40 
M. DE LA RIVE’S RESEARCHES ON THE VOLTAIC ARC. 
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 state of the wire submitted to the 
experiment. I had particularly mentioned the influence of temper and annealing, 
of greater or less tension, and of temperature. I had shown that iron wire, when 
under the influence of an action which renders it magnetic, does not emit the same 
sound as when it is in its natural state. Finally, by modifying, through the agency 
of heat, the molecular arrangement of some metals, such as platinum and brass, I 
had succeeded in obtaining from them, during the passage of the interrupted current, 
sounds, which, though feeble, were yet distinct. 
The preceding reflections tended to confirm me in my opinion, that sounds pro- 
duced under the influence of magnetism in the experiments on the voltaic arc, are 
owing to a molecular modification effected by the action of the magnet, and the 
more so inasmuch as the voltaic arc may be regarded as produced by a succession 
of interrupted currents, following each other with extreme rapidity, rather than by 
a perfectly continuous current. I accordingly took bars of other metals besides iron, 
as of tin, zinc, lead, bismuth, &c. I placed them on the poles of the electro-magnet, 
and I caused an interrupted current from a Grove’s battery of from five to ten pairs 
to traverse them. They emitted no sound as long as the electro-magnet was not 
magnetized, but as soon as it was, sounds were very distinctly heard, composed of a 
series of strokes corresponding to the interruptions of the current, and analogous to 
that produced by a toothed wheel. The bars were 18 inches long, and from 9 to 10 
lines square. Rods of copper, platinum, and silver produced a similar effect ; a red 
of iron did not emit a much louder sound under the influence of the magnet than it 
did when not exposed to this action. 
What appeared to me most remarkable was, to find lead, a body so little elastic, 
yield a sound as powerful as those proceeding from the other metals, when placed 
under the same circumstances. The position of metallic bars with respect to the 
poles of the electro-magnet did not in any way modify the result of the experiment; 
they might be placed axially, that is to say, in the direction of the poles, or equatori- 
ally, that is, across the poles ; the effect remained the same, being merely weakened as 
the distance between the bar and the poles increased. In order to hear the sound 
distinctly, when not very powerful, it was sufficient to establish a communication be 
tween the metallic bar and the ear by means of a wooden rod. In this manner the 
sound was not unfrequently heard prolonged some seconds, though growing con- 
stantly feebler, until it ceased entirely, after the source of magnetism had been with- 
drawn from the electro-magnet. Mr. Faraday has remarked an analogous fact in 
the action of the transparent medium on the polarized ray, an action which does not 
cease immediately with the magnetism of the electro* magnet. Is this prolongation 
owing to the magnetization of the electro-magnet not ceasing in a sudden manner ; 
or to its return to its primitive molecular state not taking place instantaneously in the 
