42 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



the truth of the dynamical theory of heat beyond all manner of 

 doubt. His results have been extended by Helmholtz, Mayer, 

 Clausius, and Thomson, till the law of conservation has been shown 

 to govern all natural forces. 



Thomson demonstrated that Faraday's discovery of the rotation of 

 the plane of polarization of a polarized ray of light, produced by 

 media under the influence of a powerful magnet, involved the 

 dependence of magnetism on motion. 



The extension of the principles of the conservation and trans- 

 formation of energy to physiological phenomena has been largely due 

 to Helmholz and Carpenter. 



There can be no doubt that Maxwell's electro-magnetic theory of 

 light is destined to play no unimportant part in the development of 

 the true theory of energy. From data supplied by Weber Maxwell, 

 found that electro-magnetic disturbances were propagated with the 

 same velocity as light. The explanation of this he held to be that 

 electricity like light was due to the undulatory vibrations of the 

 medium, which is beyond question necessary for the propagation of 

 light. Should this hypothesis be found to be a valid one, a very 

 clear insight will be obtained into the real connexion between 

 electricity, light, and radiant heat. 



From what has preceded, it will be seen that the mysterious, 

 all-pervading ether plays an increasingly important part in the 

 prevailing physical theories. To such an extent is this the case that 

 Tyndall has justly remarked that its relations to the matter of the 

 universe must mainly occupy the investigations of future scientists. 

 In order to form a more definite idea of the properties of this highly 

 attenuated substance, which is yet so elastic and incompressible, that 

 Stallo has characterised it as an adamantine solid, it is now proposed 

 to attempt a calculation of its probable density. To accomplish this 

 object, it is necessary to know the amount of radiant energy emitted 

 by the sun. This determined by careful observations with the 

 pyrheliometer, and expressed by means of Joule's mechanical equiva- 

 lent of heat, amounts to 5,500,000 foot-pounds per second from every 

 square foot of the sun's surface. 



Now, the velocity of light is 186,000 miles per second. Therefore 

 the radiant energy sent forth by the sun dui'ing any given second of 

 time will at the end of that second be contained between two spheres, 

 the smaller 433,000 miles, or the semi-diameter of the sun for l'adius, 



