1852.] OF THE ROYAL INSTITUTION. 217 enters in any way into the exercise of this power, whatever the distance between the acting bodies, as that from the sun to the earth, or from star to star. We can hardly conceive of this force in one particle by itself ; it is when two or more are present that we comprehend it : yet in gaining this idea we perceive no difference in the character of the power in the different particles ; all of the same kind are equal, mutual, and alike. In the case of gravitation, no effect which sustains the idea of an independent or physical line of force is presented to us ; and as far as we at present know, the line of gravitation is merely an ideal line representing the direction in which the power is exerted. Take the Sun in relation to another force which it exerts upon the earth, namely, its illuminating or warming power. In this case rays (which are lines of force) pass across the intermediate space ; but then we may affect these lines by different media applied to them in their course. We may alter their direction either by re- flection or refraction ; we may make them pursue curved or angular courses. We may cut them off at their origin and then search for and find them before they have attained their object. They have a relation to time, and occupy 8 minutes in coming from the sun to the earth : so that they may exist independently either of their source or their final home, and have in fact a clear distinct physical existence. Thev are in extreme contrast with the lines of gravitating power in this respect ; as they are also in respect of their condition at their terminations. The two bodies terminating a line of gravitating force are alike in their actions in every respect, and so the line joining them has like relations in both directions. The two bodies at the terminals of a ray are utterly unlike in action ; one is a source, the other a destroyer of the line ; and the line itself has the relation of a stream flowing in one direction. In these two cases of gravity and radiation, the difference between an abstract and a physical line of force is immediately manifest. Turning to the case of Static Electricity we find here attractions (and other actions) at a distance as in the former cases ; but when we come to compare the attraction with that of gravity, very striking distinctions are presented which immediately affect the question of a physical line of force. In the first place, when we examine the bodies bounding or terminating the lines of attraction, we find them as before, mutually and equally concerned in the action ; but they are not alike : on the contrary, though each is endued with a force which speaking generally is of the like nature, still they are in such contrast that their actions on a third body in a state like either of them are precisely the reverse of each other, — what the one attracts the other repels ; and the force makes itself evident as one of those manifestations of power endued with a dual and antithetical condi- tion. Now with all such dual powers, attraction cannot occur unless the two conditions of force are present and in face of each other through the lines of force. Another essential limitation is that 218 NOTICES OF THE MEETINGS [June 1 ] , these two conditions must he exactly equal in amount, not merely to produce the effects of attraction, but in every other case ; for it is im- possible so to arrange things that there shall be present or be evolved more electric power of the one kind than of the other. Another limitation is that they must be in physical relation to each other ; and that when a positive and a negative electrified surface are thus associated, we cannot cut off this relation except by transferring the forces of these surfaces to equal amounts of the contrary forces provided elsewhere. Another limitation is that the power is defi- nite in amount. If a ball a be charged with 10 of positive electricity it may be made to act with that amount of power on another ball b charged with 10 of negative electricity; but if 5 of its power he taken up by a third ball c charged with negative electricity, then it can only act with 5 of power on hall a, and that ball must find or evolve 5 of positive power elsewhere : this is quite unlike what occurs with gravity, a power that presents us with nothing dual in its character. Finally the electric force acts in curved lines. If a hall be electrified positively and insulated in the air, and a round metal- lic plate be placed about 12 or 15 inches off, facing it and uninsulated, the latter will be found, by the necessity mentioned above, in a negative condition ; but it is not negative only on the side facing the ball but on the other or outer face also, as may be shewn by a carrier applied there, or by a strip of gold or silver leaf hung against that outer face. Now the power affecting this face does not pass through the uninsulated plate, for the thinnest gold leaf is able to stop the inductive action, but round the edges of the face and therefore acts in curved lines. All these points indicate the ex- istence of physical lines of electric force : — the absolutely essential relation of positive and negative surfaces to each other, and their dependence on each other contrasted with the known mobility of the forces, admit of no other conclusion. The action also in curved lines must depend upon a physical line of force. And there is a third important character of the force leading to the same result, namely, its affection by media having different specific inductive capacities. When we pass to Dynamic Electricity the evidence of physical lines of force is far more patent. A voltaic battery, having its ex- tremities connected by a conducting medium, has what has been expressively called a current of force running round the circuit, but this current is an axis of power having equal and contrary forces in opposite directions. It consists of lines of force which are com- pressed or expanded according to the transverse action of the con- ductor, which changes in direction with the form of the conductor, which are found in every part of the conductor, and can be taken out from any place by channels properly appointed for the purpose ; and nobody doubts that they are physical lines of force. Finally as regards a Magnet, which is the object of the present discourse. A magnet presents a system of forces perfect in itself, 1852.] OF THE ROYAL INSTITUTION. 219 and able, therefore, to exist by its own mutual relations. It has the dual and antithetic character belonging to both static and dynamic electricity ; and this is made manifest by what are called its polarities, l i. e. by the opposite powers of like kind found at and towards its extremities. These powers are found to be absolutely equal to each other ; one cannot be changed in any degree as to amount without an equal change of the other ; and this is true when the opposite i polarities of a magnet are not related to each other, but to the polarities of other magnets. The polarities, or the northness and i southness, of a magnet are not only related to each other, through i or within the magnet itself, but they are also related externally to opposite polarities, (in the manner of static electric induction) or they cannot exist ; and this external relation involves and necessi- tates an exactly equal amount of the new opposite polarities to which those of the magnet are related. So that if the force of a magnet a is related to that of another magnet b, it cannot act on a 1 third magnet c without being taken off from b, to an amount pro- portional to its action on c. The lines of magnetic force are shewn by the moving wire to exist both within and outside of the magnet ; also they are shewn to be closed curves passing in one part of their course through the magnet ; and the amount of those within the magnet at its equator is exactly equal in force to the amount in any section including the whole of those on the outside. The lines of ; force outside a magnet can be affected in their direction by the use i of various media placed in their course. A magnet can in no way be procured having only one magnetism, or even the smallest excess of northness or southness one over the other. When the pola- rities of a magnet are not related externally to the forces of other magnets, then they are related to each other : i. e. the northness and southness of an isolated magnet are externally dependent on and sustained by each other. Now all these facts, and many more, point to the existence of physical lines of force external to the magnets as well as within. They exist in curved as well as in straight lines ; for if we conceive of an isolated straight bar magnet, or more especially of a round disc of steel magnetised regularly, so that its magnetic axis shall be in one diameter, it is evident that the polarities must be related to each other externally by curved lines of force ; for no straight line can at the same time touch two points having northness and south ness. Curved lines of force can, as I think, only consist with physical lines of force. The phenomena exhibited by the moving wire confirm the same conclusion. As the wire moves across the lines of force, a current of electricity passes or tends to pass through it, there being no such current before the wire is moved. The wire when quiescent has no such current, and when it moves it need not pass into places where the magnetic force is greater or less. It may travel in such a course that if a magnetic needle were carried through the same course it would 220 NOTICES OF THE MEETINGS {June 1 1 , be entirely unaffected magnetically, i.