110 CONDUCTION AND RADIATION [CH. x. 



accelerative connection ; it is only when the charge 

 vibrates, and during its accelerative periods, that it is 

 able to influence the ether at a distance by emission of 

 waves.* These waves consist probably of alternations 

 of shear, with no motion of the ether as a whole, 

 but only a to-and-fro quiver of its equal opposite 

 constituents over some excessively small amplitude : 

 a kind of motion which constitutes what we know as 

 radiation. It is not the atom pulsating as a whole 

 which disturbs the ether, but the pulsations or 

 vibrations, or the startings and stoppings and 

 revolutions, of its electric charge. Acceleration of 

 electric charge is the only known mode of originat- 

 ing ether disturbance. But normal or centripetal 

 acceleration, involving nothing more than change of 

 direction, is just as effective as actual change of 

 speed. If an electric charge is able to describe a 

 small orbit four-hundred-billion times a second, it 

 will emit the lowest kind of visible red light. 

 This number of revolutions is equal to the number 

 of seconds in about fourteen million years, or in 

 the time since some early geologic period. If 

 it revolves faster it will emit light of higher re- 

 frangibility ; and the particular kind of radiation 

 emitted by the atom of any substance, when in a 

 fairly free state, will depend on the orbital period 

 of its electrons, if they could be considered as inde- 

 pendent. But if that were so, every atom would soon 

 radiate itself to destruction. The condition that an 

 atom must fulfil in order to have a chance of survival 

 by retaining its energy, was given by Larmor (Phil. 

 Mag., Dec. 1897) in the form that the vector sum of 

 the accelerations of all its electrons, with due regard 

 to their signs, should be permanently null. This 



* See Chaps. I. and IX., also Appendix G. 



