discoverer himself, there is no investigator in this branch of science whose labours have been so richly rewarded as those of Professor Plücker of Bonn.

In 1847 M. Plücker had a magnet constructed of the same size and power as that described by Mr. Faraday,* his object being to investigate the influence of the fibrous constitution of plants upon their magnetic deportment. While conducting these experiments, he was induced to try whether crystalline structure exercised an influence. The first experiment,' says M. Plücker,' gave an immediate and decided reply.'

Following up his investigations with crystals, he was led to the affirmation of the following two laws :

When any crystal whatever with one optic axis is brought between the poles of a magnet, the axis is repelled by each of the poles; and if the crystal possess two axes, each of these is repelled, with the same force, by the two poles.

"The force which causes this repulsion is independent of the magnetism or diamagnetism of the mass of the crystal; it decreases with the distance more slowly than the magnetic influence exerted by the poles.'†

It is, perhaps, worth explaining that if, on exciting the magnet, the optic axis take up the axial position, it is said to be attracted; if the equatorial, it is said to be repelled.

The first experiment of M. Plücker, which led to the affirmation of these laws, was made with tourmaline. A plate of the crystal which had been prepared for the purposes of polarisation, twelve millimetres long, nine wide, and three thick, was suspended by a silk fibre between the poles of an electromagnet. On sending a current round the latter, the plate, which was magnetic, set itself as an ordinary magnetic substance would do, with its longest dimension from pole to pole. The optic axis of the crystal, thus suspended, was vertical.

On hanging the crystal, however, with its optic axis horizontal, when the magnet was excited, the plate stood no longer as a magnetic substance, but as a diamagnetic; its longest dimension being at right angles to the line joining the poles. The optic axis of the crystal was found to coincide with its

* Phil. Mag., vol. xxviii. p. 396.

+ Poggendorff's Annalen, vol. lxxii. p. 75.

length, and the peculiar deportment was considered as a proof that the optic axis was repelled.

This law was further established by experiments with Iceland spar, quartz, zircon, beryl, &c., and, as above stated, included crystals of all kinds, both optic positive and negative. It has, however, lately undergone considerable modification at the hands of M. Plücker himself. In a letter to Mr. Faraday, which appears in page 450, vol. xxxiv. of the 'Philosophical Magazine,' he expresses himself as follows:

"The first and general law I deduced from my last experiments is the following one :-"There will be either repulsion or attraction of the optic axes by the poles of a magnet, according to the crystalline structure of the crystal. If the crystal is a negative one, there will be repulsion; if it is a positive one, there will be attraction.""*

This law applies to crystals possessing two optic axes, each of the said axes being attracted or repelled according as the crystal is positive or negative. It will simplify the subject if we regard the line bisecting the acute angle enclosed by the two axes as the resultant of attraction or repulsion; for the sake of convenience, we shall call this the middle line. In positive crystals, therefore, the middle line, according to the above law, must stand axial; in negative crystals, equatorial. It is also evident that the plane passing through the optic axes must, in the one class of crystals, stand from pole to pole, in the other class at right angles to the line joining the poles.

In explaining this new modification of the law, M. Plücker lays particular emphasis upon the fact that the attraction or repulsion is the result of an independent force, connected in no way with the magnetism or diamagnetism of the mass of the crystal; and this view is shared by Mr. Faraday, who, in expressing his concurrence with M. Plücker, denominates the force in question an optic axis force.'t

The experiments described in our first paper upon this subject furnish, we conceive, sufficient ground of dissent from these views. In the case of five crystals of pure carbonate of lime (Iceland spar), we found the law of Plücker strictly verified, all five crystals being diamagnetic; on replacing,

* Phil. Mag., vol. xxxiv. p. 450.

+ Phil. Trans. 1849, p. 32. Phil. Mag., vol. xxxvi. p. 178. A short preliminary notice printed further on!

however, a portion of the carbonate of lime by carbonate of iron, nature herself being the chemist in this case, the crystal was no longer diamagnetic, but magnetic; in every other respect it was physically unchanged; its optical properties remained precisely as before, the crystal of carbonate of lime and the crystal of carbonate of lime and iron being both negative. In the one case, however, the optic axis was attracted; in the other the said axis was repelled, the attraction being evidently caused by the passage of the crystal from the diamagnetic into the magnetic state.

We have examined other crystals of the same form as Iceland spar, both magnetic and diamagnetic. In all cases the former act in a manner precisely similar to the carbonate of lime and iron already described, while the latter behave as the pure carbonate of lime. The following are examples.

Nitrate of Soda.-This crystal is of the same form as carbonate of lime, and, like it, diamagnetic. Its deportment is in every respect the same. A rhombus cloven from the crystal and suspended horizontally between the poles sets its longer diagonal axial. Suspending the full crystal between the poles, with its optic axis horizontal, on exciting the magnet this axis sets itself equatorial.

