438 Mr. J. Larmor. A Dynamical Theory of [Dec. 7. 



III. " A Dynamical Theory of the Electric and Luminiferous 

 Medium." By JOSEPH LARMOR, F.R.S., Fellow of St. 

 John's College, Cambridge. Received November 15, 

 1893. 



(Abstract.) 



Ever since the causes of natural phenomena began to attract atten- 

 tion, the interaction of the different classes of physical agencies has 

 been taken to suggest that they are all manifestations in different 

 ways of the energy of some fundamental medium ; and the efforts of 

 the more sanguine class of naturalists have always been in some 

 measure directed towards the discovery of the properties of this 

 medium. It is only at the end of the last century that the somewhat 

 vague principle of the economy of action or effort in physical actions 

 which, like all other general principles in the scientific explanation 

 of Nature, is ultimately traceable to a kind of metaphysical origin 

 has culminated in the hands of Lagrange in his magnificent mathem- 

 atical generalisation of the dynamical laws of material systems, 

 Before the date of this concise and all-embracing formulation of the 

 laws of dynamics there was not available any engine of sufficient 

 power and generality to allow of a thorough and exact exploration of 

 the properties of an ultimate medium, of which the mechanism and 

 mode of action are almost wholly concealed from view. The precise 

 force of Lagrange's method, in its physical application, consists in 

 its allowing us to ignore or leave out of account altogether the 

 details of the mechanism, whatever it is, that is in operation in the 

 phenomena under discussion ; it makes everything depend on a single 

 analytical function representing the distribution of energy in the 

 medium in terms of suitable co-ordinates of position and of their 

 velocities ; from the location of this energy, its subsequent play and 

 the dynamical phenomena involved in it are all deducible by straight- 

 forward mathematical analysis. 



The problem of the correlation of the physical forces is thus 

 divisible into two parts, (i) the determination of the analytical 

 function which represents the distribution of energy in the primordial 

 medium which is assumed to be the ultimate seat of all phenomena, 

 and (ii) the discussion of what properties may be most conveniently 

 and simply assigned to this medium, in order to describe the play of 

 energy in it most vividly, in terms of the stock of notions which we 

 have derived from the observation of that part of the interaction of 

 natural forces which presents itself directly to our senses, and is 

 formulated under the name of natural law. It may be held that the 

 first part really involves in itself the solution of the whole problem ; 

 that the second part is rather of the nature of illustration and ex- 



