to any one who carefully considers the experiments described in this memoir. The further examination of this deeply interesting subject we defer to another occasion.

Nature acts by general laws, to which the terms great and small are unknown; and it cannot be doubted that the modifications of magnetic force, exhibited by bits of copperas and sugar in the magnetic field, display themselves on a large scale in the crust of the earth itself. A lump of stratified grit exhibits elective polarity. It is magnetic, but will set its planes of stratification from pole to pole, though it should be twice as long in the direction at right angles to these planes. A new element appears thus to enter our speculations as to the position of the magnetic poles of our planet; the influence of stratification and plutonic disturbance upon the magnetic and electric forces.

MARBURG: May, 1850.

Note, 1870.-I wish to direct attention here to a paper written by M. Plücker, and translated by myself, for the new series of 'Scientific Memoirs,' published by Taylor and Francis (1853). In this paper M. Plücker approached much more closely than he had previously done to the views expressed in the foregoing memoir. But his paper, which had been written in December, 1849, remained unprinted till 1852.-J. T.

SECOND MEMOIR.

ON DIAMAGNETISM AND MAGNE-CRYSTALLIC
ACTION.

[This investigation was conducted in the laboratory of Professor Magnus, of Berlin, during the spring of 1851, and was communicated to the British Association at its meeting at Ipswich the same year. It was also published in the Philosophical Magazine' for September, 1851.-J. T. 1870.]

§ 1. On Diamagnetism.

FIVE years ago Faraday established the existence of the force called diamagnetism, and from that time to the present some of the first minds in Germany, France, and England have been devoted to the investigation of this subject. One of the most important aspects of the inquiry is the relation which subsists between magnetism and diamagnetism. Are the laws which govern both forces identical? Will the mathematical expression of the attraction in the one case be converted into the expression of the repulsion in the other by a change of sign from positive to negative ?

6

The conclusions arrived at by Plücker in this field of inquiry are exceedingly remarkable and deserving of attention. His first paper, On the relation of Magnetism and Diamagnetism,’ is dated from Bonn, September 8, 1847, and will be found in Poggendorff's Annalen and in Taylor's Scientific Memoirs.' He sets out with the question, 'Is it possible, by mixing a magnetic substance with a diamagnetic, so to balance the opposing forces that an indifferent body will be the result?' This question he answers in the negative. The experiments,' he writes, which I am about to describe, render it necessary that every thought of the kind should be abandoned.'

One of these experiments will serve as a type of the whole, and will show the foundation on which the negative reply of

M. Plücker rests. A piece of cherry-tree bark, 15 millims. long and 7 millims. wide, was suspended freely between the two movable poles of an electro-magnet; on bringing the points of the poles so near each other that the bark had barely room to swing between them, it set itself, like a diamagnetic substance, with its length perpendicular to the line which united the two poles. On removing the poles to a distance, or on raising the bark to a certain height above them, it turned round and set its length parallel to the line joining the poles. As is usual, we shall call the former position the equatorial, and the latter position the axial. Thus when the poles were near, diamagnetism was predominant, and caused the mass to set equatorial; when the poles were distant, magnetism, according to the notion of M. Plücker, was predominant, and caused the mass to set axial. From this he concludes, That in the cherry-tree bark two distinct forces are perpetually active; and that one of them, the magnetic, decreases more slowly with the distance than the other, the diamagnetic.

In a later memoir this predominance of the diamagnetic force at a short distance is affirmed by M. Plücker to be due to the more general law, that when a magnet operates upon a substance made up of magnetic and diamagnetic constituents, if the power of the magnet be increased, the diamagnetism of the substance increases in a much quicker ratio than the magnetism; so that without altering the distance between it and the magnet, the same substance might at one time be attracted and at another time repelled by merely varying the strength of the exciting current.

This assertion is supported by a number of experiments, in which a watch-glass containing mercury was suspended from one end of a balance. The watch-glass was magnetic, the mercury was diamagnetic. When the glass was suspended at a height of 3.5 millims. above the pole of the magnet, and the latter was excited by a battery of four cells, an attraction of one milligramme was observed; when the magnet was excited by eight cells, the attraction passed over into a repulsion of the same amount.

