646 Dr. C. V. Burton on the 



of every element until it is„?i times as great as before, the 

 virtual work corresponding to 86 will be n 2 times as great 

 as before. Now on the Faraday-Maxwell view, since we 

 regard the aether as simply partaking of the normal dis- 

 placement of each electrified surface, the displacement of the 

 aether * involved in the change of configuration 86 will 

 depend on 89 only, and not on the electrical charges. Hence 

 when all the charges are raised to n-t'old, involving an 

 w 2 -fold value for the virtual work in 86, we must have n 2 -fold 

 values for the aetherial stresses. 



8. But on the electronic theory, any displacement 86 of 

 the bodies in question is a displacement of the electrons 

 which make up those bodies and their surface charges, and 

 the corresponding displacement of the aether at any point f 

 is the" vector-increment of electric polarization at that point. 

 On this view, then, the w 2 -fold virtual work corresponding 

 to 86, due to n-icAA charges throughout the system, is 

 accounted for hj ??-fold virtual displacement everywhere in 

 the aether surrounding the bodies, together with n-fold 

 setherial stress. On the electronic theory, therefore, the 

 stress in the aether is simply proportional to the electromotive 

 intensity, conformably to the linear character of the equations 

 of^electromagnetism. 



9. Maxwell's investigation of the stress in a magnetic 

 field X follows essentially the same course as that relating to 

 an electrostatic field, and for non-magnetizable media an 

 identical type of stress is found. Here again the assumption 

 referred to in § 3 above is implicitly made, and it is further 

 implicitly assumed that, in the magnetic as in the electro- 

 static case, the medium which transmits the forces in question 

 may be treated as being at rest. The conclusion that the 

 forces mutually exerted by electrified bodies on the one hand 

 and by magnets on the other hand, are due to aetherial 

 stresses of identical type is indeed somewhat disconcerting ; 

 but for isotropic media Maxwell's " mechanical stress " must 

 be regarded as a perfect mathematical analogy, affording in 

 certain cases a ready means of reaching results which are 

 not so immediately obvious when otherwise approached. 

 The most conspicuous example is Maxwell's own discovery 

 of " radiation pressure," the general phenomena of which 



* Thb term (infinitesimal) displacement is here used in the generalized 

 sense commonly understood in relation to dynamical systems ; not in the 

 special sense associated with Maxwell's " electric displacement." 



t See preceding footnote. 



+ ' Treatise/ vol. ii. chap. xi. 



