Dr. P. L. Rijke on the Inductive Spark. 441 



stroys the symmetrical arrangement of the density of the sether 

 about them ; and since when they are distant from each other 

 the density decreases towards their axes, on their being brought 

 near, the space between them becomes a position of minimum 

 density, towards which they are consequently attracted. If the 

 currents pass in opposite directions, by their interference they 

 produce an excess of density between the electrodes, which are 

 consequently impelled, as is known experimentally. The inter- 

 ference of the transverse motions would produce the contrary 

 effects. It seems, therefore, that the observed attractions and 

 repulsions are due to the excess of the longitudinal above the 

 transverse action of the currents. 



20. It is scarcely necessary to say that the velocity of a gal- 

 vanic current in an electrode is very different from the velocity 

 with which the commencement of the current, or any interruption 

 of it, as in the use of the electric telegraph, would be propagated 

 along the electrode through space. The latter velocity, on hy- 

 drodynamical principles, would be comparable with the velocity 

 of transmission of light through solid substances, and would 

 probably exceed the velocity with which light would traverse 

 a substance of the same density as the electrode. 



The foregoing explanations seem to justify the conclusion that 

 the facts of galvanism may with much probability be referred 

 to the dynamical action of the setherial fluid, and consequently 

 be included within the proposed hydrodynamical theory of the 

 physical forces. 



Cambridge Observatory, 

 November 16, 1860/ 



LX. Note on the Inductive Spark. By Dr. P. L. Rijke, Pro- 

 fessor of Natural Philosophy in the University of Ley den*. 



1. \ BOUT five years ago Vicomte du Moncel first called 

 XjL attention to the fact f that the inductive spark obtained 

 from RuhmkorfTs apparatus differs materially from the spark 

 obtained on discharging, by ordinary means, a common electri- 

 fying machine or a Leyden jar. In the latter case the spark is 

 homogeneous, and consists of a simple point of light, while the 

 spark obtained from RuhmkorfTs apparatus is composed of two 

 parts altogether different from each other, — the one being a point 

 of light precisely similar to the ordinary spark, the other a lumi- 

 nous atmosphere that admits of being displaced by means of a 



* Communicated by the Author. 



t Comptes Rendus, vol. xl. p. 313 ; and Phil. Mag. S. 4. vol. ix. p. 546. 



Phil. Mag. S. 4. Vol. 20. No. 135. Dec. 1860. 2 G 



