774 



valoe of ihis teeter for all the molecules within an element of volume be the 



oh w* mil light, then the equations which express this average 



be of the same form as that which expresses the displacement in the 



theory. 



It is often uscrtH that the mere fact that a medium is elastic or com- 

 M proof that the medium is not continuous, but is composed of 

 part* having void spaoea between them. But there is nothing incon- 

 with experience in supposing elasticity or compressibility to be properties 

 of ewty portion, however small, into which the medium can be conceived to 

 be di\ull. in which case the medium would be strictly continuous. A medium, 

 | a<| ^ irt though homogeneous and continuous as regards its density, may be 

 rendered heterogeneous by its motion, as in Sir W. Thomson's hypothesis of 

 vorlex-moleculea in a perfect liquid (see Art ATOM)'. 



The wilier, if it is the medium of electromagnetic phenomena, is probably 

 molecular, at least in this sense. 



Sir W. Thomsont has shewn that the magnetic influence on light discovered 

 by Faraday depends on the direction of motion of moving particles, and that it 

 indicate* a rotational motion in the medium when magnetized. See also 

 Maxwell's Electricity and Mmjin'tism, Art. 806, &c. 



Now, it is manifest that this rotation cannot be that of the medium as a 

 whole about an axis, for the magnetic field may be of any breadth, and there is 

 no evidence of any motion the velocity of which increases with the distance from 

 single fixed line in the field. If there is any motion of rotation, it must be a 

 rotation of very small portions of the medium each about its own axis, so 

 that the medium must be broken up into a number of molecular vortices. 



We have as yet no data from which to determine the size or the number 

 of these molecular vortices. We know, however, that the magnetic force in the 

 region in the neighbourhood of a magnet is maintained as long as the steel 

 retains its magnetization, and as we have no reason to believe that a steel 

 magnet would lose all its magnetization by the mere lapse of time, we 

 conclude that the molecular vortices do not require a continual expenditure of 

 work in order to maintain their motion, and that therefore this motion does 

 not necessarily involve dissipation of energy. 



' (p. 445 of the present volume.] t Proceedings of llie Royal Society, June, 1856. 



