Electricity and Magnetism afford us a very instructive example of the belief in secondary causation. Subsequent to the discovery by Oersted of Electro-Magnetism, and prior to that by Faraday of Magneto-Electricity, Electricity and Magnetism were believed by the highest authorities to stand in the relation of cause and effect, i.e., electricity was regarded as the cause, and magnetism as the effect, and where magnets existed without any apparent electrical currents to cause their magnetism, hypothetical currents have been supposed, for the purpose of carrying out the causative view; but magnetism may now be said with equal truth to be the cause of electricity, and electrical currents may be referred to hypothetical magnetic lines; again, if electricity cause magnetism, and magnetism cause electricity, why then electricity causes electricity, which is absurd.

To take another instance, which may render these positions more intelligible. By heating two bars of Bismuth and Antimony in contact, a current of electricity is produced, and if their extremities be united by a fine wire, the wire is heated. Now, here the electricity in the metals is said to be caused by heat, and the heat in the wire to be caused by electricity, and in a concrete sense this is true; but can we thence say abstractedly that heat is the cause of electricity, or that electricity is the cause of heat? Certainly not,for if either be true, both must be so, and the effect then becomes the cause of the cause, or, in other words, a thing causes itself. If you will put any other proposition on this subject, you will find it involve similar difficulties, until, at length, your minds will become convinced that abstract secondary causation does not exist, and that a search after essential causes is vain.

The position which I seek to establish in this Essay is, that the various imponderable agencies, or the affections

of matter which constitute the main objects of experimental physics, viz., Heat, Light, Electricity, Magnetism, Chemical Affinity, and Motion are all Correlative, or have a reciprocal dependence. That neither, taken abstractedly, can be said to be the essential or proximate cause of the others, but that either may, as a force, produce or be convertible into the other; thus heat may mediately or immediately produce electricity, electricity may produce heat; and so of the rest.

The term Force, although used in very different senses by different authors, in its limited sense may be defined as that which produces or resists Motion. Although strongly inclined to believe that the five other affections of matter, which I have above named, are, and will ultimately be resolved into, modes of motion, it would be going too far, at present, to assume their identity with it; I therefore use the term Force, in reference to them, as meaning that active principle inseparable from matter, which induces its various changes.

Let us begin with MOTION-the most obvious, the most distinctly conceived of all the affections of matter. It is a proposition universally received since the time of Newton, that a body in motion will continue so for ever, in the same direction, and with the same velocity, unless impeded by some other body, or affected by some other force than that which originally impelled it; but it is also very generally believed, that if the visible or palpable motion be arrested by impact on another body, the motion ceases, and the force which produced it is annihilated. On the other hand, the view which I venture to submit to you in these lectures is, that force cannot be annihilated, but is merely subdivided ⚫ or altered in direction or character.

First, as to direction. Wave your hand; the motion

1

which has apparently ceased is taken up by the air, from the air by the wall of the room, and so, by direct and reacting waves, continually comminuted, but never destroyed. It is true that, at a certain point, we lose all means of detecting the motion, from its minute subdivision, which defies our most delicate means of appreciation, but we can indefinitely extend our power of detecting it, accordingly as we confine its direction or increase the delicacy of our examination. Thus, if the hand be moved in unconfined air, the motion of the air would not be sensible to a person at a few feet distance, but if a piston of the same extent of surface as the hand be moved with the same rapidity in a tube, the blast of air may be distinctly felt at several yards' distance. There is no greater absolute amount of motion in the air in the second than in the first case, but its direction is restrained, so as to make its means of detection more facile; by carrying on this restraint, as in the air-gun, we get a power of detecting the motion, and of moving other bodies at far greater distances; while the same puff of air which would, in the air-gun, project a bullet a quarter of a mile; if allowed to escape without restraint, as steam does in the candle bomb, would not be perceptible at a yard distance, though the same absolute amount of motion is impressed on the surrounding air.

It may, however, be asked, what becomes of force when motion is arrested or impeded by the counter-motion of another body? This is generally believed to produce rest or entire destruction of motion, and consequent annihilation of force; so indeed it may as regards the motion of the masses, but a new force or new mode of force now ensues, the exponent of which, instead of visible motion, is Heat. I venture to regard the heat which results from friction or percussion, as a continuation of the force which was previously associated with

the moving body, and which when this impinges on another body, ceasing to exist as gross palpable motion, continues to exist as heat. Thus, let two bodies, A and B, be supposed moving in opposite directions (putting for the moment out of the question all resistance, such as that of the air, &c.), if they pass each other without contact, each will move on for ever in its respective direction with the same velocity, but if they touch each other, the velocity of the movement of each is reduced, and each becomes heated; if this contact be slight, or such as to occasion but a slight diminution of their velocity, as when the surfaces of the bodies are oiled, then the heat is slight; but if the contact be such as to occasion a great diminution of motion, as in percussion, or as when the surfaces are roughened, then the heat is great, so that in all cases, the resulting heat is in direct proportion to the diminished velocity. Where instead of resisting and consequently impeding the motion of the body A, the body B gives way, or itself takes up the motion originally communicated to A, then we have less heat in proportion to the motion of the body B, for here the operation of the force continues in the form of palpable motion; thus the heat resulting from friction in the axle of a wheel is lessened by surrounding it by rollers, these take up the primary motion of the axle, and the less, by this means, the initial motion is impeded, the less is the resulting heat. Again, if a body move in a fluid, the heat produced is very trifling, because the particles of the fluid themselves move, and continue the motion originally communicated to the moving body; for every portion of motion communicated to them this loses an equivalent, and where both lose, then an equivalent of heat results.

As the converse of this proposition it should follow, that the more rigid the bodies impinging on each other, the

greater should be the amount of heat developed by friction, and so we find it. Flint, steel, hard stones, glass, and metals, are those bodies which give the greatest amount of heat from friction or percussion, while water, oil, &c., give little or no heat, and from the ready mobility of their particles, lessen its developement when interposed between rigid moving bodies. Thus, if we oil the axles of wheels, we have more rapid motion of the bodies themselves, but less heat; if we increase the resistance to motion, as by roughening the points of contact, so that each particle strikes against and impedes the motion of others, then we have diminished motion, but increased heat. I cannot present to I cannot present to my mind any case of heat resulting from friction which is not explicable by this view; friction, according to it, is simply impeded motion, and the resulting heat is a continuation of indestructible force, capable, as we shall presently see, of reproducing palpable motion, or motion of definite masses.

Hitherto I have taken no distinction as to the physical character of the bodies impinging on each other, but Nature gives us a remarkable difference in the character or mode of the force eliminated by friction, accordingly as the bodies which impinge are homogeneous or heterogeneous; if the former, Heat alone is produced; if the latter, Electricity.

We find, indeed, instances given by authors, of electricity resulting from the friction of homogeneous bodies, but, as I stated in these Lectures, I have not found such facts confirmed by my own experiments, and I am happy to see this conclusion corroborated by some experiments of Professor Erman, communicated to the last meeting of the British Association, in which he finds no electricity to result from the friction of perfectly homogeneous substances, as, for instance, the ends of a broken bar. Such experiments as these will, indeed, be seldom free from slight electrical currents,