bably not exactly that of the wire of measured resistance. The error, however, must have been very slight.

The course of procedure was as follows:-The wire was heated red hot in an alcohol flame. After cooling, its specific gravity was determined. It was then fastened by the copper plates to the thick copper wires, and a slight weight was attached just sufficient to straighten the wire, that its length might be accurately ascertained (it was straightened as much as possible by being drawn between the fingers before being fastened in the apparatus). When straight enough for the determination of its length, its resistance was also measured by the method of double observation, as described in the "Galvanismus" (see above).* After the determination of resistance, weights were carefully piled upon the carrier, which during the operation was held fast from below. They were then allowed to stretch the wire gradually until it hung quite free, and its elongation had ceased. Then the length, the resistance, and finally the length a second time, were determined, the second measurement of length being made in order to be certain that there had been no further elongation. The weights were now carefully taken off, the carrier being again supported from below, and the specific gravity of the wire was measured as before, the compressed ends, however, having been cut off. A large number of experiments were rendered useless by the fact that too great weights were attached. Either the wires broke, or they were found on inspection to have too variable a diameter to be regarded as uniform. In the result given below such small weights were used that almost no difference of diameter throughout the whole length of the wire could be noticed by means of a magnifying instrument. Thus the wire could be treated as uniform, and the specific gravity method assumed to give its diameter.

The results of the examination of three wires are given in the following table:

Instead of the formula given in "Galvanismus,” the following was used:— s3+s2 (3d12− dɩ ~ d2) +8(d¿d12+d,d ̧1⁄2 – 3d ̧d ̧) — d ̧dd12 =0.

The length of the German-silver wire as found by this formula was 1108-795 As measured by the cathetometer its length was 1108.8 mm.

mm.

These results agree, as might be expected, with those which Mousson has published on steel, iron, and copper wires, in the fact that the resistance increases very much faster than the length. This must be the case unless there be a diminution of resistance, due to tension, sufficient to neutralise the increase of resistance due to decrease of the cross section of the wire. It is interesting to ask, then-Does the decrease in the diameter of the wire account for that part of the increase of its resistance which is not due to the increase of its length? The following table answers this question. The column headed "calculated resistance" contains the resistance as it ought to have been if its increase had been due only to change of dimensions:

The agreement of the figures in the observed and calculated columns is very close, notwithstanding the many sources of error to which the experiments were liable, such as the change in

* See Wiedemann's "Galvanismus," vol. i. p. 255.

+

Galvanismus," vol. i. p. 310; "Neue Schweizerische Zeitschrift," vol. xiv. (1855), p. 33.

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.

VOL. IX.

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

M

* 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.