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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 suffi. ciently 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.
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
MAGNETIC POWER OF LIGHT.
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 3 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
MAGNETISATION OF LIGHT.
dip of the needle. The other six needles were washed with dilute nitric acid, and arranged three on a card, in the same manner as the former. After having been exposed to sunshine for some hours, they were examined, and it was found that those needles which had been placed in the direction of the dip had acquired polarity; but no change could be detected in any of the others.
(499.) In 1845, Dr. Faraday communicated to the Royal Society a Memoir on the “ Magnetisation of Light, &c.," and on the “ Action of Magnets on Light." These subjects are intimately connected with our present consideration, and require some attention. Dr. Faraday's view will be best understood from the following note of explanation.
“Neither accepting nor rejecting the hypothesis of an ether, or the corpuscular, or any other view that may be entertained of the nature of Light; and, as far as I can see, nothing being really known of a ray of Light more than of a line of magnetic or electric 'force, or even of a line of gravitating force, except as it and they are manifest in and by substances; I believe that, in the experiments I describe in the paper, Light has been magnetically affected, i. e. that that which is magnetic in the forces of matter has been affected, and in turn has affected that which is truly magnetic in the force of Light: by the term magnetic, I include here either of the peculiar exertions of the power of a magnet, whether it be that which is manifest in the magnetic, or the diamagnetic class of bodies. The phrase “illumination of the lines of magnetic force' has been understood to imply that I had rendered them luminous. This was not within my thought; I intended to express that the line of magnetic force was illuminated as the earth is illuminated by the sun, or, the spider's web is illuminated by the astronomer's lamp. Employing a ray of light, we can tell by the eye, the direction of the magnetic lines through a body; and by the alteration of the ray and its optical effect on the