Breunnerite. This is a crystal composed principally of carbonate of lime and carbonate of magnesia, but containing a sufficient quantity of the carbonate of iron to render it magnetic. Suspended in the magnetic field, the optic axis sets from pole to pole.

Dolomite. In this crystal a portion of the lime is replaced by protoxide of iron and protoxide of manganese, which ingredients render it magnetic. The optic axis sets from pole to pole.

Carbonate of Iron.-In the cases cited, the substitution of iron for calcium was partial; in the case now before us the substitution is complete. This crystal differs in nothing, save in the energy of its action, from the magnetic crystals already described. If a full crystal be hung between the poles, with its optic axis. horizontal, on closing the circuit and sending a current round the magnet, the axis sets strongly in the line joining the poles, vibrates through it quickly for a time, and finally comes to rest there. If a thin rhombus be cloven from the crystal and

suspended from one of its obtuse angles with its parallel faces vertical, it will set itself exactly equatorial. In this case it is easy to see that the horizontal projection of the optic axis, which passes through the obtuse angle of the crystal, stands axial. Hung from its acute angle, the rhombus takes up an oblique position, making a constant angle with the line joining the poles. To this position, if forcibly removed from it, it will invariably return. The position may be either right or left of the axial line; but the angle of obliquity is always the same, being the angle which the optic axis makes with the face of the rhombus. Hung from the obtuse angle the obliquity is nothing-from the acute angle it is a maximum; the rhombus is capable of all degrees of obliquity between these extremes, the optic axis setting in all cases from pole to pole.

Oxide of Iron.-The above phenomena are exhibited even in a more striking manner by this crystal. So strong is the directive power that a rhombus, suspended from one of its obtuse angles, will set itself strongly equatorial, though its length may be fifteen or twenty times its breadth.

What is the conclusion to be drawn from these experiments? We have first of all a diamagnetic crystal of pure carbonate of lime, which sets its optic axis equatorial. On substituting for a portion of the lime a quantity of protoxide of iron sufficient to render the crystal weakly magnetic, we find the position of the axis at once reversed. Replacing a still further quantity of the diamagnetic lime by a magnetic constituent, we find the action stronger, the force with which the optic axis takes up the axial position increasing as the magnetic constituents increase. These experiments appear to be irreconcilable with the statement, that the position of the optic axis is independent of the magnetism or diamagnetism of the mass of the crystal. Turning now to crystals possessing two optic axes, we find the law of Plücker equally untenable.

Dichroite. This crystal, as is well known, receives its name from its ability to transmit light of two different colours. The specimen examined by us is a cube. In the direction of the 'crystallographic' axis, which coincides with the middle line,' the light transmitted is yellowish; through the other four sides of the cube it is a deep blue. Suspended with the middle line horizontal, whatever be the position of that line before

closing the circuit, the instant the magnetic force is developed it turns with surprising energy into the axial position and becomes fixed there. According to the law, however, the middle line should stand equatorial, for the crystal is negative.*

Sulphate of Baryta (Heavy spar).-The form of this crystal is a prism whose base is a rhombus, the four sides being perpendicular to the base. It cleaves parallel to the sides and base. Suspended between the poles, with the axis of the prism vertical, on exciting the magnet, though the crystal is diamagnetic, the long diagonal sets itself axial. It agrees thus far with the carbonate of lime. Suspended from the acute angle formed by two sides of the prism, on closing the circuit the axis sets parallel to the line joining the poles, and remains there as long as the force is present. Suspending the crystal from its obtuse angle, the axis being still horizontal, on closing the circuit the axis sets itself equatorial. A plane perpendicular to the rhombic base, and passing through the long diagonal, contains the two optic axes, which are inclined to each other at an angle of 38°. The middle line bisecting this angle is parallel to the axis of the prism, and hence stands axial or equatorial, according as the prism is suspended from its acute or its obtuse angle. The position of the middle line is therefore a function of the point of suspension, varying as it varies ; at one time supporting the law of Plücker, and at another time contradicting it. Heavy spar is positive.

Sulphate of Strontia (Cœlestine).-This is also a positive crystal, its form being precisely that of heavy spar; the only difference is this, that, in Coelestine, the optic axes enclose an angle of 50° instead of 38°. The corroboration and contradiction exhibited by heavy spar are exhibited here also.

Sulphate of Zinc.-Suppose the crystalline prism to be hung from its end, and the line which stands equatorial when the magnet is excited carefully marked. A plate taken from the crystal parallel to this line, and to the axis of the prism, displays, on examination with polarised light, the ring systems surrounding the ends of the two optic axes. The middle line, which bisects. the acute angle enclosed by these axes, is perpendicular to the

*Brewster's list.