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, to the phenomena of 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, than any 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.)

WE 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 one 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

solid bodies, though the proofs are not so common; since, as I have just observed, the particles of solid bodies have less power of movement, and, consequently, of adaptation to each other, than those of liquids. Thus, two plates of lead, whose opposite. surfaces correspond so exactly that every particle of each surface shall have a bearing upon the particle opposed to it, when once united by pressure, assisted by a little friction, cohere so powerfully as to require a very considerable force to separate them. And it may be shown, either by measuring this force, or by suspending the lead in the vacuum of an air-pump, that the pressure of the atmosphere is not materially concerned in producing this effect. A cohesion of this kind is sometimes of practical utility in the arts; little ornaments of laminated silver remaining attached to iron or steel, with which they have been made to connect themselves by the powerful pressure of a blow, so as to form one mass with it. And it is now a well-known fact, and of a most curious nature, that one of the causes by which eight-day clocks go at times irregularly, and monthly clocks, whose weights are much larger and heavier, often amounting to not less than thirty pounds, stop suddenly, proceeds from the attraction which takes place between their leaden weights and the leaden ball of the pendulum, when the weights have descended just so low as to be on a level, and, consequently, very nearly in a state of contact, with the pendulum-ball. And hence the reason why both these kinds of clocks, if the pendulum have not actually stopped, seem gradually, a few days afterward, to recover their former accuracy; the attraction diminishing as the distance once more increases, In like manner, Studor remarks that beams of steel become sometimes erroneous by acquiring magnetic polarity.†

It is by the same means that the greater number of rocks seem to be produced that enter into the substance of the earth's solid crust. The lowermost of these, as I shall have occasion to observe in an ensuing lecture, are united by an intimate crystallization, which is the most perfect form of aggregate or homogeneous attraction that can exist between solid bodies, and which must have commenced while such bodies were in a fluid state. Some of the upper kinds or families are united by a particular cement, which is nothing more than a substance possessing a peculiar attraction, or, if I may be allowed the expression, physical partiality to the rudimental corpuscles of which the rock consists; and others by nothing more than the law of aggregation or homogeneous attraction in its simplest state; whence earths unite to earths in consequence of mutual approximation, assisted by their own or a superincumbent pressure, in the same manner as I have just stated that plates of lead or other metals unite to metals.

II. But there are substances that are UNLIKE IN THEIR NATURE, as solids and fluids, for instance, that under particular circumstances are often found to exhibit a mutual attraction; whence this mode of union is called HETEROGENEOUS ATTRACTION, and from its occurring most palpably between liquids and solid substances possessing small capillary or hair-tubes, CAPILLARY

TRACTION.

AT

The cause of this attraction is obvious; and it is still more clearly a mere modification of the general attraction of gravitation, than the preceding power of homogeneous attraction. It is the common attractive property of material substance for material substance; the liquid, or that whose particles are easily separable, pressing toward the solid, whose parts are by any action of their own altogether inseparable. Hence the reason why water or any other liquid hangs about the sides of a wine-glass: hence, partly, the reason why a wineglass, when somewhat more than brim-full of a liquid, does not overflow; the co-operative reason being, as I have already stated, the homogeneous attraction of the corpuscles of the fluid for each other, which prevents them from separating readily and hence also the reason why a liquid contained in a narrownecked and inverted phial does not obey the common attraction of gravitation, and fall to the earth, although the stopper be removed to allow it, till we † Gilb. xiii. 124. Young's Nat. Phil. ii. 159.

Reid, in Nicholson's Journal, vol. xxxiii. p. 92.

aid the power of gravitation, or rather loosen the power of the peculiar attraction, by shaking the phial.

In this last case it is manifest that the heterogeneous attraction, or that between the two different substances, is stronger than the common force of gravity. In minute capillary tubes or pores this is still more obvious. Such are the pores of a piece of sponge, when pressed or softened, so as to become more pliable to the action of water or of any other liquid within its reach. For, in this case, the water being minutely divided by the pores of the sponge into very small portions, and still surrounded by the pores in every direction after such division, has its common force of gravitation and its peculiar force of homogeneous attraction equally overpowered; and ascends from the surface of the earth, instead of descending to it, or uniting into a spherical form; and the same kind of pores, and, consequently, the same kind of power, being continued to the utmost height of the sponge, it will rise to the full extent of its column. The tubes of various imperfect crystals, as those of sugar, for example, are still smaller; and hence the lateral attraction must be still stronger; and any liquid within its reach will rise both higher and more freely, till the sugar at length becomes dissolved, and, consequently, its pores are totally destroyed. The cause of capillary attraction is therefore obvious: and the reasoning and phenomena now submitted may be applied to an explanation of every other species of the same kind that may occur to us.

