660 



stances, in the form either of points or of plates. He examines mi- 

 nutely the transfer of particles which takes place from one pole to 

 the other under these various circumstances, and the differences 

 which occur when the currents are reversed. He observed that 

 when a positive metallic point is presented to a negative plate, par- 

 ticles of the former are transported by the voltaic arc, and deposited 

 on the latter, forming a ring of a regular form, having as its centre 

 the projection of the point on the plate. This happens in atmo- 

 spheric air whether highly rarified or of the ordinary density, but 

 not in hydrogen gas. This deposit consists always of oxidized par- 

 ticles of the positive metal which forms the pointed electrode. In 

 the case of platinum, the circular spot is of a blue colour, and pre- 

 sents the appearance of the coloured rings of Nobili. This effect 

 the author is disposed to ascribe to the action of the oxygen brought 

 by the voltaic current into that particular condition which Schoen- 

 bein first described under the name of Ozone. While this deposit 

 is taking place, a vivid blue light is emitted. 



In the second section the author investigates the action exerted by 

 a powerful electro-magnet on the voltaic arc. He describes the re- 

 markable modifications which the length, the form, and even the 

 nature of the arc undergoes when the electro-magnet is brought ex- 

 tremely near to it, and the magnetization of the electrodes them- 

 selves, w^hen they are susceptible of that affection by their approxi- 

 mation to the electro-magnet. He notices the singular phenomenon 

 of a peculiar sound emitted by the luminous arc, when subjected to 

 this magnetic influence ; a sound which varies both in its nature and 

 its intensity according to the nature, the form and the temperature 

 of the electrodes, consisting sometimes of a shrill whistle, and at 

 other times of a series of slight detonations. 



The third section is devoted to the investigation of a remarkable 

 phenomenon presented by all the conducting bodies while transmit- 

 ting discontinuous electric currents, under the influence of a power- 

 ful electro-magnet ; namely, the emission of a sound resembling that 

 of the revolving toothed-wheel in Savart's experiments. This sound 

 is distinctly heard, and is peculiarly loud with prismatic bars of lead, 

 bismuth, tin, &c., about three-quarters of an inch square and a foot 

 and a half long, whether placed in the direction of a line joining the 

 poles of an electro-magnet, or in a direction at right angles to such 

 line : it was weakened only by increasing the distance between the 

 poles and the bar. The intensity of the sound appeared to depend 

 much less on the nature of the substance which was subjected to 

 this action, than on its form, its volume, and its mass. All conduct- 

 ing bodies, whatever may be their nature, or state of aggregation, 

 are capable of yielding these sounds. They are produced by char- 

 coal of all kinds and shape. Mercury contained in a cylindrical 

 glass tube, of similar dimensions with the metallic bars, emits a 

 sound of great intensity ; and a still louder sound arises from a wire 

 coiled as a helix around a cylinder of wood, and also by tubes formed 

 of different metals. Similar phenomena are also observable by the 

 action of a helical coil substituted for the electro-magnet. 



