530 JAMES CLERK MAXWELL. 



glass ends be placed within a coil of wire through which an 

 electric current is passing, and plane polarised light be trans- 

 mitted through the tube, the plane of polarisation will be 

 turned through an angle in the direction in which the current 

 circulates, and this angle will be proportional to the current, 

 Verdet showed that in the case of a transparent (para-) 

 magnetic substance the rotation is in the opposite direction 

 to that of the current. 



The curious effect of a magnet upon the luminous dis- 

 charge in a vacuum tube and the recent experiments of 

 Dr. Kerr, may indicate other relations between light and 

 electricity and magnetism. 



Having thus very briefly referred to the principal pheno- 

 mena of magnetism and electromagnetism, we may proceed 

 to give a short explanation of the medium or mechanism by 

 which Maxwell accounted for these phenomena and their 

 mutual interdependence. 



From the well-known laws of the propagation of light, 

 Maxwell assumed "as a datum derived from a branch of 

 science independent of that with which we have to deal, the 

 existence of a pervading medium, of small but real density, 

 capable of being set in motion, and of transmitting motion 

 from one part to another with great, but not infinite, velocity." 

 Inasmuch as this medium can transmit undulations with 

 finite velocity, it follows that it possesses a property analo- 

 gous to mass, so that its motion implies kinetic energy ; in 

 addition to elasticity, in virtue of which its deformation 

 implies potential energy. 



It is well known that if a body rotate about a fixed centre 

 there will be a tension along any radius drawn in the plane of 

 rotation. The form which the earth would assume under the 

 action of gravity only, if there were no rotation, would be 

 that of a sphere. The diurnal rotation tends to cause the 

 polar axis to contract and the equatorial diameter to increase ; 

 and this action would go on indefinitely were it not that at 

 a certain early stage it is balanced by the attraction of gravi- 

 tation, and thus the earth assumes a nearly spherical form, in 

 which the polar axis is shorter than the equatorial diameter. 



