the force acting upon s, on account of its greater distance from the axis of rotation, causes the lever to recede from N, and we obtain a result perfectly analogous to that obtained with the bar of bismuth (fig. 2).

The metallic moulds referred to in the foregoing abstract are sketched in the annexed three figures:

In Fig. 1, A', B', C' represent the three parts of the mould used for forming cubes of compressed bismuth, whether solid and coherent, or in powder. Fig. 3, A, B, C, represent the three parts of the mould employed to form rectangular bars. In Fig. 2, X, the three parts of Fig. 1 are put together. In Fig. 2, Y, the three parts of Fig. 3 are put together. In experimenting, B' or B is first placed upon its base, C' or C; the solid or the powder is then placed within B' or B, the plunger A' or A is then introduced, and the whole squeezed between the plates of a small hydraulic press. The compressed substance is of course firmly jammed in the mould, and to remove it a perforated base (not shown in the figure) is employed, on which B' or B is placed, and the squeezed metal forced out by the plunger A' or A, acted on by the hydraulic press. The drawings are half the linear size of the moulds themselves.

XII.-DISPOSITION OF FORCE IN PARAMAGNETIC AND DIAMAGNETIC BODIES.*

THE notion of an attractive force, drawing bodies towards the centre of the earth, was entertained by Anaxagoras and his pupils, by Democritus, Pythagoras, and Epicurus; and the conjectures of these ancients were renewed by Galileo, Huyghens, and others, who stated that bodies attract each other as a magnet attracts iron. Kepler applied the notion to bodies beyond the surface of the earth, and affirmed the extension of this force to the most distant stars. Thus it would appear, that in the attraction of iron by a magnet originated the conception of the force of gravitation. Nevertheless, if we look closely at the matter, it will be seen that the magnetic force possesses characters strikingly distinct from those of the force which holds the universe together. The theory of gravitation is, that every particle of matter attracts every other particle; in magnetism also we have the phenomenon of attraction, but we have also, at the same time, the fact of repulsion, and the final effect is always due to the difference of these two forces. A body may be intensely acted on by a magnet, and still no motion of translation will follow, if the repulsion be equal to the attraction. A dipping needle was exhibited: previous to magnetisation, the needle, when its centre of gravity was supported, stood accurately level; but, after magnetisation, one end of it was pulled towards the north pole of the earth. The needle, however, being suspended from the arm of a fine balance, it was shown that its weight was unaltered by its magnetisation. In like manner, when the needle was permitted to float upon a liquid, and thus to follow the attraction of the north magnetic pole of the earth, there was no motion of the mass towards the pole referred to; and the reason was known to be, that although the marked end of the needle was attracted by the north pole,

*Proceedings of the Royal Institution, vol. ii. p. 159.

the unmarked end was repelled equally, and these two equal and opposite forces neutralised each other as regards the production of a motion of translation. When the pole of an ordinary magnet was brought to act upon the swimming needle, the latter was attracted,-the reason being that the attracted end of the needle being much nearer to the pole of the magnet than the repelled end, the force of attraction was the more powerful of the two; but in the case of the earth, the pole being so distant, the length of the needle was practically zero. In like manner, when a piece of iron is presented to a magnet, the nearer parts are attracted, while the more distant parts are repelled; and because the attracted portions are nearer to the magnet than the repelled ones, we have a balance in favour of attraction. Here then is the most wonderful characteristic of the magnetic force, which distinguishes it from that of gravitation. The latter is a simple unpolar force, while the former is duplex or polar. Were gravitation like magnetism, a stone would no more fall to the ground than a piece of iron towards the north magnetic pole: and thus, however rich in consequences the supposition of Kepler and others may have been, it was clear that a force like that of magnetism would not be able to transact the business of the universe.

