Here it would appear that the maximum effect was produced by the green rays.

The plate of brass being oxidised the effects were as follows:

Plates exposed to iodine, chlorine, and bromine, were tried by M. E. Becquerel, and similar results obtained.

(490.) Not considering these results so satisfactory as I could desire them to be, I instituted during the summer of 1843 a series of experiments with plates of different metals, which were excited in several ways. These plates were connected with wires from the galvanometer, the different prismatic rays were passed separately through a slit in a card, and the transient and permanent deflections carefully noted. The galvanometer, which was by no means a sensitive one, was never deflected by any ray below the green, unless by the extreme red, and this was no doubt a thermo-electrical action; but in the green a weak action was always detected, which increased powerfully as we ascended into the rays of still greater refrangibility; the maximum shifting with the kind of preparation employed, between the mean blue ray and the most refrangible violet.

(491.) This action is only to be regarded as one of the evidences of chemical disturbance, exciting electrical currents; yet at the same time, it opens the question of the identity of the agent producing this disturbance and

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electricity. In the present state of the inquiry, we are bound to regard Light, Heat, and Electricity, as distinct elements; and possibly we have now to add a fourth to this list of imponderable agents. In the examples already brought forward in this treatise, we see the extreme difficulty which exists in keeping separate from each other the Light, Heat, and ACTINISM of the solar rays, and the results just stated, involve the element of Electricity in the already complicated consideration.

A few experiments made more recently with a sufficiently delicate galvanometer, prove that every ray of the spectrum produces an electrical disturbance. The rays, however, at the least refrangible end, produce a deflection of the needle in one direction, whilst the most refrangible rays set up a disturbance in an opposite direction. There are many indications of a condition analogous to polarity in the action of the prismatic rays.

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CHAPTER X.

MAGNETISING POWER OF THE SOLAR RAYS.

(492.) HAVE the sun's rays the power of developing the phenomena of polarity in steel? This question has been agitated for upwards of twenty years. Dr. Morichini was the first to announce, that the violet rays of the solar spectrum had this power. The experiments were tried by collecting the violet rays in the focus of a convex lens, and exposing one half of fine needles, previously proved to be entirely free of magnetism, to the influence of these rays for half an hour. MM. Carpa and Ridolfi repeated these experiments of Morichini with the most satisfactory results, and the Italian philosopher succeeded in magnetising several needles, before Professor Playfair, and others. It must, however, be stated that Berard, Professor Configliachi of Pavia, and Dr. Faraday failed in producing the same effects. Mrs. Mary Somerville restored the confidence of the scientific world in the results of Morichini, by a series of exceedingly beautiful experiments which were published in the "Philosophical Transactions," for 1826. Needles were ascertained to be entirely free of magnetism; they were then half covered with paper, and the exposed end placed in the violet ray of the spectrum, about five feet from the prism. In two hours, the needle was magnetised, the exposed end being the north pole. The indigo ray gave nearly the same result. The magnetic influence was imparted by the blue and green rays, but in a much less degree. The yellow, orange, and the calorific rays were tried for several days; but no magnetism was developed. Pieces of clock and watch springs gave similar results. It was also found

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that the same effects could be produced by exposing needles partly covered with paper to the Light which permeated cobalt blue and green glasses. Green and blue riband produced similar changes.

Baumgartner, of Vienna, discovered that a steel wire, polished in some parts and oxidised in others, became magnetic, exposed to the white light of the sun, the polished part becoming the north pole. The concentrated rays acted more rapidly, and in this way eight poles were obtained on as many inches of wire.

(493.) Barlocci and Zantedeschi found that an armed natural loadstone had its power nearly doubled in twentyfour hours, if exposed to the strong Light of the sun, and that an artificial magnet which carried 13 oz. supported 31 oz. more, after three days' exposure to sunshine; and it at last supported 31 oz. by continuing the solar action. Zantedeschi found that while the strength increased in oxidised magnets, it diminished in those highly polished. He also discovered that by concentrating the sun's rays, the magnet acquires strength when its north pole is exposed to them; and loses it when their south pole is acted on by them.

(494.) Mr. Christie found that when a magnetised needle, or a needle of copper, or of glass, vibrated by the force of torsion in white Light, the arch of vibration was more rapidly diminished in the sun's Light than in the shade; this effect being more evident on the magnetised needle.

*

(495.) Riess and Moser published a series of experiments conducted with great care, which seem to throw much doubt on the results of other philosophers. They examined the number of oscillations performed in a given time, before and after the needle was submitted to the influence of the violet rays. A focus of violet Light concentrated by a lens, was made to traverse one-half of the

* Edinburgh Journal of Science, New Series, No. IV. p. 225.; Annales de Chimie et de Physique, November, 1829.

needle 200 times. They, however, could not detect any difference in the oscillations, which could be at all attributable to any magnetising property of the solar rays. These experiments were tried at different seasons of the year, and at all hours of the day. They also endeavoured to verify the results of Baumgartner, but without success.

(496.) Connected with these researches, there appear to be many almost inexplicable phenomena, which have probably led to these discordant results. The whole series certainly require a fresh investigation. Mr. R. W. Fox, in pursuing his investigations on terrestrial magnetism, was led to observe the fact, that the oscillating needle was much affected by the sun's rays; the arch of vibration being more rapidly diminished, as was observed by Mr. Christie. These experiments were repeated by Mr. W. Snow Harris, who adopted the plan of swinging the needles in a vacuum, and this talented electrician came to the conclusion that no such retardation took place, under the exhausted receiver of an air pump. It must, however, be borne in mind, that it is impossible to avoid the leaking in of air even with the best instruments; and this would give rise to currents which would materially influence the results.

(497.) Berzelius has stated that the results of Seebeck's experiments show, that in no circumstances do the sun's rays develope polarity in steel, which did not, previously to exposure to their influence, possess magnetic properties. He therefore considers the experiments of Mrs. Somerville as illusive.

(498.) I will now name an experiment of my own, and leave the matter for still further investigation. Twelve sewing needles were carefully examined, and found to be without any polarity whatsoever. These were stuck through four cards, and one half being thus screened from the Light, the other was covered with deep blue coloured glasses. Three of the needles were placed magnetic E. and W., and three others in the direction of the