RECENT PE0GP.E3S IlSr THEORETICAL PHYSICS. 221 



ments are once set up in a frictionless fluid they are absolutely in- 

 destructible save by the power that originated them. Only an in- 

 finite power can set up vortex movement in a perfect fluid without 

 friction, and only an infinite power can destroy such motion when once 

 set up. On this idea Sir Wm. Thompson has based his famous spec- 

 ulation that the atoms of matter are merely so many vortex-rings of 

 variable but definite shape for each elementary kind of matter. Such 

 rings possess all the qualities usually attributed to the atoms of - 

 matter, being absolutely impenetrable, and possessing when set in 

 vibration that characteristic periodicity of vibration which the spec- 

 troscope shows to be the case with the atoms of the elements of matter^ 

 As Professor Tait says, " not only can these vortex-rings in a perfect 

 fluid not be cut, but we cannot even so much as get at them, to try 

 to cut them." They rebound from the sharpest edge. 



Thus it will be seen that there is at least an analog ij heiween vor- 

 tex-filaments in a perfect fluid and magnetism caused by electric 

 currents. The equations of the electro-magnetic field show this, 

 when compared with the equations of vortex filaments. But this 

 is by no means all. 



In Faraday's beautiful experiment of the rotation of the plane of 

 polarized light when passing through a medium which is under the 

 influence of magnetic strain, we have a means of testing whether an}^- 

 thing of the nature of rotation of small elements, either of gross mat- 

 ter or of some incompressible frictionless fluid be going on in the 

 magnetic field. For, if the magnetic force be in any way the con- 

 sequence of such minute rotations, we might expect a priori that 

 the minute motions which cause light, at least those circular oscil- 

 lations that constitute circularly polarized light, could in some way 

 be compounded with the minute rotations involved in magnetic 

 phenomena, and be influenced by them. And thus, although we 

 could not directly observe these vortex movements by the senses, 

 we yet might have the means of exploring the magnetic field, by 

 an agent of almost superhuman delicacy in the shape of the oscilla- 

 tions of light. The possibility of the compounding of the mag- 

 netic rotations with those of circularly polarized light, wdiich con- 

 stitutes the explanation Thompson gives of the Magnetic Rotation 

 of Polarized Light, I will take up next.* 



* This subject was fally treated in the paper as read before the Academy, but its 

 publication is delayed until cuts can be prepared to illustrate it, and Greek type 

 obtained for the formulae. 



