M. DE LA RIVE’S RESEARCHES ON THE VOLTAIC ARC. 
39 
I have satisfied myself that in order to obtain the hissing- sounds, it is necessary that 
the positive electrode should be at a sufficiently high temperature to experience a com- 
mencement of liquefaction ; for without this condition, only a series of detonations 
are heard. The hissing would be the result of the easy and continuous transport of 
matter more or less liquefied from the positive electrode, whilst the detonations would 
be the effect of the resistance opposed by the same matter to the disintegration of its 
particles when it is not sufficiently heated. Numerous experiments made with me- 
tallic points, whether of the same or different natures, as silver, iron, brass, as also 
platinum and copper, some of which become heated sooner than others under the same 
circumstances, have quite confirmed me in this view of the subject. It is merely 
necessary to be careful, in order to produce the hissing noise, to maintain as much 
as possible the continuity of the arc when once the positive electrode becomes incan- 
descent ; while, on the other hand, to obtain the detonations, one of the electrodes 
must be held in the hand, and then the arc frequently made and broken without 
waiting till the metallic points acquire too high a temperature. 
It remains now to be considered why the influence of powerful 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 appearance ; it is therefore not surpri- 
sing that it should also be capable of producing a phenomenon 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 particular attention: the results at which 
I have arrived, in pursuing it more closely, form the subject of the following section. 
§ 3. Influence of the 'permanent action of Magnetism on conducting bodies traversed by 
interrupted electric currents. 
Faraday’s brilliant discovery of the action exerted by magnetism on a ray of pola- 
rized 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 con- 
sequently attributed the phenomenon 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 molecular 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 founding 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 per- 
forming the same function for opake conducting bodies that polarized light does for 