III. The third particular attraction I have noticed, is that of PECULIAR BODIES FOR PECULIAR BODIES, and which has hence been denominated ELECTIVE OF CHEMICAL ATTRACTION; as the tendencies they have to each other have been denominated AFFINITIES. Thus lime has a strong affinity for carbonic acid, and greedily attracts it from the atmosphere, which hence becomes purified by being deprived of it. But the same substance has a still stronger affinity for sulphuric acid, and hence parts with its carbonic acid, which flies off in the form of gas, in order to unite with the sulphuric whenever it has a possibility of doing so. It is highly probable that this kind of attraction is also nothing more than a peculiar modification of that of gravitation, more select in its range, but more active in its power. To trace out the various substances that are possessed of this peculiar property, and to measure the degrees of their affinities, is one of the chief branches of chemistry, but of too voluminous a nature to touch farther upon at present.

:

IV. V. The two remaining kinds of attraction to which I have adverted, those of ELECTRICITY and of MAGNETISM, are still more select, and perhaps still more powerful than even the preceding but the phenomena to which they give rise cannot, I think, be attributed to any modification of a gravitating ethereal medium. We call the medium in both these cases a fluid, but we know little or nothing of the laws by which they are regulated; whether they be different substances, or, according to M. Ampore, the same substance under different modifications, or whether, in reality, they be material substances at all. They are certainly deficient in the most obvious properties of common matter, and may be another substrate of being united to it.

There are also two other substances, or which are generally conceived to be substances, in nature, of a very attenuate texture, which largely contribute to the changes of material bodies. I mean LIGHT and HEAT, of the general nature of which we are still also in a considerable degree of ignorance. Like the powers of magnetism and electricity, we only know them, and can only reason concerning them, by their effects. These effects, indeed, are of a most curious and interesting character, but spread too widely to be followed up in the course of the present lecture, though we may endeavour to pursue them, and, as far as we are able, to develope them, hereafter.

All these four powers or essences, for we know not which to call them, concur in exhibiting none of the common properties of matter; their respective particles repel each other at least as powerfully as they attract, and in the cases of light and heat repel alone, and without attracting. They may, possibly, be ponderable; but if so, we have no instruments fine enough to detect their

relative weights; and we are hence incapable of determining, as I took leave to observe on a former occasion, whether they be matter at all, whether mere properties of matter, or whether modifications of some etherealised and incorporeal substrate, combining itself with the material mass, and exciting many of its most extraordinary phenomena. It is at present, however, very much the habit to generalise them into one common origin; and to conceive the whole as modified results of matter, or of the gravitating property of matter. Thus, the attractive powers of chemical affinity and of electricity are identified in the following passage of Sir Humphry Davy's valuable "Elements of Chemical Philosophy :"-" Electrical effects are exhibited by the same bodies when acting as masses, which produce chemical phenomena when acting by their particles; it is not improbable, therefore, that the primary cause of both may be the same."* And in like manner, in an adjoining passage, he suggests that all the various properties or essences that have thus far passed in survey before us, may be nothing more than the general attractive power of matter, though he admits that at present we are incompetent to determine upon the subject. "With regard to the great speculative questions, whether the electrical phenomena depend upon one fluid in excess in the bodies positively electrified, and in deficiency in the bodies negatively electrified, or upon two different fluids capable by their combination of producing heat and light, or whether they may be particular exertions of the general attractive power of matter, it is, perhaps, impossible to decide, in the present imperfect state of our knowledge."t

And hence, heat, in the view of Sir Humphry Davy, Count Rumford, and various other justly celebrated chemists and philosophers of the present day, coincidently with the doctrine of the Peripatetic school, is a mere property of matter, and not a substance sui generis, as was contended for by the Epicureans, in opposition to the disciples of Aristotle, and is contended for by the disciples of Boerhaave, Black, Crawford, and most of the chemists of our own times. The cause of heat, among those who deny it a substantive existence, consists in a vibrating motion of the constituent particles of the heated body, too rapid to be traced by the eye. And as it is known to every one that bodies in general, as they become heated, occupy a larger space, and have their particles more widely repelled and separated from each other than in a colder temperature, it has of late become a favourite doctrine that the repulsive power, which in our last lecture we noticed to exist throughout matter, depends altogether upon the property of heat; in consequence of which Sir Humphry Davy uses heat and calorific repulsion as synonymous terms, and hence regards heat and gravitation, or general attraction, as antagonist

powers.

There is much plausible reasoning to be urged in favour of this hypothesis. It will as readily account for many, perhaps most, of the phenomena which accompany bodies in their change from one temperature to another, as the position of the substantive form of heat, and has some advantage in point of simplicity; but it is opposed by a variety of facts of so stubborn and intractable a nature, that no efforts of ingenuity have hitherto been capable of bending them into the service of the new doctrine. I observed, for instance, in our last lecture, that when two plates of glass are within a ten thousandth part of an inch of each other, they cannot be made to approach nearer without a strong additional pressure. I observed, farther, that Professor Robison has calculated the extent of this pressure from actual experiment, and finds it amount to not less than a thousand pounds weight for every square inch of the glass. Now this resistance or repulsive power between the two plates of glass takes place equally under an air-pump and in the fullest exposure to the air of the atmosphere, but it appears to cease under water. By what cause the repulsion is excited in the two former instances, or disappears in the latter, we know not; but it does not seem possible for any ingenuity of argument to connect this repulsive power with heat, whether regarded as a substance or a mere property.