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specific gravity produced by the rolling up of the wires, their extension by weight between the first determination of specific gravity and the first determination of resistance, the irregularity in their cross section produced by stretching, and the slight contraction of the wires after the removal of the weights and before the second determination of specific gravity,-all of which, however, must have been exceedingly slight. It seems to warrant the statement that if tension has any effect upon silver wires at all the effect is exceedingly small. This differs from Mousson's conclusion as to steel, iron, and copper wires. He found that the increase in their resistance produced by stretching was not fully accounted for by the change of their dimensions.

In the course of the experiments I found that by raising a silver wire, which had been stretched, to a red heat, its resistance was very slightly diminished. A wire of about the dimensions of No. III., which, after having been stretched by 6985 grms. had a resistance of 1.8135, had, after being heated red hot, a resistance of 1-8103. This is again different from what Mousson has found to be true of steel, iron, and copper wires, but it agrees with a determination made by Becquerel on silver wires.*

The following tables contain series of observations made for the purpose of finding the relation between the stretching weight and the total increase in the resistance of the silver wires used. In these determinations, the constant resistance with which the resistances of the stretched wires were compared was that of a silver wire. Both wires were surrounded by a coating of steam. The stretched wire, in order that, by its being kept at a high temperature, greater elongations might be produced by the same weights; the constant wire in order that thermo-electric effects might be eliminated. The steam coating was formed by enclosing the wires in glass tubes, and these tubes in a much larger tube, and conducting steam between them. In other respects the apparatus and mode of procedure were quite the same as before. The observations were made when the appended weights had ceased to produce any appreciable elongation, and with the steam coating half-anhour was generally found to be a sufficient length of time for the production of the total stretching effect.

* "Ann. de Chimie et de Physique" (3), xvii. 1846, p. 253.

VOL. IX.

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* This measurement is marked in my notes as "inaccurate, owing to an error of observation."

It will be seen that the relation between appended weights and thereby increased resistances is not that of simple proportion. In this respect silver wires appear again to differ from copper wires. Some experiments made by Messrs Meik and Murray having shown that the changes of resistance of copper wires, when stretched by weights, are directly proportional to the weights.

*

I am deeply indebted to Professor Wiedemann of Leipzig, in whose laboratory these experiments were performed, for the excellent apparatus which he kindly placed at my disposal, and for the advice and assistance with which he favoured me.

2. On the Defoliation of the Coniferæ. By Dr Stark.

3. On Diamagnetic Rotation. By George Forbes, Esq., M.A., F.R.A.S.

Faraday's discovery of the magnetic rotatory polarisation of light may be expressed in the following manner:-Let two electromagnets, in the form of iron tubes, surrounded by helices of wire, be placed end to end, so that in the space between them the lines of force are very intense. Let a rod of dense glass be placed in this space, so that a ray of light may pass through the two tubes and the rod of glass. Let such a ray on entrance be plane-polarised, so that the direction of vibration is in a vertical direction. If the electro-magnet be now magnetised, the emergent ray will be polarised, so that its vibrations are inclined to the vertical at a small angle. The direction in which the line of vibration has been rotated is the same as the direction of the positive current in the helices.

The same effect might be produced without the aid of magnetism if the rod were rotated round the axis of the ray of light with great velocity. The rotation of the plane of polarisation

* Proc. Roy. Soc. Edin., Session 1869-70, p. 3.

would be the angle rotated whilst the light traverses the glass rod. This is on the assumption that the ether within the rod is likewise rotated. In the magnetic experiment it is easy to produce a rotation of 1° in a piece of glass three inches long. Light takes 7.000.000.000 of a second to traverse this distance. Hence, to produce an effect equal to the magnetic effect, the glass rod would require to be rotated 10,000,000 times in a second. We cannot determine with great precision the plane of polarisation of a ray of light, hence we cannot measure any rotation of the plane of polarisation which might be thus produced.

In the same manner, if we suppose the molecules of glass and the accompanying ether to be rotated round the lines of magnetic force, and in the direction of the positive current producing the given magnetic field, then the phenomena observed by Faraday would be explained; and we should be able to determine the number of rotations per second induced in any specimen of glass with a given intensity of magnetic field.

Hence we

So soon as the electro-magnet is demagnetised, the rotation of the molecules ceases. It seems, then, that there is a friction among the molecules tending to stop the rotation. should be led to conclude that the energy of the magnet is gradually used up by the presence of the piece of glass.

Assuming, then, that there is a friction among the molecules of glass, it follows that when the electro-magnet is magnetised the rod of glass has a tendency to turn bodily round an axis through its centre of gravity parallel to the lines of force; and if the rod of glass were free it would turn round this axis.

In the winters 1872-3 and 1873-4, I made a number of experiments to put this hypothesis to the test. The general idea of the experiments was this. A rod of glass was suspended by a fine skein of silk fibres between two poles of an electro-magnet, one pole above, the other below. A small mirror was fixed on the rod, and a lamp and scale arrangement was mounted for measuring rotations. Readings were taken when there was no current, and also in the two positions of the commutator.

The result of these experiments seems to be that there is an effect of the kind anticipated. Sometimes, it is true, a deflection was produced by a diamagnetic repulsion, owing to a want of

absolute symmetry in the arrangements. This rotated the glass rod slightly, and produced a little confusion. But beyond this there was an undoubted effect, for the nature of the deflection was found to depend upon the polarity of the two poles, the effect being different according as the upper pole was a north or a south one. I was sometimes able, simply by timing the reversals of the commutator, to get up a very large rotation-swing, and by the same means I was able to stop the rotation swinging.

I delayed the publication of these experiments with the view of establishing more certain results. But much time has elapsed, and I have still been unable to find time for this. Hence, I am unwilling any longer to withhold their publication. I will now give the experiments in detail, and will conclude by collecting the general results.

Description of the Apparatus,

A rod of glass was suspended by a strand of silk fibres attached to one end. This was supported on a stone imbedded in the wall of the laboratory. The rest of the apparatus was on a separate stand. A horse-shoe electro-magnet was placed so that the poles were vertically over each other, and as nearly above and below the axis of the glass rod as possible. Thus the axis of the glass rod lay along the lines of magnetic force. The electro-magnet was connected through a commutator with a battery of Grove cells. Upon the glass rod was attached a small piece of silvered glass, by means of which the light reflected from a paraffin lamp fell upon a scale divided to millimetres. The distance of the scale from the glass rod was about seven decimetres. The apparatus was arranged so that the glass rod was suspended within a glass jar or bottle, to get rid of currents of air.

During the session 1872-73 thick flint glass tubes were employed for suspension, and an electro-magnet weighing about 6 lbs. After that a rod of Faraday's heavy glass was used, and an electro-magnet weighing about 50 lbs.

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