The object of the evening's discourse was to inquire whether the force of diamagnetism, which manifested itself as a repulsion of certain bodies by the poles of a magnet, was to be ranged as a polar force, beside that of magnetism; or as an unpolar force, beside that of gravitation. When a cylinder of soft iron is placed within a helix, and surrounded by an electric current, the antithesis of its two ends, or in other words, its polar excitation, is at once manifested by its action upon a magnetic needle; and it may be asked why a cylinder of bismuth may not be substituted for the cylinder of iron, and its state similarly examined. The reason is, that the excitement of the bismuth is so feeble, that it would be quite masked by that of the helix in which it is enclosed; and the problem that now meets us is, so to excite a diamagnetic body that the pure action of the body upon a magnetic needle may be observed, unmixed with the action of the body used to excite the diamagnetic.

How this may be effected, was illustrated in the following manner:-An upright helix of covered copper wire was placed

upon the table, and it was shown that the top of the helix attracted, while its bottom repelled the same pole of a magnetic needle; its central point, on the contrary, was neutral, and exhibited neither attraction

at their centres, and suspended from the point a so that both magnets swung in the same horizontal plane. It was so arranged that the poles N'S' were opposite to the central or neutral point of the helix, so that when a current was sent through the latter, the magnets were unaffected by the current. Here then we had an excited helix which itself had no action upon the magnets, and we were thus at liberty to examine the action of a body placed within the helix and excited by it, undisturbed by the influence of the latter. The helix was 12 inches high, and a cylinder of soft iron 6 inches long suspended from a string and passing over a pulley could be raised or lowered within the helix. When it was so far sunk that its lower end rested upon the table, the upper end found itself between the poles N'S' attracting one of them, and repelling the other, and consequently deflecting the astatic system in a certain direction. When the cylinder was raised so that the upper end was at the level of the top of the helix, its lower end was between the poles N'S'; and a deflection opposed in direction to the former one was the immediate consequence. To render these deflections more visible, a mirror m, was attached to the system of magnets; a beam of light thrown upon the mirror was reflected and projected as a bright disk against the wall of the theatre; the distance of this image from the mirror being considerable, and its angular motion double that of the latter, a very slight motion of the magnet was sufficient to produce a displacement of the image through several yards. This then is the principle of the beautiful instrument* by which the investigation now brought forward was conducted.

It is manifest that if a second helix be placed between the

* Devised by Prof. W. Weber, and constructed by M. Leyser, of Leipzig.

poles SN with a cylinder within it, the action upon the astatic magnet may be exalted. This was the arrangement made use of in the actual inquiry. Thus to intensify the feeble action, which it is here our object to seek, we have in the first place neutralised the action of the earth upon the magnets, by rendering them astatic. Then, by permitting two cylinders to act simultaneously on the four poles of the magnets, we have rendered the deflecting force four times what it would be if only a single pole were used. The whole apparatus was enclosed in a suitable case, which protected the magnets from air currents, and by means of a distant telescope and scale the deflections were read off through a small glass window in the case of the instrument.

A pair of bismuth cylinders was first examined. Sending a current through the helices, and observing that the magnets swung perfectly free, it was first arranged that the cylinders within the helices bad their central points opposite to the poles of the magnets. All being at rest the number on the scale marked by the cross wire of the telescope was 572. The cylinders were then moved so that the two ends were brought to bear simultaneously upon the magnetic poles: the magnet moved promptly, and after some oscillations* came to rest at the number of 612; thus moving from a smaller to a larger number. The other two ends of the bars were next brought to bear upon the magnet: a prompt deflection was the consequence, and the final position of equilibrium was 526; the movement being from a larger to a small number. We thus observe a manifest polar action of the bismuth cylinders upon the magnet; one pair of ends deflecting it in one direction, and the other pair deflecting it in the opposite direction.

Substituting for the cylinders of bismuth thin cylinders of iron, of magnetic slate, of sulphate of iron, carbonate of iron, protochloride of iron, red ferrocyanide of potassium, and other magnetic bodies, it was found that when the position of the magnetic cylinders was the same as that of the cylinders of bismuth, the deflection produced by the former was always opposed in direction to that produced by the latter; and hence the disposition of the force in the diamagnetic body must have been precisely antithetical to its disposition in the magnetic ones.

To lessen these a copper damper was made use of.