We

the case of carbonic acid, and 40° in the case of hydrogen. see by this that the electric conductivity of the rarefied gas contained in the axial tube is notably augmented by the magnetizing. At the same time we ascertain that the effect is not the same in the different gases: it is maximum in hydrogen, minimum in air; the order of the three gases is the same here as in the experiments with the transverse discharge. The less the pressure, the more marked is the effect; at millim. pressure we have seen the deflection of the galvanometer pass, in consequence of the magnetizing, from 15° to 26° with air, to 30° with carbonic acid, and to 38° with hydrogen: it is seen that the intensity of the current was more than doubled in hydrogen.

2nd position. The negative electrode is in the immediate vicinity of one of the magnetic poles. The modification induced in this case in the appearance of the electric jet is much more remarkable than in the preceding. In proportion as the pressure diminishes below 2 millims., the luminous envelope which surrounded the negative electrode lengthens more and more, invading the dark space; at the lowest pressures we have been able to attain, the negative part of the jet formed at last a very elongated frustum of a cone, filling the whole interval between the two electrodes, the positive part having been driven back into the interior of the soft-iron cylinder. In this position of the tube the increase observed in the conductivity of the rarefied gas is a little less than that obtained in the first position. By slowly moving the tube, it may be made to take all the positions between the two on which we have particularly dwelt. Starting from the second position, we see the negative luminous cone shorten more and more, then give place to the positive dart, which advances as far as behind the negative electrode. The direction of the magnetization has no influence, either on the increase of conductivity or on the appearance of the electric jet.

M. de la Rive had already, in his study on the same subject (cited at the beginning of our memoir), described a special case in which the action of magnetism appeared to him to increase the electric conductivity of the gas instead of diminishing it it was where the discharge was transmitted through a spiral tube placed in a peculiar manner between the poles of the electromagnet; but he did not dwell on it, reserving the study of it for a future memoir*.

IV. The Action of Magnetism on the Electric Jet when this is rotated continuously round the pole of the Electromagnet.

The rotatory movement of the jet may either be performed in a plane perpendicular to the axis of the electromagnet which * Archives des Sci. Phys. et Nat. Dec. 1866, vol. xxvii. p. 296.

produces the rotation (which takes place when the spark strikes between a metallic ring perpendicular to the axis of the magnet and an electrode placed in its centre in the continuation of that axis*), or take place vertically round the axis of a small cylinder of soft iron magnetized by contact with one of the poles of the electromagnet, and of which the extremity constituted one of the electrodes.

In the first case there is no sensible variation of conductivity in the gas when the jet is put in movement by the action of magnetism; the conductivity remains exactly what it was when the jet was not under the influence of the magnet and consequently was at rest. It is the same when, aqueous or alcoholic vapour being introduced into the rarefied gas, under the magnetic influence the jet, previously single, is divided into several resembling the spokes of a wheel.

With aqueous vapour the medium at the same degree of tension is more conductive than the dry gas; with the vapour of alcohol it is less so, in the same conditions; but with neither of these two vapours, any more than with the dry gas, does the magnetism, when it determines the rotation of the jet, influence the electric conductivity.

It is quite different when the jet describes round the rod of magnetized soft iron a cylinder whose axis is that of the rod. In this case there is a very sensible increase of resistance to the electric conduction when the jet, instead of being motionless, rotates from the effect of the magnetism. But this increase is sensibly greater when it is positive electricity that issues from the apex of the soft iron than when it is negative. Thus in the first case we have seen the deflection of the galvanometer diminish from 65° to 45°, while in the second case it diminished only from 65° to 55°. Let us remark that in the case in which the diminution of conductivity is the greatest the rotation of the jet seems to be effected with more difficulty, in the same conditions of intensity of the electric discharge, intensity of magnetism, and rarefaction of the gaseous medium, which is simply atmospheric air at 4 millims. pressure: not only is the rotation much less rapid, but the jet itself, instead of remaining vertical, takes during its rotation an inclined position; this is observed to a certain degree in the other case, but is much more pronounced in that in which the conductivity is most diminished.

It would therefore seem that this diminution of conductivity corresponds to the constrained position which the electric jet is forced to take under the influence of the magnetizing in the case in which it is naturally vertical; whereas, when it is naturally * In these experiments the magnet was arranged as a vertical column, instead of in a horseshoe-form as in the preceding experiments.

horizontal and turns in the same direction as the hands of a watch, the magnetism merely impresses on it a continuous movement of rotation, without making any alteration in its form, its direction, or its appearance.

Conclusions.

It follows from the experiments described in this memoir :— 1. That the action of magnetism, when it is exerted only on a portion of an electric jet transmitted through a rarefied gas, determines in that portion an increase of density.

2. That the same action, when it is exerted on an electric jet placed equatorially between the poles of an electromagnet, produces in the rarefied gas in which it is propagated an increase of resistance which is as much greater as the gas itself is more conductive.

3. That this action, on the contrary, determines a diminution of resistance when the jet is directed axially between the two magnetic poles, this diminution being as much greater as the gas is more conductive.

4. That when the action of the magnetism consists in impressing a continuous movement of rotation on the electric jet, it has no influence on the resistance to conduction, if the rotation is effected in a plane perpendicular to the axis of the magnetized soft-iron cylinder which determines the rotation; while it notably diminishes it if the rotation takes place so that the electric jet describes a cylinder round the axis of the rod.

