may also be studied, and the remarkable effect produced by connecting the two ends of one of the wires, or the spark of the extra-current in the other, may be exhibited.

The milled head a, fig. 7, can be screwed off, and the rod ac pushed downwards into the case; the helix in the centre can also be readily lifted out of its position and removed from the glass case--we will suppose this done. The two electro-magnets alone are now within the case, and the view is uninterrupted, which would not be the case if the helix had been permitted to remain.

The fibre hanging from the groove d, fig. 7, can be so arranged that any substance attached to it shall hang between the moveable masses of soft iron which surmount the electro-magnets, and the same arrangement can be made for the fibre suspended from the groove d'.

But a body suspended between the movable masses of soft iron would be hidden by these masses from the audience, and hence to render the motions of the body visible the following expedient was adopted :-Fig. Sa represents a thin index of ivory about 4 inches long, and shaped as in the figure; from the centre depends the stem dc, which is terminated by a tongs-shaped arrangement which can clasp the body to be submitted to experiment; to the right of the index a section of the little ivory pliers, by a plane passing through the stem at right angles to in, is given; the stem is slit up to a, so as to allow the pliers being opened to receive the body to be examined, which they then clasp in virtue of the elasticity of the ivory. The stem de is of such a length, that when the body is in the centre of the space between the poles, the index i n is seen above them; and, as the index follows all the motions of the body underneath, these motions are recognised by all who see the index.

5. If an ordinary magnetic bar, sufficiently feeble, be suspended between one pair of poles, and an ordinary diamagnetic bar between the other pair, on sending the same current round both magnets, the index of the former sets itself parallel to the polar line, while the index of the latter sets itself perpendicular to the polar line, and thus the phænomena of magnetism and diamagnetism address the eye simultaneously.

6. In the same way, if a normal magnetic bar be suspended between one pair of poles, and an abnormal magnetic bar

between the other, the antithesis of their deportment may be made manifest. The same antithesis is exhibited when we compare a normal diamagnetic bar with an abnormal one.

7. And when between one pair of poles is suspended a normal magnetic bar, and between the other pair an abnormal diamagnetic one, the apparent identity of deportment of both bars is rendered evident at once. The same identity is shown when we compare the abnormal magnetic bar with the normal diamagnetic one.

8. Causing the points to face each other, instead of the flat ends of the poles, and observing the directions given in the paper spoken of, the curious phænomena of rotation on raising or lowering the body from between the points, first observed by M. Plücker, and explained in the paper referred to, may be exhibited.

9. To show that a bar of bismuth, suspended within a helix and acted upon by magnets, presents phænomena exactly analogous to those of soft iron, only always in opposite directions, let the flat helix be replaced between the two electromagnets. The bar of bismuth used in experiments with the instrument now described is 6 inches long and 0.4 of an inch in diameter. Suspended so as to swing freely within the helix, its ends lie between the movable masses of iron which rest upon the electro-magnetic cores. Four poles are thus brought simultaneously to bear upon the bar of bismuth, and its action is thereby rendered both prompt and energetic. The two poles to the right of the bar must both be of the same name, and the two to the left of the bar of the opposite quality. If those to the right be both north, those to the left must be both south, and vice versa. On sending a current from 10 or 15 cells round the helix, and exciting the magnets by a battery of 4 or 5 cells, the current reversers place the deflections of the bar entirely under the experimenter's control. By changing the direction of the current in the helix by means of its reverser, a change of deflection is produced; the same is effected if the polarity of the magnets be changed by the reverser which belongs to them. For a full description of all these phænomena I must refer the reader to the paper on the nature of the diamagnetic force already mentioned.

10. To those acquainted with what has been done of late

years in diamagnetism, numerous other experiments will suggest themselves. The antithesis of two isomorphous crystals, one magnetic and the other diamagnetic, the general phænomena of magnecrystallic action, and the analogous effects produced by pressure, may all be exhibited.

By placing one of the helices of the electro-magnet upon the other, a coil of double length is obtained, and two such coils may be formed from the four which we have described. For the additional expense of the iron merely, a single electromagnet, far more powerful than either of the others, because excited by twice the quantity of coil, may be obtained.

