substances, such as sulphate of iron, very appreciable induced currents are obtained, which shows the sensibility of the apparatus; with regard to non-magnetic metals, the intensity of the currents induced during the movement, when the plate is very near to the line of the poles, shows that the effects depend only on the conductibility of the metals, and not at all on their diamagnetic power. M. Verdet, in order to analyse the phenomenon in its details, added to the machine a commutator, which allowed the current to reach the galvanometer only during the second part of a rotation of the plate; he thus recognised, as Faraday had done, the influence of time upon the induction, which explains why the induced currents are not distributed in a symmetrical manner, during the period when the plate is withdrawing from the line of the poles, and during the period when it is approaching it; a dissymetry, which is the more marked in proportion as the velocity is greater.

We see, therefore, that we must renounce the idea of a diamagnetic polarity, analogous, but of a contrary name to magnetic polarity;-that it is in like manner impossible to admit the existence in diamagnetic bodies of a polarity similar in all respects, save in regard to its intensity, to that of magnetic bodies. It therefore appears to us probable that the force which impels substances when they are suspended freely in the field of magnetic forces, to pass from the more powerful to the more feeble points, is of a very different nature from magnetism; moreover, that it is general, and that, if magnetic bodies do not obey it, it is, that in them it is counteracted by that very special property with which they are endowed, and which we term magnetism. We have, in fact, seen that Mr. Thomson succeeded in impressing upon magnetic bodies, by a special arrangement, which neutralised the effect of their magnetism, a position similar to that acquired by diamagnetic bodies under the same circumstances. The point, in our opinion, upon which the attention of the philosopher should be directed, should therefore be a careful study of the magnetic field, and, consequently, a minute analysis of the forces by which it is traversed, and of the

circumstances that may make these forces vary, in order, if possible, to arrive by these means at the discovery of the cause which leads them to act upon bodies.

Let us add, further, that it would be important to study, better than has hitherto been done, the conditions arising from the very constitution of bodies, in regard to the form, the molecular arrangement, and the chemical constitution which renders the action exercised upon them by these forces more or less energetic. The influence of temperature would be valuable to know; M. Plucker has already proved that a mass of bismuth weighing 2223 grains Troy, required at the ordinary temperature a weight of 25.78 grains Troy to counterbalance the effect of diamagnetic repulsion; whilst it required no more than 4:32 grains Troy, when the temperature was raised to the point of the fusion of the metal. The solid or liquid state exercises no change over this property; ice, according to the observations of M. Brunner, Jun., is as diamagnetic as water, whether in a state of liquid or of vapour. These facts, and others also, are neither sufficiently numerous, nor sufficiently in accordance, to enable us to draw from them any general consequence; it is possible we may arrive at some such, when they shall have been more multiplied.

Influence of Magnetism upon Flames and Gases, and upon Liquids.

Mr. Faraday had thought he was able to conclude from his first experiments that air and the different gases do not differ from each other in regard to their magnetic or diamagnetic properties, and that they ought, in this respect, to be placed, as well as vacuum, almost in the middle of the scale, that is to say, between magnetic and diamagnetic bodies. I was induced to remark that the results obtained by Mr. Faraday were perhaps only apparent, and that they simply depended upon the material of the tube in the interior of which the vacuum or the gas was placed, as well as upon the influence of the surrounding medium. A curious experiment made at Genoa by Father Bancalari had demonstrated that elastic

fluids are not so indifferent as might have been supposed to magnetic action. M. Bancalari found that the poles of an electro-magnet have a decided repulsive action upon the flame of a lamp, upon smoke, as well as upon the vapour of water and of alcohol. M. Zantedeschi, in repeating and confirming these experiments, proved that flame is repelled equally by each of the poles:that the effect is not due to currents of air, that the repulsion is accompanied by a depression of the flame. The same philosopher further observed that the smoke that rises from the snuff of an extinct flame, fed by oil, alcohol, or wax, is subject to the same repulsive force.

Mr. Faraday, as soon as he became acquainted with these experiments of MM. Bancalari and Zantedeschi, took up, by a new method of experimenting, his researches upon gases, and arrived at results that showed him, conformably to what he had thought, that elastic fluids are not insensible to the action of the magnet, but, contrary to his former experiments, that there exist sensible differences between different elastic fluids with regard to their magnetic or diamagnetic properties.