5. That these different effects apparently cannot be attributed to variations of density produced in the gaseous medium by the magnetic action, but very probably their explanation will be found in the perturbation induced by that action in the arrangement (or disposition of the particles of the rarefied gas) necessary for the propagation of electricity.

XXVII. Proceedings of Learned Societies.

ROYAL SOCIETY.

[Continued from p. 155.]

May 25, 1871.-General Sir Edward Sabinė, K.C.B., President, in the Chair.

THE following communication was read :

:

"Note on the Spectrum of Uranus and the Spectrum of Comet I., 1871." By Willian Huggins, LL.D., D.C.L., V.P.R.S. In the paper "On the Spectra of some of the Fixed Stars"*, presented conjointly by Dr. Miller and myself to the Royal Society Phil. Trans. 1864, p. 413; and for Mars, Monthly Notices R. Astr. Soc. vol. xxvii. p. 178.

in 1864, we gave the results of our observations of the spectra of the planets Venus, Mars, Jupiter, and Saturn; but we found the light from Uranus and Neptune too faint to be satisfactorily examined with the spectroscope.

By means of the equatorial refractor of 15 inches aperture, by Messrs. Grubb and Son, recently placed in my hands by the Royal Society, I have succeeded in making the observations described in this paper of the remarkable spectrum which is afforded by the light of the planet Uranus.

It should be stated that the spectrum of Uranus was observed by Father Secchi in 1869*. He says, "le jaune y fait complétement défaut. Dans le vert et dans le bleu il y a deux raies très-larges et très-noires." He represents the band in the blue as more refrangible than F, and the one in the green as near E.

The spectrum of Uranus, as it appears in my instrument, is represented in the accompanying diagram. The narrow spectrum placed above that of Uranus gives the relative positions of the principal solar lines and of the two strongest absorption-bands produced by our atmosphere-namely, the group of lines a little more refrangible than D, and the group which occurs about midway from C to D. The scale placed above gives wave-lengths in millionths of a millimetre.

The spectrum of Uranus is continuous, without any part being wanting, as far as the feebleness of its light permits it to be traced, which is from about C to about G.

On account of the small amount of light received from this planet, I was not able to use a slit sufficiently narrow to bring out the Fraunhofer lines. The positions of the bands produced by planetary absorption, which are broad and strong in comparison with the solar lines, were determined by the micrometer and by direct comparison with the spectra of terrestrial substances.

The spectroscope was furnished with one prism of dense flintglass, having a refracting-angle of 60°, an observing telescope magnifying 5 diameters, and a collimator of 5 inches focal length. A cylindrical lens was used to increase the breadth of the spectrum.

The remarkable absorption taking place at Uranus shows itself in six strong lines, which are drawn in the diagram. The least * Comptes Rendus, vol. lxviii. p. 761, and 'Le Soleil,' Paris, 1870, p. 354.

refrangible of these lines occurs in a faint part of the spectrum, and could not be measured. Its position was estimated only, and on this account it is represented in the diagram by a dotted line. The positions of the other lines were obtained by micrometrical measures on different nights. The strongest of the lines is that which has a wave-length of about 544 millionths of a millimetre. The band at 572 of the scale is nearly as broad but not so dark; the one a little less refrangible than D is narrower than the others.

The measures taken of the most refrangible band showed that it was at or very near the position of F in the solar spectrum. The light from a tube containing rarefied hydrogen, rendered luminous by the induction-spark, was then compared directly with that of Uranus. The band in the planet's spectrum appeared to be coincident with the bright line of hydrogen.

Three of the bands were shown by the micrometer not to differ greatly in position from some of the bright lines of the spectrum of air. A direct comparison was made, when the principal bright lines were found to have the positions, relatively to the lines of planetary absorption, which are shown in the diagram. The band which has a wave-length of about 572 millionths of a millimetre is less refrangible than the double line of nitrogen which occurs near it. The two planetary bands at 595 and 618 of the scale appeared very nearly coincident with bright lines of air. The faintness of the planet's spectrum did not admit of certainty on this point; I suspected that the planetary lines are in a small degree less refrangible. There is no strong line in the spectrum of Uranus in the position of the strongest of the lines of air, namely the double line of nitrogen.

As carbonic acid gas might be considered, without much improbability, to be a constituent of the atmosphere of Uranus, I took measures with the same spectroscope of the principal groups of bright lines which present themselves when the induction-spark is passed through this gas. The result was to show that the bands of Uranus cannot be ascribed to the absorption of this gas.

There is no absorption-band at the position of the line of sodium. It will be seen by a reference to the diagram that there are no lines in the spectrum of Uranus at the positions of the principal groups produced by the absorption of the earth's atmosphere.

Spectrum of Comet I., 1871.

On April 7 a faint comet was discovered by Dr. Winnecke. I observed the comet on April 13 and May 2. On both days the comet was exceedingly faint, and on May 2 it was rendered more difficult to observe by the light of the moon and a faint haze in the atmosphere. It presented the appearance of a small faint coma, with an extension in the direction from the sun.

When observed in the spectroscope, I could detect the light of the coma to consist almost entirely of three bright bands.

A fair measure was obtained of the centre of the middle band, which was the brightest; it gives for this band a wave-length of about 510 millionths of a millimetre. I was not able to do more than estimate roughly the position of the less refrangible band. The result Phil. Mag. S. 4. Vol. 42. No. 279. Sept. 1871.

Q