Referring to fig. 27a, page 140, it will be seen that we have here dealt with the second and gravest objection of M. Matteucci, and converted the facts upon which the objection is based into a proof of diamagnetic polarity, so cogent that it alone would seem to be sufficient to decide this important question.

Holding the opinion entertained by M. Matteucci regarding the nature of diamagnetic force,* his objection must have appeared to him to be absolutely unanswerable: I should be glad to believe that the remarks contained in this Appendix furnish, in the estimation of this distinguished philosopher, a satisfactory explanation of the difficulty which he has disclosed.

Let me, in conclusion, briefly direct the reader's attention to the body of evidence laid before him in the foregoing pages. It has been proved that matter is repelled by the pole of a magnet in virtue of an induced condition into which the matter is thrown by such a pole. It is shown that the condition evoked by one pole is not that which is evoked by a pole of an opposite quality -that each pole excites a condition peculiar to itself. A perfect antithesis has been shown to exist between the deportment of paramagnetic and diamagnetic bodies when acted on by a magnet alone, by an electric current alone, or by a magnet and an electric current combined. The perplexing phenomena resulting from molecular structure have been laid open, and the antithesis between paramagnetic and diamagnetic action traced throughout. It is further shown, that whatever title to polarity the deportment of a bar of soft iron, surrounded by an electric current, and acted on by other magnets, gives to this substance, a bar of bismuth possesses precisely the same title: the disposition of forces, which in the former case produces attraction, produces in the latter case repulsion, while the repulsion of the iron finds its exact complement in the attraction of the bismuth. Finally, we have a case adduced by M. Matteucci which suggests a crucial experiment to which all our previous reasoning has been submitted, by which its accuracy has been proved, and the insufficiency of the assumption, that the diamagnetic force is not polar, is reduced to demonstration. When we remember that against all this no single experimental fact† or theoretic

* "Il ne peut exister dans les corps diamagnétiques une polarité telle qu'on la conçoit dans le fer doux."-Cours spécial, p. 201.

I refrain from alluding to the negative results obtained by Mr. Faraday in repeating

argument which can in any degree be considered as conclusive, has ever been brought forward, nor do I believe can be brought forward, the conclusion seems irresistible, that we have in the agency by which bodies are repelled from the poles of a magnet, a force of the same dual character as that by which bodies are attracted; that, in short, 'diamagnetic bodies possess a polarity the same in kind but the opposite in direction to that possessed by magnetic ones."

[The experiments and reasonings recorded in the foregoing memoir left no shadow of doubt upon my mind as to the polar character of the diamagnetic force. Throughout the most complex series of actions, the doubleness of action to which the term polarity has been applied, was manifested in a clear and conclusive manner. Still I thought it would contribute to the final settlement of the question if I were to take up the subject after the method of Weber, and satisfy all the demands which had been made upon him by the opponents of diamagnetic polarity. Here, as in the foregoing inquiry, it was my wish to render the experiments exhaustive, and to employ apparatus which should place it definitely within the power of all investigators to subject the question to experimental demonstration. I devised a scheme of experiment, but, previous to putting it into execution, wrote to Prof. Weber asking him whether he did not think it possible so to improve his apparatus as materially to exalt the action. Weber's own experiments had been made with bismuth solely. It was objected that his results were due to ordinary induced currents, and he was called upon to produce the same effects with insulators. This demand it was my object to meet, and to do so more delicate and more powerful means than any previously employed were necessary.

Professor Weber replied to me immediately, stating that the employment of an astatic system of magnets would be a great improvement; and he had the exceeding kindness to devise in detail the apparatus for me. It was admirably constructed under Prof. Weber's own superintendence by Leyser of Leipzig, and with it the following inquiry was conducted.-J. T., 1870.]

M. Weber's experiments; for though admirably suited to the exhibition of certain effects of ordinary induction, Mr. Faraday himself has shown how unsuitable the apparatus employed would be for the investigation of the question of diamagnetic polarity. See Experimental Researches (2653, 2654), vol. iii. p. 143.—J. T., May 9,

1855.

FIFTH MEMOIR.

FURTHER RESEARCHES ON THE POLARITY OF THE

DIAMAGNETIC FORCE.*

Introduction.