He first satisfied himself that hot air is powerfully diamagnetic in respect to cold air. He arrived at this result by placing between the two poles of the electro-magnet, but a little below their surface, a helix of platinum, rendered powerfully incandescent by an electric current. So long as the electro-magnet was not magnetised, the current of hot air rose regularly between the two poles; but immediately magnetisation was produced, it was perceived by means of the thermometer, and even simply by the sensation experienced by the fingers, that the ascending current of hot air divided itself into two currents, mounting separately on the two sides of the axial line; and that there was between them a current of cold air, descending between the poles. The converse experiment was made; namely, by passing a current of air into a tube, surrounded by a freezing mixture; and it was found, by means of a thermoscope placed below the poles of the electro-magnet, that this current was carried upon the axial

line; a further proof that cold air is more magnetic than hot air.

In order to operate upon the different gases, and to discover the direction they assume in the magnetic field, Mr. Faraday employed glass tubes open at both ends, about in. ៖ wide, and 2 in. long, arranged in different ways around, above, or below the poles of the electro-magnet, and containing within them a piece of paper moistened with ammonia. Each gas, when submitted to experiment, was itself mixed with a very slight quantity of muriatic acid, a quantity not sufficient of itself to give white vapours in air, but capable of producing them by its mixture with ammonia. In this way, the appearance of the white cloud indicated in which tube the gas had passed; whence it was easy to conclude what direction it had followed, and, consequently, if it were magnetic or diamagnetic in respect to the surrounding medium. A current of oxygen, which descended vertically between the poles, was in no way affected by the magnetisation of the electro-magnet; but the current of the gas having been slightly displaced and put outside the axial line, the oxygen was seen, under the magnetic influence, to approach this line and to descend in the tube placed directly below, and not in that in which it had previously descended. Thus oxygen is powerfully magnetic in respect to air. The reverse is the case for nitrogen. It was also found with hydrogen, notwithstanding the difficulty of operating with it, on account of its specific lightness, that it is very diamagnetic in ordinary air. He did the same, in respect to carbonic acid, which places itelf in the equatorial direction in a very decided manner. Lime water, by becoming white, indicated in an elegant manner into which tube the carbonic acid was directed. All the other gases, to the number of fifteen, that were submitted to experiment were found by Faraday to be diamagnetic, with the exception, perhaps, of nitrous gas, which appeared to be magnetic.

M. E. Becquerel, when observing the repulsion exercised by the poles of the electro-magnet upon different bodies, such as cylinders of wax or glass, placed in vacuum or in different

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gases, succeeded in recognising, as Mr. Faraday had done, the magnetic power of oxygen. The fraction which, according to M. Becquerel's accurate experiments, expresses the specific magnetism of this gas for equal masses, in respect to soft iron, places it among the most powerfully magnetic fluids; in fact, concentrated solution of protochloride of iron, the most magnetic liquid known, is nearly three times less attracted than oxygen for equal weights. The magnetic power of oxygen increases with the elastic force; and the effect appears to be very exactly proportional to this force, and consequently to the quantity of material particles contained under a given volume. Atmospheric air presents the same effects as oxygen, and nearly in the same intensity; which latter is less on account of the presence of nitrogen. M. E. Becquerel has made some further experiments upon the gases, by condensing them by means of fragments of charcoal, which he then suspends between the poles of the electro-magnet: he has again proved by this method the magnetism of oxygen, which, by its presence in the pores of carbon, renders this substance magnetic from being diamagnetic, which it is naturally; whilst other gases, condensed in the same manner, increase its diamagnetic power. With regard to nitrogen and hydrogen, they do not become sufficiently condensed in carbon to enable us to detect any sensible effect within the limits of the observations.

Faraday conceived a very elegant and very sensitive method of showing the magnetic and diamagnetic properties of gases; it consists in employing soap-bubbles of the gas that is to be experimented upon. When placed in the magnetic field, they are attracted or repelled according as the gas is magnetic or diamagnetic in respect to air; thus a bubble filled with oxygen is powerfully attracted. But, in order to operate in a more convenient and accurate manner, Faraday replaced the soap-bubbles by little balls of very thin glass, into which the gas is introduced; he places two of these balls, each filled with a different gas, upon one of the extremities of a horizontal wooden lever, suspended delicately to a torsion thread; and he so arranges his apparatus, that