diamagnetic, but the line of compression, instead of setting equatorial as in the former instance, set decidedly axial.

A quantity of the mixed powder was next taken, in which the magnetic constituents amounted to 1 per cent. The mass was still diamagnetic, but the line of compression set axial; it did so when the influence of exterior form was quite neutralised, so that the effect must be referred solely to the compression of the mass. With 2 per cent. of carbonate of iron powder the mass was magnetic, and set with increased energy its line of compression axial; with 4 per cent. of carbonate of iron the same effect was produced in a still more exalted degree.

Now, why should the addition of a quantity of carbonate of iron powder, which is altogether insufficient to convert the mass from a diamagnetic to a paramagnetic one, be able to overturn the tendency of the diamagnetic body to set its line of compression equatorial? The question is puzzling at first sight, but the difficulty vanishes on reflection. The repulsion of the mass of bismuth, suspended before a pointed pole, depends upon the general capacity of the mass for diamagnetic induction, while its position as a magne-crystal between the flat poles depends on the difference between its capacities in two different directions. The diamagnetic capacity of the mass may be very great while its capacity in different directions may be nearly alike, or quite so: the former, in the case before us, came into play before the pointed pole; but between the flat poles, where the directive, and not the translative energy is great, the carbonate of iron powder, whose directive power, when compressed, far exceeds that of bismuth, determined the position of the body. In this simple way a number of perplexing results obtained with bodies formed of a mixture of paramagnetic and diamagnetic constituents, is capable of satisfactory explanation.

Finally, inasmuch as the set of the mass in the magnetic field depends upon the difference of its excitement in different direetions, it will follow that any circumstance which affects all directions of a magne-crystallic mass in the same degree will not disturb the differential action upon which its deportment depends. This seems to me to be the explanation of the results recently obtained by Mr. Faraday with such remarkable uniformity, namely, that, no matter what the medium may be in which the magne-crystallic body is immersed, whether air or

liquid, paramagnetic or diamagnetic, it requires, in all cases, the same amount of force to turn it from the position which it takes up in virtue of its structure.*

I have thus dwelt upon instances of magne-crystallic action which have revealed themselves in actual practice, as affording the best examples for the application of the knowledge which the demonstration of the polarity of the diamagnetic force places in our possession; and I believe it has been shown that these phenomena, which were in the highest degree paradoxical when first announced, are deducible with as much ease and certainty from the action of polar forces, as the precession of the equinoxes is from the force of gravitation. 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.

ROYAL INSTITUTION:
December, 1855.

I need hardly draw attention to the suggestive beauty of this experiment.-J. T., 1870.

LETTERS, ESSAYS, AND REVIEWS

RELATING TO

MAGNETISM AND ELECTRICITY.

The following pages embrace letters on diamagnetism, some of which are of permanent interest; a number of essays, to be regarded in part as lecture-room summaries, useful, I trust, to the practical scientific teacher; two reviews relating to electricity, and extracted from a series which under the title Reports on the Progress of the Physical Sciences,' were written in those early days for the Philosophical Magazine.' The volume ends with a memoir on Electro-magnetic Attractions, intended to prepare me for the investigation of diamagnetic phenomena, but which of itself constituted a healthful discipline at the time.

201

I.-FARADAY ON MAGNETIC POLARITY.

[In the Philosophical Magazine' for February 1855, Faraday published an exceedingly interesting paper On Some Points of Magnetic Philosophy,' in which he discusses the question of Diamagnetic Polarity, and also describes some beautiful experiments relating to the influence of various media surrounding magnetic and diamagnetic bodies. He then applies his experiments to support a theory of diamagnetic polarity similar to that advocated by Edmond Becquerel, and referred to at page 47 of this book. The portion of Faraday's paper necessary to explain what follows is here given.-J. T., 1870.]

THE meaning of this phrase is rapidly becoming more and more uncertain. In the ordinary view, polarity does not necessarily touch much upon the idea of lines of physical force; yet in the one natural truth it must either be essential to, and identified with it, or else absolutely incompatible with, and opposed to it. Coulomb's view makes polarity to depend upon the resultant in direction of the action of two separated and distant portions of two magnetic fluids upon other like separated portions, which are either originally separate, as in a magnet, or are induced to separate, as in soft iron, by the action of the dominant magnet;-it is essential to this hypothesis that the polarity force of one name should repel polarity force of the same name and attract that of the other name. Ampère's view of polarity is, that there are no magnetic fluids, but that closed currents of electricity can exist round particles of matter (or round masses), and that the known experimental difference on the opposite sides of these currents, shown by attraction and repulsion of other currents, constitutes polarity. Ampère's view is modified (chiefly by addition) in various ways