other line. If instead of increasing the density in one direction we increase it in all directions, we thereby augment the general magnetic power of the body.* Anything therefore which tends to increase density increases magnetic power; and whatever diminishes density diminishes magnetic power also. Knowing this, the conclusion is inevitable, that the local action of the sun upon the earth's crust must influence, in some degree, the resultant effect. The action here meant is wholly different from the generation of thermo-electric currents which affect the needle. The simple mechanical change of density is what is meant. It is a true cause, and no complete theory can omit taking it into account. The Lecturer then proceeded to remark on the influence of geologic changes upon the earth as a magnet, and concluded as follows: "This evening's discourse is, in some measure, connected with this locality; and thinking thus, I am led to enquire wherein the true value of a scientific discovery consists. Not in its immediate results alone, but in the prospect which it opens to intellectual activity, in the hopes which it excites, in the vigour which it awakens. The discovery which led to the results brought before you to night was of this character. That inagnet† was the physical birthplace of these results; and if they possess any value they are to be regarded as the returning crumbs of that bread which in 1846 was cast so liberally upon the waters. I rejoice. in the opportunity here afforded me of offering my tribute to the greatest worker of the age, and of laying some of the blossoms of that prolific tree which he planted, at the feet of the discoverer of diamagnetism.' * Some time subsequent to this discourse, I tried in vain to augment sensibly the density of bismuth by pressure.-J. T., 1870. The instrument with which Faraday first produced the rotation of the plane of polarisation lay on the table. S XI.-ON DIAMAGNETIC REPULSION.* It was stated at the commencement of the discourse that bodies are repelled by the poles of a magnet, in virtue of a state of excitement into which they are thrown by the latter. The repulsion of bismuth, and the attraction of soft iron, followed precisely the same laws when the strength of the influencing magnet was augmented, the respective forces being proportional, not simply to the strength, but, within wide limits, to the square of the strength of the magnet. The result is explained in the case of iron by the fact of its being converted, while under magnetic influence, into a true temporary magnet, whose power varies with that of the influencing one; and in the case of bismuth, the result can only be explained by the fact that the diamagnetic mass is converted into a true diamagnet. It was next shown that the condition of excitement evoked by a magnetic pole was not the same as that evoked by another pole of an opposite quality. If the repulsion were independent of the quality of the pole, then two poles of unlike names ought to repel the bismuth, when brought to act upon it simultaneously. This is not the case. Two poles of the same name produce repulsion; but when they are of equal powers and opposite names, the condition excited by one of them, as shown by Reich, is neutralised by the other, and no repulsion follows. Bars of magnetic and diamagnetic bodies were next submitted to all the forces capable of acting upon them magnetically; first, to the magnet alone; secondly, to the electric current alone; and thirdly, to the magnet and current combined. Attention to structure was here found very necessary, and the neglect of it appears to have introduced much error into this portion of science. Powdered bismuth, without the admixture of any foreign ingredient, was placed in a strong metallic mould, * Proceedings of the Royal Institution, vol. ii. p. 13. and submitted to the action of a hydraulic press; perfectly compact metallic masses were thus procured, which, suspended in the magnetic field with the line of compression horizontal, behaved exactly like magnetic bodies, setting their longest dimensions from pole to pole. This identity of deportment with an ordinary magnetic substance was also exhibited in the case of the current singly, and of the current and the magnet combined. In like manner, by the compression of a magnetic powder, magnetic bars were produced, which, between the two poles of a magnet, set exactly like ordinary diamagnetic ones; this identity of deportment is preserved when the bars are submitted to the action of the current singly, and of the current and magnet combined. Calling those bars which show the ordinary magnetic and diamagnetic action normal bars, and calling the compressed bars abnormal ones, the law follows, that an abnormal bar of one class of bodies exhibits precisely the same deportment, in all cases, as the normal bar of the other class; but when we compare normal bars of both classes together, or abnormal bars of both classes, then the antithesis of action is perfect. The experiments prove that, if that which Gauss calls the ideal distribution of magnetism in magnetic bars be inverted, we have a distribution which will produce all the phenomena of diamagnetic ones. Fig. 