epicurus, was mere CHANCE, OF FORTUNE. There is nothing, however, in those fragments of his works which have descended to us, that can in any way countenance so opprobrious an opinion, but various passages that distinctly controvert it,―passages in which he peremptorily denies the existence of CHANCE OF FORTUNE, either as a deity or a cause of action; and unequivocally refers the whole of those complex series of percussions and repercussions, interchanges and combinations, exhibited by the elementary seeds or atoms of matter during the creative process, to a chain of immutable laws which they received from the Almighty Architect at the beginning, and which they still punctually obey, and will for ever obey, till the universe shall at length cease to exist.* "Whom," says Epicurus, in a letter to his disciple Menæceus, that has yet survived the preying tooth of time, and will be found in Diogenes Laertius," do you believe to be more excellent than he who piously reveres the gods, who feels no dread of death, and rightly estimates the design of nature? Such a man does not, with the multitude, regard CHANCE as a god, for he knows that God can never act at random; nor as A CONTINGENT CAUSE OF EVENTS; nor does he conceive, that from any such power flows the good or the evil that measures the real happiness of human life." He held, however, that the laws which govern the universe were altogether arranged and inposed upon it by the Creator at its first formation, and that the successive train of events to which they have given rise, have followed as the necessary result of such an arrangement, and not as the immediate superintendence of a perpetually controlling Providence. For it was the opinion of Epicurus, as well as of Aristotle, that perfect rest and tranquillity are essential to the perfect happiness even of Him, who, to adopt his own language in another place, possesses all immortality and beatitude. "Think not," says he, "that the different motions and revolutions of the heavens, the rising, setting, eclipses, and other phenomena of the planets, are produced by the immediate control, superintendence, or ministration of Him who possesses all immortality and beatitude; it is from the immutable laws which they received at the beginning, in the creation of the universe, that they punctually fulfil their several circuits."

The origin of this calumny upon the character of Epicurus it is by no means difficult to trace, and it has been sufficiently traced, and sufficiently exposed, by Diogenes Laertius, Gassendi, Du Rondelle, and other distinguished writers, who have done ample justice to his memory; and upon the confessions of Plutarch, Cicero, and Seneca, abundantly proved, that it was the same rancorous spirit of envy among many of his competitors for public fame, and especially among the Stoic philosophers, which strove to fix upon

For a more extensive inquiry into this subject, the reader is referred to the author's Prolegomena to his translation of "The Nature of Things," from which this summary is drawn D

him the charge of voluptuous living, though the most temperate and abstemious Athenian of his day; that thus, with yet keener malevolence, endeavoured to brand him with the still fouler reproach of the grossest impiety and atheism. It is, indeed, scarcely to be believed, if the fact were not concurrently attested by all the writers of antiquity, that the philosopher whose name, from the low and malignant spirit I have just adverted to, has been proverbialised for general licentiousness and excess, drew the whole of his daily diet from the plainest pottage, intermixed with the herbs and fruits of his pleasant and celebrated garden. "I am perfectly contented," says he, in an epistle to another friend," with bread and water alone; but send me a piece of your Cyprian cheese, that I may indulge myself whenever I feel disposed for a luxurious treat." Such, too, was the diet of his disciples. Water, says Diocles, was their common beverage; and of wine they never allowed themselves more than a very small cup. And hence, when the city of Athens was besieged by Demetrius, and its inhabitants reduced to the utmost extremity, the scholars of Epicurus bore up under the calamity with less inconvenience than any other class of citizens; the philosopher supporting them at his own expense, and sharing with them daily a small ration of his beans. The pleasure of friendship, the pleasure of virtue, the pleasure of tranquillity, the pleasure of science, the pleasure of gardening, the pleasure of studying the works of nature, and of admiring her in all the picturesque beauty of her evolutions, formed the sole pursuit of his life. This alone, he affirmed, deserves 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 transcendant 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 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, 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 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 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 synonymes, 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 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 volcanoes than any which are known to exist on the earth. And hence it requires no great stretch of imagination to conceive that bodies