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Dr. J. Larmor. A Dynamical Theory 



interactions through the aether ; its continuity of motion is, in the 

 case of fluids, of limited character, being maintained only by viscosity 

 and other such causes. The elasticity of matter in bulk is to be 

 based on the distribution of organised material energy per unit 

 volume : the material energy depends on the relative positions of 

 the atoms, therefore this organised or mechanical part of it depends 

 on the change of their mutual configurations expressed with refer- 

 ence to the deformation of the element of volume, that is on the 

 strain. It follows, as Grreen was the first to show with logical sim- 

 plicity and precision, that the stress in an element of volume is self- 

 conjugate. But none of these ideas have any application to the sethereal 

 continuum : in it there is no question of change of mutual configura- 

 tion of physical parts ; the energy of strain is not thus restricted to 

 be a function of deformation only, in fact after MacCullagh it 

 assumes the geometrically more simple form of a function of the 

 absolute position, more precisely of the rotational displacement, of 

 the element ; the resulting stress need not be self-conjugate, — nor is 

 it in fact so even in a material medium that has gyrostatic quality. 

 An electric field would consist of rotational strain in the sether, a 

 magnetic field of irrotational flow, each in actual cases extremely 

 slight : the motion involved in a permanent magnetic field com- 

 bined with a permanent electric field would not become jammed in 

 course of time, because it can relieve itself by a slight separation of 

 free electrifications, which will again neutralise each other after this 

 object has been attained. 



4. An atom of matter has been represented by a collocation of 

 electrons describing stable orbits round each other. The discussion 

 of the internal vibrations of such an atom and the consequent radia- 

 tion will follow the lines of Laplace's general analysis of the oscilla- 

 tions about steady motion of a system of connected bodies like the 

 Solar system. When the gyrations or orbital motions are sufficiently 

 rapid, there will be two types of vibrations produced by disturbance 

 of the system ; very rapid ones which radiate light, and very slow 

 ones like the precession of a spinning top which do not involve 

 appreciable internal deformation of the system. In gases it is only 

 these latter that would be excited sensibly by the comparatively 

 gentle encounters between the molecules : these are in relation to 

 the thermal energy, but are only in indirect connexion with radia- 

 tion. The difficult outstanding problem of the theory of gases, that 

 namely of the connexion between temperature and internal thermal 

 energy, involving the relation of the two specific heats, would on 

 this view take on a form different from the usual one. In various 

 other respects, a recognition that the motions which constitute heat 

 are not the vibrations w T hich feed radiation seems to extend and 

 improve the capabilities of molecular theory. 



