NOTICES OF SCIENTIFIC WORKS.

Researches on Diamagnetism and Magne-crystallic Action, including the Question of Diamagnetic Polarity. By JOHN TYNDALL, LL.D., F.R.S., Professor of Natural Philosophy in the Royal Institution. London: Longmans, 1870.

THIS work is the first instalment of a complete collection of the original memoirs on experimental physics, which the learned author has published during the last eighteen years. It contains not only a record of his own work on the subject of diamagnetism, but also extracts from the writings of Faraday, Plucker, Becquerel, Matteucci, Weber, and other experimental philosophers, bearing upon the same phenomena, so that the reader has before him everything necessary for a complete understanding of this very intricate subject. The second part of the book contains letters, essays, and reviews, relating to magnetism and electricity, and includes among others a discussion on the existence of a magnetic medium in space, the relation between magnetism and the electric current, an account of the polymagnet, and one of the clearest descriptions of Ohm's theory which we have ever read,

We are so accustomed to see a magnetic substance like iron or a magnetic needle point north and south, rush to the poles of a magnet when brought near to one, or arrange itself axially when suspended between the poles, that it is difficult to imagine that the vast majority of substances possess almost diametrically opposite qualities. When brought near to a magnet of sufficient power they are repelled from it, and when suspended freely between its poles they swing round, if of an elongated form, and arrange themselves equatorially or at right angles to the line joining the two poles, apparently with the object of getting as far away from them as possible. This action was named by Faraday "Diamagnetism," the common phenomena exhibited by iron being named "Paramagnetism," whilst Magnetism is used as a general term to include the whole range of both phenomena. Paramagnetic bodies are few in number, but they include some of extraordinary energy, iron, nickel, cobalt, and oxygen for instance; whilst diamagnetic bodies include the greater number of the metals, and such substances as rock crystal, heavy spar, sulphate of magnesia, marble, alum, common salt, saltpetre, carbonate of soda, Iceland spar, tartaric acid, citric acid, water, alcohol, ether, the mineral acids, glass, iodine, phosphorus, sulphur, resin, spermaceti, sealing-wax, turpentine, indiarubber, sugar, starch, gum arabic, wood, fresh beef, blood, apple, bread, &c. In fact, could a marble statue, or its living prototype,

be suspended between the poles of a sufficiently powerful magnet it would set equatorially, or east and west instead of north and south. Of all diamagnetic bodies bismuth has attracted the most attention, owing to the comparative power which it exhibits. Its diamagnetic properties, although vastly inferior to the paramagnetism of iron, are yet sufficiently marked to enable its properties to be observed with small permanent magnets weighing a few ounces; whilst a bismuth needle freely suspended will set itself parallel to the wires of a galvanometer. The property is not one possessed permanently by the bismuth, but is simply induced by the proximity of the magnet, nothing being communicated which the bismuth can carry

away.

Having shown almost complete antithesis between the magnetism of iron and bismuth, the question naturally arose, Is this extended to polarity? Faraday worked long and earnestly at this question, and we believe to the last he was not satisfied that the question of polarity in diamagnetic bodies was settled, although the experiments with Weber's exquisitely beautiful apparatus were tried in his presence by Professor Tyndall. In a letter to Matteucci, dated November 2, 1855, Faraday wrote, "All Tyndall's results are to me simple consequences of the tendency of paramagnetic bodies to go from weaker to stronger places of action, and of diamagnetic bodies to go from stronger to weaker places of action, combined with the true polarity or direction of the lines of force in the places of action." On the other hand, it would appear as if these two philosophers were looking at the subject from entirely different points of view. Faraday had his mind fixed on lines of magnetic force, the use of which, as true representations of nature, he said never failed him; whilst Tyndall limited his view to that doubleness of action in which the term polarity originated. But these were apparent differences only, not differences in reality, for in the letter just quoted, Faraday said, "I differ from Tyndall a good deal in phrases, but when I talk with him I do not find that we differ in facts. That phrase polarity in its present undefined state is a great mystifier."

Considerable space is given to the description of the beautiful instrument devised by M. Weber in order to submit this question to a crucial test, the design of which was ably carried out by M. Leyser, of Leipzig. Clear engravings of it are given, and the experiments are described in full detail. With it not only has diamagnetic polarity been proved to exist in the case of bismuth, but the same result was obtained with cylinders of calcareous spar, statuary marble, phosphorus, sulphur, heavy glass, distilled water, bisulphide of carbon, and other non-conductors of electricity, removing the scruples of those who saw in the first experiments of this sort an action produced by induced currents. By these experiments,

Professor Tyndall concludes that a body of evidence has accumulated in favour of diamagnetic polarity, which places it among the most firmly-established truths of science.

This being the case, it would be of interest to ascertain on which side exists the fallacy of reasoning by which Professor Thompson has reduced the existence of diamagnetic polarity to an apparent absurdity; the paradox is well stated in the following quotation from a paper by Faraday, "On some Points of Magnetic Philosophy," published in the Philosophical Magazine' for February, 1855:—“ If a globe of bismuth be placed without friction in the middle of the magnetic field, it will not point or move because of its shape; but if it have reverse polarity, it will be in a state of unstable equilibrium; and if time be an element, then the ball, being once moved on its axis ever so little, would then have its polarity inclined to the magnetic axis, and would go on revolving for ever, producing a perpetual motion. I do not see how this consequence can be avoided, and therefore cannot admit the principles on which it rests. The idea of a perpetual motion produced by static forces is philosophically illogical and impossible, and so I think is the polarity opposed or adverse static condition to which I have already referred.'

