Recent Theories of Electricity. 203 



Professor Lorentz recognizes the growth o£ the idea (hat it 

 is unnecessary for the physicist to dwell on the mechanism 

 of the coefficients introduced into our equations, but he 

 adheres to the view * that " we cannot be satisfied with 

 simply introducing for each substance these coefficients, 

 whoso values are to be determined by experiment ; we shall 

 be obliged to have recourse to some hypothesis about the 

 mechanism that is at the bottom of the phenomena." In 

 respect to the aether he is exceeding^ vague, and so far as a 

 mechanism of it is concerned, gives nothing. For example, 

 while speaking of the state of this medium when it is the 

 seat of an electromagnetic field, he says f : " We need by 

 no means go far in attempting to form an image of it and, 

 in fact, we cannot say much about it." In agreement with 

 Sir J. Larmor, his aether is a plenum always at rest, 

 capable of maintaining and of transmitting electric strains, 

 and containing electrons, or extremely small particles 

 charged with electricity. Sometimes he gives the impres- 

 sion that these electrons are electricity only. The aether 

 not only penetrates the spaces between the electrons but 

 also pervades them: " We J can reconcile ourselves w ith 

 this, .... by thinking of the particles of matter as of some 

 local modifications in the state of the aether. - " Here also, 

 mechanical forces and attributes are discarded, and he 

 holds : " That § the phenomena going on in some (any ?) part 

 of the aether are entirely determined by the electric and 

 magnetic force existing in that part," Now we are indebted 

 to Maxwell, and to him alone, for a set of electromagnetic 

 stresses and strains which will satisfy the requirements of 

 the electromagnetic field. In his mind, there is no doubt, 

 these stresses produced a material strain in the aether wmich 

 was communicated physically to dielectrics existing in the 

 aether and materially distorted them. It is, of course, 

 impossible to test experimentally the existence of these 

 strains in the free aether, but in a long series of papers in 

 this journal I have shown that no distortion is produced by 

 them in dielectrics. These experiments were undertaken 

 when the real existence of the aetherial stresses was generally 

 accepted. I was led to the opposite opinion, because in no 

 other case had any static connexion been found between 

 aether and matter, and because Helmholtz had shown that 

 such stresses would produce motion in the aether, an effect 

 unlikely to be true. Professor Lorentz certainly concurs in 

 this opinion. He states || : '• While thus denying the real 



* 'Theory of Elections,' r, 8. 



t L. c. p. 1. { Z. c. p. 11. 



§ L. c. p. '26. || L. c, p. 31. 



