Mr. N. Campbell on the ^Ether. 183 



has led to the belief that the energy of radiation is essentially 

 of the same nature as that which is localized around an elec- 

 trically charged body, at rest or in motion. The aether is 

 regarded as the vehicle, not only of the energy of radiation, 

 but also of all forms of electromagnetic energy, and we may 

 define it simply as " the body in which electromagnetic 

 energy is localized."" 



So rough a definition will doubtless not be found satisfactory 

 by all, but it will suffice for our purpose because it draws 

 attention clearly to the features of the conception of the 

 a:ther, as generally understood, which it is my present object 

 to discuss. 



§ 3. Of course a definition is not a proposition, and is 

 incapable of being either true or false. Whatever definition 

 of a scientific concept is adopted, it is always possible by 

 framing suitably the propositions concerning it to state a 

 theory in accordance with the observations. But, as a matter 

 of fact, in science, as well as in other studies, the propositions 

 are usually historically prior, though logically subsequent, 

 to the definitions. The propositions are chosen for their 

 simplicity, their suitability for mathematical development, or 

 for some such reason, and the first requisite of the defini- 

 tions of the concepts concerned in the propositions is that 

 they should be such as to make these propositions true. (An 

 obvious case of such a procedure is afforded by the concept 

 "a perfect gas.") 



In the case of the aether the proportions which have to be 

 true are represented by the six equations of Maxwell; the 

 definition of the aether has to be chosen so that these proposi- 

 tions are true, when the axes of reference are '* fixed in the 

 aether." If it turns out that, with the definition adopted, 

 these equations are not true when the axes of reference are 

 " fixed in the aether," then we may say roughly that the 

 definition is false, though strictly the falsity should be 

 attributed to the equations. For the purposes of our dis- 

 cussion it will be convenient and will involve no loss of 

 generality if we replace the set of equations by a single 

 simple deduction from them — the proposition that an electric 

 charge e moving with a velocity u relative to the axes of 

 reference is equivalent to a current element of strength 

 (K, the direction of which is coincident with the path of 

 the charge. 



§ I. It might seem at first sight that such a definition of 

 the aether as has been given could not possibly render such 

 a proposition untrue, but attention must be drawn to the 

 first words of the definition — "the body" — and to the 