I think it would be an improvement if the suspensions were independent of the glass case, so as to permit of the entire removal of the latter. The best way of showing the deflection of the bismuth bar within the central helix to a large audience, is to attach a long, light index to the bar itself, and permit this index to enter a French shade which will protect it sufficiently from currents of air. With this arrangement the motions are strikingly evident, and may be seen by hundreds at once. The instrument above described was constructed by Mr. Becker, of Newman Street, and its cost is about twentyfour pounds. It was not my intention originally to have so much wire round the electro-magnets; and the effects may also be made manifest with a smaller central coil. I have no doubt that with 8 lbs. of wire round each limb of the electromagnets, and a central coil weighing 4 lbs., the experiments might be exhibited to a large audience with perfect distinctness. A sensible diminution of cost would of course accompany this diminution of material and labour.

XVIII.-ON MAGNUS'S INVESTIGATION OF THERMOELECTRIC CURRENTS.*

EXACTLY thirty years have flown by since the discovery of thermo-electricity by Seebeck in Berlin. Since that time our knowledge of facts in connection with this subject has been enriched by the labours of Becquerel, Sturgeon, Matteucci, Henrici, and others; but our advance towards principles has been slow. Indeed, some of the facts at present generally accepted are of so incomprehensible a nature; the results of various experimenters—and even of the same experimenter at different times-are so perplexing and contradictory, as pressingly to indicate the necessity of further and stricter examination. In the production of thermo-currents and the determination of their directions, so many hidden influences come into play, that if one subject more than another require the exercise of patience and experimental tact it is this. Until very lately every attempt at progression in this department of inquiry was accompanied by the unpleasant conviction that there was no sure starting-point; and hence he that would advance had to begin afresh, and jealously test every result of his predecessors. This is the state of things which the investigation of M. Magnus is intended to remedy, and his memoir on the subject furnishes internal evidence of the precision with which the inquiry has been conducted. The investigation is far from exhausting the subject, but it lets us know precisely where we are; new and striking facts have been added, errors have been corrected, anomalies accounted for, and the first great step made towards the reduction to law of phænomena which have hitherto perplexed philosophers.

* Philosophical Magazine, vol. iii. p. 81.

That

The wire usually applied in the construction of galvanometers often presents a difficulty in enquiries like the present. purchased at the merchants is so magnetic as greatly to interfere with the purity of the experiments. To obviate this defect, some precipitated copper was obtained from a galvano-plastic manufactory; but the metal, after having been cast into cylindrical moulds, was found so magnetic as to necessitate its rejection. The pure metal was finally obtained in the following manner: an excess of ammonia was added to a solution of sulphate of copper, the precipitated oxide being thus redissolved, and the iron mixed with the salt separated; the solution was filtered, evaporated to dryness, and the ammonia expelled; the sulphate thus procured was redissolved in water and precipitated by the voltaic current. This metal, however, was exceedingly brittle, and required to be melted eight times in succession before it could be drawn into wire; when drawn, however, it was found to answer its purpose perfectly.*

In the following pages we shall often have occasion to speak of the direction of the current, and it is therefore prudent to define clearly in the first instance what is meant by this expression. If a strip of copper and a strip of zinc be immersed in a conducting fluid, and the exposed ends be united by a copper wire, the current is said to proceed from the copper through the uniting wire to zinc, and from the zinc through the fluid to the copper. Supposing a bit of antimony to be put in the place of the copper, and a bit of bismuth in the place of the zinc, and doing away with the fluid, that the free ends of both are brought into contact and the place of contact heated; the consequent thermo-electric current will act upon a magnetic needle exactly like that generated by the zinc and copper pair. The current therefore passes from antimony through the wire to bismuth (from A to B), but from bismuth to antimony (against the alphabet) across the place of junction. Whenever it is stated in this Report that the current passes from one metal to another, the words across the place of junction' are always implied.

It was soon ascertained by Magnus that a difference in point

The magnet is an admirable test of the fitness of a wire for a galvanometer. If the wire be feebly repelled it is all right; if attracted, it ought to be rejected. See the commencement of a paper On the Absorption and Radiation of Heat by Gases and Vapours-Phil. Trans., 1861, and Phil. Mag., September 1861.-J. T., 1870.