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 elective 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 aré 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 heterogeneous 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