Of course if time does not enter as an element in diamagnetic induction the above argument falls to the ground; but it appears to be so firmly established a fact that an exertion of physical force occupies time, that it can scarcely be doubted that it is concerned here also; that was Faraday's opinion, although he admitted that it seemed to be so brief in period as to be inappreciable by the means he had employed.

We should have liked to give an extended notice of the second subject included in the title of this work, namely, Magne-crystallic action, the phenomena of which were at first so paradoxical as to baffle the ingenuity of the most acute experimentalists, but, thanks to the labour of Professor Tyndall and other physicists, now deducible with as much care and certainty from the action of polar forces as the precession of the equinoxes is from the force of gravitation. In the author's language, "The whole domain of magne-crystallic action is thus transferred from a region of mechanical enigmas to one in which our knowledge is as clear and sure as it is regarding the most elementary phenomena of magnetic action."

The magne-crystallic force is one by which certain crystals are caused to set themselves with certain of their axes parallel or transverse to the lines of magnetic force acting on them. This force acts at a distance, and is by no means so weak as might be at first supposed, for just as a crystal is moved by the magnet at a distance, so can the crystal also move the magnet at a distance. Faraday obtained the latter result by converting a steel bodkin into a magnet

and suspending it freely in the neighbourhood of the crystal. The tendency of the needle was always to place itself parallel to the magne-crystallic axis.

Neither will space permit us to refer, except in the briefest manner, to the results obtained by preparing bars of magnetic and diamagnetic substances, by reducing them to fine powder, and then compressing them in moulds in such a manner that the line of greatest compression is in different directions along or across the bar. A bismuth bar so prepared, squeezed flat within the jaws of a vice and suspended between the poles, will turn with the energy of a magnetic substance into the axial position; whilst a bar made up of powdered carbonate of iron (magnetic) compressed in this manner will recoil from the poles as if violently repelled. It thus appears that the line of magnetic action has a near relation to that of the closest contact among the material particles, and this relationship is traced in many different ways, and appears related to the cleavage of crystals.

It would be of interest to try some of these experiments on diamagnetism with the metal Thallium, a metal which, whilst it rivals, if it does not surpass, bismuth in diamagnetic energy, is as soft and amorphous as lead, and lends itself with the same facility to moulding and compression. Probably many of the apparent anomalies of diamagnetism which observers at first encounter, owing to the highly crystalline nature of bismuth, would disappear if thallium were the metal selected for experiment.

We cannot close this book without expressing the profound admiration which it leaves in the mind for the author's philosophical acumen and experimental skill. He moreover possesses one valuable quality, which we regret to say is as rare amongst scientific men as the combination of the two former,-that of placing his views and describing his experiments in such clear language that the profoundest mysteries of nature seem under his treatment to become clear and simple to a child's comprehension. Speaking as one who never loses an opportunity of listening to this philosopher on whom the mantle of Faraday has so worthily descended, the writer scarcely knows which gives him greater pleasure-to listen to one of Dr. Tyndall's lucid expositions of some hitherto hidden mystery of nature, or to hear him in his clear logical manner quietly put down a scientific opponent who has ventured to differ from some of his conclusions.

ON SAVAGES.*

WHAT Sir Charles Lyell has accomplished for the student of Geology, Sir John Lubbock is now achieving for the student of Ethnology. His 'Pre-historic Times '† first excited and awakened public attention by the clearness of its descriptions and the able and masterly manner in which the author dealt with the questions relating to primitive man.

In the present work Sir John Lubbock has adopted the same inductive method of reasoning which has been so ably applied to geological investigations by the illustrious Lyell in his Principles,' viz. that of explaining the monuments of the earth's past history by the "living present." Thus, from the habits and customs of modern savages we are enabled to understand the meaning and uses of the various relics of early man met with in civilized countries where no primitive races now exist, and we can thus more accurately picture and more vividly conceive the manners and customs of our ancestors in bygone ages.

Founded upon a course of lectures, originally delivered at the Royal Institution in 1868, the author proposes in the present volume "more particularly to describe the social and mental condition of savages, their art, their systems of marriage and of relationship, their religions, language, moral character, and laws." Sir John promises in a future volume to publish those portions of his lectures which have reference to their houses, dress, boats, arms, implements, &c.

"The study of the lower races of man," writes the author, "apart from the direct importance which it possesses in an empire like ours, is of great interest from three points of view. In the first place, the condition and habits of existing savages resemble in many ways, though not in all, those of our own ancestors in a period now long gone by; in the second, they illustrate much of what is passing among ourselves, many customs which have evidently no relation to present circumstances, and even some ideas which are rooted in our minds, as fossils are imbedded in the soil; and thirdly, we can even, by means of them, penetrate some of that mist which separates the present from the future."

On the subject of savage intellect, it seems difficult to realize the extreme mental inferiority of the lower aborigines; the mind of the savage, like that of the child, is of wonderfully small capacity and limited in its powers of taking in ideas; it is easily fatigued by exercise, and is generally in a dormant state. Curious instances

*The Origin of Civilization, and the Primitive Condition of Man' (Mental and Social Condition of Savages). By Sir John Lubbock, Bart., M.P., F.R.S., &c. 8vo. Pp. 380. London, 1870. Longmans and Co. † Originally published (in part) in the Natural History Review.'