J. J. Thomson Quote

The whole mass of any body is just the mass of ether surrounding the body which is carried along by the Faraday tubes associated with the atoms of the body. In fact, all mass is mass of the ether; all momentum, momentum of the ether; and all kinetic energy, kinetic energy of the ether, This view, it should be said, requires the density of the ether to be immensely greater than that of any known substance.

Electricity and Matter (ed. 1904)