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that beams of steel become sometimes erroneous by acquiring magnetic polarity.*

It is by analogous 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 venture on such an 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 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 ATTRACTION.

The cause of this attraction is obvious; and it is

* Gilb. xiii. 124. Young's Nat. Phil. ii. 159.

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 towards 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 wine-glass, 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, in part, the reason why a liquid contained in a narrow-necked 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 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 phænomena 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 or 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 phænomena 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. Ampere, 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 observed 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 phænomena. 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 phænomena 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

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Elem. p. 164, 165.

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