from Professor Weber to myself, this hypothesis of nonresisting circuits is also included in the theory of Ampère. Nobody, of course, who accepts unreservedly this theory, as applied to iron and steel, will find any difficulty in the conception that these channels of perfect conductivity surround atoms which, in their aggregate form, constitute the most powerful insulators. Shell-lac, sulphur, and glass, for example, which are all diamagnetic, must be assemblages of such atoms with their circuits. As a speculation, Weber's theory is beautiful and consistent, and if it affords repose and satisfaction to his powerful mind, it is sure to do the same to the minds of others. But, being a matter of fact, the question of diamagnetic polarity lies apart from these theoretic considerations. The knowledge that a magnet has two poles does not require to be prefaced by a general theory of magnetism. The essence of magnetic polarity consists in the simultaneous and inseparable existence, or development, of two hostile powers which, in action, always resolve themselves into mechanical couples. Here, it may be said in passing, the key of all Faraday's difficulties-the solution of all the mechanical paradoxes which so perplexed him-is to be found. The facts of magnetic polarity can be mastered and made sure of by anybody possessing a bar magnet and a magnetic needle, or even two magnetic needles. And passing from steel magnets to bars of iron in helices through which electric currents flow, the polarity of the iron is as much a matter of experimental certainty as the polarity of the magnetized steel. The question to be decided was: Do diamagnetic bodies, under magnetic influence, show this doubleness of action? To put the case strongly, iron is repelled by a magnet, as well as attracted; is bismuth attracted by a magnet, as well as repelled? That it is so is abundantly proved in the following pages. Faraday, over and over again, observed this attraction; but it came to him in the disguise of magne-crystallic action, in which, according to his view, neither attraction nor repulsion had any share. The subject of diamagnetic polarity was first definitely approached by me in the investigation described in the "Third Memoir' of this series but I had not, at the time, the apparatus and material needed to carry the enquiry out. Thanks to the Council of the Royal Society, this want was soon supplied; and I faced the investigation recorded in the Fourth Memoir,' with the resolution to leave no stone unturned in the effort to arrive at the truth. The deportment of diamagnetic bodies was subjected to an exhaustive comparison with that of magnetic bodies, and the antithesis between them, when acted on by all possible combinations of electro-magnets and electric currents, was proved to be absolute and complete. Under the same conditions of excitement the repulsion of the one class of bodies had its complement in the attraction of the other; the north and south magnetism of the one class had its complement in the south and north magnetism of the other. When the end of an excited iron bar was repelled by a magnetic pole the end of a bismuth bar, under the same influence, was attracted by the same pole; every deflection, moreover, produced by the combined action of magnets and helices, in the one case, had its exact complement in an opposite deflection in the other. No reasonable doubt, therefore, could rest upon the mind that the diamagnetic force possessed precisely the same claim to the title of a polar force as the magnetic. This conclusion is further illustrated and enforced by the experiments recorded in the Fifth Memoir.' These experiments were executed with a most delicate apparatus, expressly devised for me by Professor Weber, of Göttingen, and constructed by Leyser, of Leipzig, with consummate accuracy and skill. With it the various objections which had been urged against Weber's own results were tirely removed. The severest conditions laid down by the en opponents of diamagnetic polarity were accepted and fulfilled. Conductors and insulators-liquids and solidswere subjected to this new test, and by it also diamagnetic polarity was shown to rest upon as safe a basis as the old and long-recognized magnetic polarity itself. The argument was rounded off by the application of the doctrine of polarity to magne-crystallic phenomena. This subject is formally approached towards the end of the 'Fourth Memoir,' where certain objections which had been urged by Matteucci are examined and removed. In the 'Sixth Memoir' the application is carried on. By combining with the doctrine of polarity, the differential attraction and repulsion, first observed in the case of bismuth by Faraday, and extended to other crystals, and to compressed substances, in the 'Second Memoir' by myself, all difficulties are caused to disappear; the cases cited by Faraday to prove that neither attraction nor repulsion was involved in these phenomena being shown to be simple mechanical consequences of the contemporaneous action of both attraction and repulsion. I have aimed at rendering this volume small and handy, by omitting various topics which were introduced in the first edition. J. TYNDALL. HIND HEAD, HASLEMERE; April, 1888. CONTENTS. THE MAGNETO-OPTIC PROPERTIES OF CRYSTALS AND THE RE- FORCE PAGE 193 |