A YEAR ago I placed before the Royal Society the results of an investigation' On the Nature of the Force by which Bodies are repelled from the Poles of a Magnet.' The simultaneous exhibition of attraction and repulsion in the case of magnetised iron is the fact on which the idea of the polarity of this substance is founded; and it resulted from the investigation referred to, that a corresponding duality of action was manifested by bismuth. In those experiments the bismuth was the moveable object upon which fixed magnets were caused to act, and from the deflection of the bismuth its polarity was inferred. But, inasmuch as the action is reciprocal, we ought also to obtain evidence of diamagnetic polarity by reversing the conditions of experiment; by making the magnet the moveable object, and inferring from its deflection the polarity of the mass which produces the deflection. This experiment would be complementary to those described in the communication referred to, and existing circumstances invested the experiment with a great degree of interest and importance.

In fact, an experiment similar to that here indicated was made by Professor W. Weber, previous to my investigation, and the result was such as to satisfy its author of the reverse polarity of diamagnetic bodies. I will not here enter into a minute description of the instrument and mode of experiment by which

* From the Philosophical Transactions for 1856, part i.; having been received by

the Royal Society November 27, 1855, and read December 20, 1855.

+ Philosophical Transactions, 1855; and Phil. Mag. for September 1855.

this result was obtained; for the instrument made use of in the present inquiry being simply a refinement of that made use of by M. Weber, its explanation will embrace the explanation of his apparatus. For the general comprehension of the criticisms to which M. Weber's results have been subjected, it is necessary, however, to remark, that in his experiments a bismuth bar, within a vertical spiral of copper wire, through which an electric current was transmitted, was caused to act upon a steel magnet freely suspended outside the spiral. When the two ends of the bar of bismuth were permitted. to act successively upon the suspended magnet, a motion of the latter was observed, which indicated that the bismuth bar was polar, and that its polarity was the reverse of that of iron.

Notwithstanding the acknowledged eminence of M. Weber as an experimenter, this result failed to produce general conviction. Mr. Faraday, in his paper On the polar or other condition of diamagnetic bodies,'* had shown that results quite similar to those obtained by M. Weber, in his first investigation with bismuth, were obtained in a greatly exalted degree with gold, silver, and copper; the effect being one of induced currents and not of diamagnetic polarity. He by no means asserted that his results had the same origin as those obtained by M. Weber; but as the latter philosopher had made no mention of the source of error which Mr. Faraday's experiments rendered manifest, it was natural to suppose that it had been overlooked, and the observed action attributed to a wrong cause. In an article published in his Massbestimmungen' in 1852, M. Weber, however, with reference to this point, writes as follows:- I will remark that the article transferred from the Reports of the Society of Sciences of Saxony to Poggendorff's Annalen was only a preliminary notice of my investigation, the special discussion of which was reserved for a subsequent communication. It will be sufficient to state here, that in the experiments referred to I sought to eliminate the inductive action by suitable combinations; but it is certainly far better to set aside this action altogether, as has been done in the experiments described in the present memoir.'

* Experimental Researches, 2640, Philosophical Transactions, 1850, p. 171.

One conviction grew and strengthened throughout these discussions-this, namely, that in experiments on diamagnetic polarity great caution is required to separate the pure effects of diamagnetism from those of ordinary induced currents. With reference to even the most recent experiments of M. Weber, referred to at the conclusion of the citation just made, it is strongly urged that there is no assurance that the separation referred to has been effected. In those experiments, as already stated, a cylinder of bismuth was suspended within a vertical helix of covered copper wire, and the action of the cylinder upon a magnet suspended opposite to the centre or neutral point of the helix was observed. To increase the action, the position of the cylinder was changed at each termination of the minute swing of the magnet, the amplitude of the oscillations being thus increased, and the effect rendered more sensible to the eye. Now, it is urged, there is every reason to believe that in these motions of a metallic mass within an excited helix induced currents will be developed, which, acting upon the magnet, will produce the motions observed. The failure indeed to demonstrate the existence of diamagnetic polarity by other means has, in the case of some investigators, converted this belief into a certainty.

Among the number whom M. Weber's experiments have failed to convince, M. Matteucci occupies a prominent place. With reference to the question before us, this philosopher writes as follows:-*

In reading the description of the experiments of M. Weber, we are struck on beholding the effects produced by moving the bismuth when there is no current in the spiral. Although the direction of oscillation in this latter case is opposed to that observed when the spiral is active, still the fact excites doubts as to the truth of the conclusions which have been drawn from

these experiments.†

To deduce

rigorously the demonstration

of diamagnetic polarity, it would be necessary to substitute for the

Cours spécial sur l'Induction, p. 206.

It is not my place to account for the effect here referred to. I may, however, remark, that there appears to be no difficulty in referring it to the ordinary action of a diamagnetic body upon a magnet. It is the result which Brugmans published upwards of half a century ago; the peculiar form of this result in one of the series of experiments quoted by M. Weber must, I think, be regarded as purely accidental.

-J. T.