the optical observations went hand in hand with our magnetic experiments. The number of these experiments was very great, but for a considerable time no fact of importance was added to those already published. At length, however, it was our fortune to meet with various crystals whose deportment could not be brought under the laws of magne-crystallic action enunciated by Plücker. We also discovered instances which led us to suppose that the magne-crystallic force was by no means independent, as alleged, of the magnetism or diamagnetism of the mass of the crystal. Indeed the more we worked at the subject the more clearly did it appear to us that the deportment of crystals in the magnetic field was due, not to a force previously unknown, but to the modification of the known forces of magnetism and diamagnetism by crystalline aggregation.

'An eminent example of magne-crystallic action adduced by Plücker and experimented on by Faraday, was Iceland spar. It is what in optics is called a negative crystal, and according to the law of Plücker, the axis of such a crystal was always repelled by a magnet. But we showed that it was only necessary to substitute, in whole or in part, carbonate of iron for carbonate of lime, thus changing the magnetic, but not the optical character of the crystal, to cause the axis to be attracted. That the deportment of magnetic crystals is exactly antithetical to that of diamagnetic crystals isomorphous with the magnetic ones, was proved to be a general law of action. In all cases, the line which in a diamagnetic crystal set equatorially, always set itself in an isomorphous magnetic crystal axially. By mechanical compression other bodies were also made to imitate the Iceland spar.

These and numerous other results bearing upon the question were published at the time in the "Philosophical Magazine" and in "Poggendorff's Annalen;" and the investigation of diamagnetism and magne-crystallic action was subsequently continued by me in the laboratory of Professor Magnus of Berlin. But it required long subsequent effort to subdue the complications of magne-crystallic action, and to bring under the dominion of elementary principles the vast mass of facts which the experiments of Faraday and Plücker had brought to light. It was proved by Reich, Edmond Becquerel, and myself, that the condition of diamagnetic bodies in virtue of which they were

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repelled by the poles of a magnet, was excited in them by those poles; that the strength of this condition rose and fell with, and was proportional to, the strength of the acting magnet. It was not then any property possessed permanently by the bismuth, and which merely required the development of magnetism to act upon it, that caused the repulsion; for then the repulsion would have been simply proportional to the strength of the influencing magnet, whereas experiment proved it to augment as the square of the strength. The capacity to be repelled was therefore not inherent in the bismuth, but induced. Thus far an identity of action was established between magnetic and diamagnetic bodies. After this the deportment of magnetic bodies, "normal" and "abnormal," crystalline, amorphous, and compressed, was compared with that of crystalline, amorphous, and compressed diamagnetic bodies; and by a series of experiments, executed in the laboratory of this Institution, the most complete antithesis was established between magnetism and diamagnetism. This antithesis embraced the quality of polarity, -the theory of reversed polarity, first propounded by Faraday, being proved to be true. The discussion of the question was very brisk. On the continent Professor Wilhelm Weber was the ablest and most successful supporter of the doctrine of diamagnetic polarity; and it was with an apparatus, devised by him, at my request, and constructed for me under his own superintendence, by Leyser of Leipzig, that the last demands of the opponents of diamagnetic polarity were satisfied. The establishment of this point was absolutely necessary to the explanation of magne-crystallic action.

With that admirable instinct which always guided him, Faraday had seen that it was possible, if not probable, that the diamagnetic force acts with different degrees of intensity in different directions, through the mass of a crystal. In his studies on electricity he had sought an experimental reply to the question whether crystalline bodies had not different specific inductive capacities in different directions, but he failed to establish any difference of the kind. difference of the kind. His first attempt to establish differences of diamagnetic action in different directions through bismuth, was also a failure: but he must have felt this to be a point of cardinal importance, for he returned to the subject in 1850, and proved that bismuth was repelled with

different degrees of force in different directions. It seemed as if the crystal were compounded of two diamagnetic bodies of different strengths, the substance being more strongly repelled across the magne-crystallic axis than along it. The same result was obtained independently, and extended to various other bodies, magnetic as well as diamagnetic, and also to compressed substances, a little subsequently by myself. The law of action in relation to this point is, that in diamagnetic crystals, the line along which the repulsion is a maximum sets equatorially in the magnetic field; while in magnetic crystals the line along which the attraction is a maximum sets from pole to pole. Faraday had said that the magne-crystallic force was neither attraction nor repulsion. Thus far he was right. It was neither, taken singly, but it was both. By the combination of the doctrine of diamagnetic polarity with these differential attractions and repulsions, and by paying due regard to the character of the magnetic field, every fact brought to light in the domain of magne-crystallic action received complete explanation. The most perplexing of those facts were shown to result from the action of mechanical couples, which the proved polarity both of magnetism and diamagnetism brought into play. Indeed the thoroughness with which the experiments of Faraday were thus explained, is the most striking possible demonstration of the marvellous precision with which they were executed.'

These researches were begun at Marburg at the close of 1849, the first paper of the series being a joint one with my friend Professor Knoblauch. The work was continued afterwards by myself in Berlin, and completed in England.

Some purely verbal alterations have been made, and here and there a superfluous passage has been omitted; but the memoirs remain, nearly in word and wholly in substance, what they were when they first appeared.

For the use of plates and blocks, my best thanks are due to the Council of the Royal Society and the proprietors of the Philosophical Magazine.'

FIRST MEMOIR.

THE MAGNETO-OPTIC PROPERTIES OF CRYSTALS AND THE RELATION OF MAGNETISM AND DIAMAGNETISM TO MOLECULAR ARRANGEMENT.

BY JOHN TYNDALL AND HERMANN KNOBLAUCH.*

In the year 1846 our views of magnetic action received, through the researches of Faraday, an extraordinary expansion. The experiments of Brugmans, Le Baillif, Seebeck, and Becquerel had already proved the power to be active beyond the limits usually assigned to it; but these experiments were isolated and limited in number. Faraday was the first to establish the broad fact, that there is no known body indifferent to magnetic influence when the latter is strongly developed. The nature of magnetic action was then found to be twofold, attractive and repulsive; thus dividing bodies into two great classes, which are respectively denominated magnetic and diamagnetic.

The representative of the former class is iron, which, being brought before the single pole of a magnet, is attracted; the representative of the latter class is bismuth, which, being brought before the single pole of a magnet, is repelled.

If a little bar of iron be hung freely between the two poles of a magnet, it will set its longest dimension in the line joining the poles; a little bar of bismuth, on the contrary, will set its longest dimension at right angles to the line joining the poles.

The position of the iron is termed by Mr. Faraday the axial; the position of the bismuth, the equatorial. We shall have occasion to use these terms.

These discoveries, opening, as they did, a new field in physical science, invited the labours of scientific men on the Continent. Weber, Ersted, Reich, and others have occupied themselves with the subject. But, if we except the illustrious

* Phil. Mag., July 1850.

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