ELECTRO-MAGNETISM. 



97 



meter.* We understand, however, that 

 Mr. Faraday is at present engaged in an 

 experimental inquiry upon this subject, 

 which cannot fail, in such able hands, 

 to lead to important results. 



CHAPTER XIV. 

 Influence of Light on Magnetism. 



(326.) PROFESSOR Morichini of Rome 

 announced, in the year 1813, his having 

 discovered that steel, exposed in a par- 

 ticular manner to the concentrated violet 

 rays of the solar spectrum, became mag- 

 netic; but the uniform failure of the 

 experiment, when tried by every other 

 person, had created great doubt of the 

 accuracy of the result as reported by 

 Morichini. In the course of some ex- 

 periments made by Mr. Christie, in the 

 year 1824, he was led to the conclusion 

 that the solar rays actually do exert a 

 sensible influence on magnetism, which 

 is shown by their affecting the vibra- 

 tions of a magnetized needle exposed to 

 them, quite independently of the effects 

 produced by the heat which they impart. 

 A needle, six inches long, contained in 

 a brass compass box with a glass cover, 

 was suspended by a fine hair, and made 

 to vibrate, alternately shaded and ex- 

 posed to the sun. He found, from a 

 number of trials, that the vibrations of 

 the needle, when exposed to the sun, 

 ceased in a much shorter time than when 

 they took place in the shade. That this 

 greater slowness of the vibrations was 

 not attributable to an increase of tem- 

 perature, was proved by the needle's 

 being observed to vibrate more rapidly 

 when its temperature was raised by 

 other means-f- . 



(327.) In the summer of 1825, Mrs. 

 Somerville was induced, by the unusual 

 clearness of the weather, to investigate 

 this subject;|;. Having at that time no 

 information of the manner in which 

 Morichini's experiments had been con- 

 ducted, it occurred to her that if the 

 whole needle were equally exposed to 

 the violet rays, it was not probable that 

 the same influence which produced a 

 south pole at one end, would, at the 

 same time, produce a north pole at the 

 other. She therefore covered half of a 

 slender sewing-needle, an inch long, with 

 paper, and fixed it in such a manner as 

 to expose the uncovered part to the 



* See Quarterly Journal of Science, xix. 338. 



f Philosophical Transactions for 1826, p. 219. 



J Ibid. p. 133. 



violet rays of a spectrum thrown, by an 

 equiangular prism of flint glass, on a 

 pannel at five feet distance. As the 

 place of the spectrum shifted by the 

 motion of the sun, the needle was moved 

 so as to keep the exposed part constantly 

 in the violet ray. The sun being bright, 

 in less than two hours the needle, which 

 before the experiment showed no signs 

 of polarity, had become magnetic, the 

 exposed end having the properties of a 

 north pole. 



The season continuing favourable, 

 afforded daily opport unities of repeat- 

 ing and varying the experiments with 

 needles of different sizes, and placed 

 in different positions with respect to 

 the meridian, and at different distances 

 from the prism. The results were nearly 

 uniform, and similar to that above 

 stated. It was not found necessary to 

 darken the room, provided the spectrum 

 was thrown out of the direct solar rays. 



(328.) Mrs. Somerville next endea- 

 voured to ascertain whether the other 

 prismatic rays had the same property 

 as the violet. Needles, previously as- 

 certained to be unmagnetic, exposed to 

 the blue and green rays, sometimes 

 acquired magnetism, though less uni- 

 formly and less quickly than in the 

 violet ray : when magnetism was thus 

 communicated, it seemed to be equally 

 strong as in the former case. The in- 

 digo ray succeeded nearly as well as 

 the violet. The exposed end, in almost 

 every case, became a north pole. In no 

 one, instance was magnetism produced 

 by the yellow, orange, or red rays, 

 though in some instances the same 

 needles were exposed to their influence 

 for three successive days ; neither did 

 the calorific rays of the spectrum pro- 

 duce any sensible effect. 



(329.) Pieces of clock and watch 

 spring were next tried with similar suc- 

 cess, and were found to be even more 

 susceptible of this peculiar magnetic 

 influence than needles, possibly on ac- 

 count of their blue colour, or greater 

 proportional surfaces. The violet rays 

 concentrated by a lens produced mag- 

 netism in a shorter time than the prism 

 alone. 



(330.) Experiments were next insti- 

 tuted by transmitting the solar rays 

 through coloured media. Needles, half 

 covered with paper, were exposed on a 

 stone outside a window, under a blue 

 glass coloured by cobalt, to a hot sun 

 for three or four hours. They were 

 found to be feebly magnetic ; but their 

 H 



