M. De la Rive's Researches on the Voltaic Arc. 331 



electrodes must be held in the hand, and then the arc fre- 

 quently made and broken without waiting till the metallic 

 points acquire too high a temperature. 



It remains now to be considered why the influence of pow-^ 

 erful magnetism, such as that exerted by the electro-magnet, 

 is necessary for the production of these sounds, which are not 

 heard in the ordinary experiment of the voltaic arc. This can 

 arise only from the change which the magnet produces in the 

 molecular constitution of the matter of the electrode, or rather 

 in the highly diffused matter which forms the voltaic arc. 

 This action is besides shown by the shortening of the arc, and 

 by the remarkable difference which it presents in its appear- 

 ance; it is therefore not surprising that it should also be capable 

 of producing a phasnomenon such as sound, which essentially 

 depends on the variations in the molecular state of bodies. 

 This view of the subject appears to me to deserve very par- 

 ticular attention : the results at which I have arrived, in pur- 

 suing it more closely^ form the subject of the following section. 



§ 3. Irifluence of the permanent actioji of Magnetism on con- 

 ducting bodies traversed hy interrupted electric currents. 



Faraday's brilliant discovery of the action exerted by mag- 

 netism on a ray of polarized light, -when that ray traverses a 

 transparent body submitted to the action of a powerful electro- 

 magnet, had no sooner been announced by its illustrious 

 author, than the majority of philosophers saw in it a proof 

 that magnetism, when at a high degree of intensity, has power 

 to modify the molecular constitution of all bodies. They 

 consequently attributed the phsenomenon observed by Faraday, 

 not to the direct action of the electro-magnet on the polarized 

 ray, but to the modification effected by this action on the mo- 

 lecular constitution of the substance traversed by the ray. I 

 was of this opinion, and communicated it to Mr. Faraday, 

 who alludes to it in his memoir. Desirous, however, of found- 

 ing this opinion on facts of a different kind, I asked myself if 

 it were not possible to find in the electric current, an agent 

 capable of performing the same function for opake conducting 

 bodies that polarized light does for transparent ones. I had 

 stated in my paper on the sound emitted by iron wires tra- 

 versed by interrupted electric currents, that the nature as well 

 as the intensity of the sounds were singularly modified by 

 the molecular .slate of the wire submitted to the experiment. 

 I had particularly mentioned the influence of temper and an- 

 nealing, of greater or less tension, and of temperature. 1 had 

 shown that iron wire, when under the influence of an action 

 which renders it magnetic, does not emit the same sound as 