It is to be regretted that M. Plücker, instead of giving us

* Poggendorff's Annalen, vol. lxxv. p. 413.

the actual strength of the exciting current, has mentioned merely the number of cells employed. From this we can get no definite notion as to the amount of magnetic force evolved in the respective cases. It depends of course upon the nature of the circuit whether the current increases with the number of cells or not. If the exterior resistance be small, an advance from four to eight cells will make very little difference; if the outer resistance be a vanishing quantity, one cell is as good as a million.*

During an investigation on the magneto-optic properties of crystals, which I had the pleasure of conducting in connection with my friend Professor Knoblauch, I had repeated opportunities of observing phenomena exactly similar to those observed by M. Plücker with the cherry-tree bark; but a close study of the subject convinced me that the explanation of these phenomena by no means necessitated the hypothesis of two forces acting in the manner described. Experiment further convinced me, that a more delicate apparatus than the balance used by M. Plücker would be better suited to the measurement of such feeble manifestations of force.

An exact acquaintance with electro-magnetic attractions appeared to be a necessary discipline for the successful investigation of diamagnetic phenomena; and pursuing this idea, an inquiry was commenced last November into the action of an electro-magnet upon masses of soft iron. I was finally led to devote my entire attention to the attraction of soft iron spheres, and the results obtained were so remarkable as to induce me to devote a special memoir to them alone. ‡

In this investigation it was proved, that a ball of soft iron, separated by small fixed distance from the pole of an electromagnet, was attracted with a force exactly proportional to the square of the exciting current. § Now this attraction is in each case the produce of two factors, one of which represents the magnetism of the magnet, and the other the magnetism of the ball. For example, if the magnetism of the magnet at any

* The usual arrangement of the cells is here assumed; that is, where the negative component of one cell is connected with the positive component of the next.

Phil. Mag., July 1850.

Phil. Mag., April 1851. Poggendorff's Annalen, May 1851.

§ This had been already proved by Lenz and Jacobi, but the employment of the iron spheres renders the result particularly sharp and exact.

given moment be represented by the number 4, and that of the ball by 3, the attraction, which is a consequence of their reciprocal action, is represented by the number 12. If we now suppose the magnetism of the magnet to be doubled by a current of double strength, the ball will have its magnetism also doubled, and the attraction resulting will be expressed by the number 48. Thus we see that a doubling of the power of the magnet causes four times the attraction; and that while the attraction increases as the square of the current, the magnetism of the ball increases in the simple ratio of the current itself.

Our way to a comparison of magnetism and diamagnetism is thus cleared. We know the law according to which the magnetism of an iron ball increases, and we have simply to ascertain whether the diamagnetism of a bismuth ball follows the same law. For the investigation of this question I constructed the following apparatus.

In two opposite sides of a square wooden box were sawn two circular holes about four inches in diameter. The holes were diagonally opposite to each other, and through each a helix of copper wire was introduced and wedged fast. Each helix contained a core of soft iron, which was pushed so far forward that a line parallel to the sides of the box through which the helices entered, and bisecting the other two sides, was a quarter of an inch distant from the interior end of each core. The distance between the two interior ends was six inches, and in this space a little beam of light wood was suspended. At the ends of the beam two spoon-shaped hollows were worked out, in which a pair of small balls could be conveniently laid. The beam rested in a paper loop, which was attached to one end of a fine silver wire. The wire passed upward through a glass tube nearly three feet in length, and was connected at the top with a torsion head. The tube was made fast in a stout plate of glass, which was laid upon the box like a lid, thus protecting the beam from currents of air. A floor of Bristol board was fixed a little below the level of the axes of the cores, the board' being so cut as to fit close to the helices: the two corners of the floor adjacent to the respective cores and diagonally opposite to each other bore each a graduated quadrant. When the instrument was to be used, two balls of the substance to be experimented with were placed upon the spoon