On the Diffusion of Sound and Light. 249 



Clausius, in the paper previously referred to, explains the 

 various phenomena produced by currents by means of this law 

 of force, and the hypothesis that a current consists of streams 

 of opposite electricities moving in opposite directions. Now, 

 since the expressions we have obtained for the force between 

 the particle do not depend on the specific inductive capacity 

 of the medium, but only on its magnetic permeability, if we 

 make this assumption about the nature of a current, it follows 

 from Maxwell's theory that the electrodynamic phenomena 

 produced by a current of given strength do not depend on the 

 specific inductive capacity of the surrounding medium, though 

 they do depend on its magnetic permeability. 



Faraday, in his ' Experimental Researches' (§ 1709 and 

 onwards), describes some experiments which he made to deter- 

 mine whether altering the surrounding medium produced any 

 change in the electromagnetic action of a current. The result 

 of the experiments was that he was unable to detect any such 

 change ; but in his experiments, though the specific inductive 

 capacities of the various media tried were very different, their 

 magnetic permeabilities were all of them very nearly unity. 



XXXIV. Theoretical Explanations of the Rectilinear Trans- 

 mission and Spontaneous Diffusion of Sound and Light. 

 By Professor Challis, M.A., F.R.S., F.R.A.S.* 



WHEN any disturbance is produced at a given position A 

 in an unlimited mass of elastic fluid of perfect fluidity, 

 defined by the relation p = a 2 p between its pressure p and 

 density p, a 2 being constant, it is found by experience that 

 there will be a resulting state of the fluid at a point P, whose 

 position is taken ad libitum, and at all intermediate points 

 between A and P. In other words, there wall be a rectilinear 

 transmission of effect from A to all surrounding points, with- 

 out respect to the particular mode of disturbing the fluid. It 

 may be that a difference of effect in different directions may 

 depend on the mode of disturbance ; at the same time it is 

 found that a resulting disturbance is produced at all points, 

 whatever be the mode of disturbance. In treatises on hydro- 

 dynamics this remarkable fact is left out of consideration. I 

 know of none in which this problem has been solved, or even 

 proposed. But it is an admitted principle that when the 

 fundamentals of any branch of applied science, after being 

 established by observation and experiment, have been ex- 



* Communicated "by the Author. 