1. S h N The important question of diamagnetic polarity was submitted to further and stricter examination. A flat helix, whose length was an inch, internal diameter an inch, and external diameter seven inches, was attached firmly to a table with its coils vertical. A mode of suspension was arranged in which a bar of bismuth, five inches long, and 0.4 of an inch in diameter, was permitted to swing freely, while surrounded by the helix. With this arrangement, the following experiments were made:-1. A voltaic current from twenty of Grove's cells was sent through the helix h, the direction of the current in the upper half of the helix being that denoted by the arrow (fig. 1). The north pole of a magnet being placed at N, the end a of the suspended bar, a b, of bismuth was attracted towards the pole N. 2. The south pole of a second magnet being placed at S, and the current being sent through the helix in the same direction as before, the bar left its central position and approached N with greater force than in the former experiment. The reason was manifest: the state of excitement which causes a to be attracted by N causes it to be repelled by S; both poles, therefore, act in unison, and a deflection of greater energy is produced. 3. The pole S being removed to the position S', the deflection was also found to be about twice as forcible as when the single pole N was employed. Here also the reason is plain: the two ends, a and b, of the bismuth bar, are in different states of excitement; the end a is attracted by a north pole, the end b is attracted by a south pole: both poles act therefore as a mechanical couple upon the bar, and produce the deflection observed. 4. The pole S' was replaced by a north pole of the same strength, thus bringing two poles of the same name to bear upon the two ends of the bar: there was no deflection by this arrangement. Here N's attraction for the end a was nullified by the repulsion of the end b by a like pole; the experiment thus furnishes an additional proof of the polar condition of a b. 5. We have supposed the pole S to be removed into the position S'; but permitting the pole S to remain, and introducing another pole (a south one) at S', a greater action than that produced with two magnets was obtained. 6. Finally, adding another north pole at N', and allowing four magnets to operate upon the bismuth bar simultaneously, a maximum action was obtained, and the bar was attracted and repelled with the greatest promptness and decision. In all these cases where an iron bar was substituted for the bismuth bar a b, a deflection precisely the opposite to that exhibited by a b was produced. A branch of the current by which the bar of bismuth was surrounded could be suffered to circulate round a bar of iron, suspended freely in an adjacent helix; when the forces acting upon the iron were the same as those acting upon the bismuth, the bars were always deflected in opposite directions. The question of diamagnetic polarity was next submitted to a test which brought it under the dominion of the principles of mechanics. A movable pole was chosen, of such a shape that the diminution of the force, as the distance was augmented, Fig. 2. was very slow; the field of force being therefore very uniform. Let the space in front of the pole P (fig. 2) be such a field. A normal bar of bismuth, a b, was attached to the end of a lever transverse to the length of the latter, and counterpoised by a weight at the other extremity: the system was then suspended from its centre of gravity g, Р so that the beam and bar swung horizontally. Supposing the bar to occupy the position shown in the figure, then if the force acting upon it be purely repulsive-that is to say, if the diamagnetic force be unpolar-it is evident that the tendency of the force acting upon every particle of the mass of bismuth tends to turn the lever round its axis of suspension, in the direction of the curved arrow. On exciting the magnetism of P, however, a precisely contrary motion is observed-the lever approaches the pole. This result, which, as far as the lecturer could see, was perfectly inexplicable on the assumption that the diamagnetic force was purely repulsive, is explained in a simple and beautiful manner on the hypothesis of diamagnetic polarity. According to this, the end b of the bar of bismuth is repelled by P, and the end a is attracted: but the force acting upon a is applied at a greater distance from the axis of suspension than that acting upon b; and as it has been arranged that the absolute intensities of the forces acting upon the two ends differ very slightly from each other, the mechanical advantage possessed by a gives it the greatest moment of rotation, and the bar is attracted instead of repelled. Let a magnetic needle n s (fig. 3) be attached like the bar a b (fig. 3) to a lever, and submitted to the earth's magnetism. the north pole of the earth be towards N; the action of the pole upon n is attractive, upon s repulsive, the absolute intensities of these forces are the same, inasmuch as the length of the needle is a vanishing quantity in comparison with its distance Let S Fig. 3. from the pole N: hence the mechanical advantage possessed by |