Fitzgerald — On a Model Illustrating Properties of Ether. 415 



permanent alteration by the matter of the nature of the medium 

 of propagation 



I need hardly say that I do not intend it to be supposed that 

 the ether is actually made up of wheels and india-rubber bands, nor 

 even of paddle-wheels, with connecting canals. I think, however, 

 that we may learn several things as to the conditions that the 

 elements of the ether should fulfil if they are to represent Maxwell's 

 equations by motions in ways analogous to those of my model. 

 This supposes that quantities, such as the magnetic displacement 

 that obey the laws of rotation, are really of the nature of rotations. 

 If this be so, it is evident that the elements of the medium must 

 be capable of rotating any number of times without any distortional 

 stress being produced, so long as the neighbouring parts all rotate 

 equally, while distortional stress must be produced and be pro- 

 portional to the difference of rotation of the neighbouring parts of 

 the medium. It seems from this almost necessary that the ether 

 must have some structure, and although Maxwell has objected to 

 its having anything like a molecular structure on account of this 

 supposition requiring it to possess specific heat, yet I venture to 

 think that it does possess the specific heat it should possess upon 

 the supposition of some sort of structure. Part of the energy in a 

 volume occupied by any substance is in the form of vibrations of 

 the ether within that volume, and the amount of this part of the 

 energy will depend on the temperature of the body. Similarly the 

 heat required to warm up a mass of silver enclosed in a non- 

 conducting envelope (so that none of the energy was spent in 

 producing vibrations in the surrounding ether) would depend on 

 ivhether the silver were all in a small lump, or were in the form of 

 a thin closed vessel, including a large volume of ether, for this 

 included ether would require some of the energy in order to give 

 it energy of vibration. If the law connecting the temperature of 

 matter with the energy of vibration of the ether in contact with it 

 were known, it would be possible to calculate in this sense the 

 specific heat of ether. As all materials are of the same tempera- 

 ture when in contact with ether in the same state of vibration, this 

 specific heat of the ether would be independent of the material of 

 which the envelope was constructed. As the energy of the ether is 

 all in the form of its own vibrations, and is not propagated by 

 conduction, but by wave propagation, it is not in exactly the same 



