FRIEDRICH HASENOHRL 373 



These endeavours only found a firm basis when the at- 

 tempt was made to connect up with mass or inertia, not the 

 ether, which is difficuh of access to us, but energy, defined 

 by Robert Mayer's well-founded and repeatedly confirmed 

 law of conservation, and always susceptible of accurate 

 measurement, in spite of the great variety of forms which it 

 assumes. Maxwell's theory already suggested that atten- 

 tion should be directed, not to the ether of the lines of force, 

 but to the energy connected with them. For electro- 

 magnetic energy can be calculated according to Maxwell's 

 equations from the course of the existing magnetic lines of 

 force, whereby each element of space traversed by lines of 

 force receives a definite fraction of the energy, so that the 

 energy actually appears as distributed in the ether of the 

 lines of force, almost as if this ether, and the energy, were 

 perhaps one and the same thing. Hence it might reason- 

 ably be attempted to ascribe to the energy of the field of 

 force of an electric current, or of moving electricity 

 generally, the mass or inertia which appears in the phe- 

 nomenon of self-induction. In this way the idea arose that 

 energy - at least electro-magnetic energy - has mass, that it 

 possesses inertia, that it resists change of velocity. 



It was first attempted to carry out this idea in the simple 

 case of a moving electron, that is to say, pure electricity 

 without matter, which could be done by means of Maxwell's 

 equations; but the attainment of a satisfactory result was pre- 

 vented by the necessity for making certain assumptions 

 about the electron which went beyond existing experience. 

 Hasenohrl was the first who put aside the electron, about 

 which too little was known, and turned to those electro- 

 magnetic fields which can exist without either matter or 

 electricity. These are the fields of force of the electric waves 

 realised by Hertz, about which it was also known that they 

 behaved according to Maxwell's equations. The fact that 

 these waves carry energy, as do all ether waves of whatever 



