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a needle, while yellow, orange, and red, produce no sen- 

 sible effect. As many philosophers have failed in repeating 

 these experiments, we were induced, in the course of the 

 summer, to undertake the investigation of this subject, 

 " which has so often disturbed science." Having procured 

 several hundred needles, of different lengths and thicknesses, 

 and having ascertained that they were perfectly free from 

 magnetism, we enveloped them in white paper, leaving one 

 of their extreme ends uncovered. Taking advantage of a 

 favourable day for trying experiments upon the chemical 

 ray, (known by the few seconds required to blacken chloride 

 of silver,) we placed the needles at right angles to the mag- 

 netic meridian, and exposed them for three hours, from 

 eleven to one, to the differently refrangible rays of the sun, 

 under coloured glasses. Those beneath the red, orange, 

 and yellow, showed no trace of magnetism, while those be- 

 neath the blue, green, and violet, exhibited, the two first 

 feeble, but the last strong traces of magnetism. 



To determine how far the oxidating power of the violet ray 

 is concerned in the phenomena, we exposed to the dif- 

 ferent coloured lights needles whose extremities had been 

 previously dipped in nitric acid, and found that they became 

 magnetic (the exposed end having been made a north pole) 

 in a much shorter time than the others, and that this effect 

 was produced in a slight degree, under the red (when ex 

 posed a sufficient length of time) strongly under white glass, 

 and so strong under violet glass, that the effect took place 

 even when the needles were placed in such a position along 

 the magnetic meridian, as would tend to produce, by the 

 earth's influence, a south pole in the exposed extremity. 



Conceiving that the inactive state produced in iron (as 

 observed by Schcenbein) when plunged into nitric acid, 

 S. G. 1.36, or by being made the positive pole of a battery, 

 or by any other means (which Dr. Faraday supposed to be 

 due to a slight oxide formed on the surface, and which may 



