96 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VoL. II. 



important properties. In the first place then ether is matter, since in 

 order to the performance of the functions which we have above described, 

 it is absolutely essential that it should possess inertia, and inertia is the 

 distinguishing test of matter. That matter is inert, and that inertia is 

 the distinguishing test of matter, are propositions, the truth of which lie 

 at the foundation of modern physics. 



Before leaving this subject we may refer, by the way, to the bearing of 

 this on the important question. Are there two essentially different kinds 

 of energy, kitietic and potential} If potential energy be defined as the 

 energy of position, its existence is utterly inconsistent with the above 

 propositions. In this sense potential energy involves a contradiction in 

 terms. But though there is and can be no fundamental difference as the 

 basis of the distinction between potential energy and kinetic energy, all 

 energy being in its nature kinetic, yet if we regard potential energy 

 as a convenient name for those kinds of energies whose nature is not 

 yet understood, the term is convenient and admissible, though liable to 

 create considerable confusion. 



From a moment's consideration of the constitution of matter it fol- 

 lows that the ether must of necessity be a highly attenuated substance. 



Yet, in order to convey heat and light with the enormous velocity with 

 which they travel, and to account for the almost instantaneous transform- 

 ations of energy observed in explosions, etc., j:he ether must be so elastic 

 and incompressible that Stallo in his " Concepts of Modern Physics " has 

 justly characterized it as an adamantine solid. 



In order that we may have a more definite idea of the properties of 

 this marvellous substance, I may mention that in a paper read before this 

 Institute some years ago I attempted a calculation of its probable 

 density. 



The calculations showed that the mass of a cubic mile of ether is not 

 less than ^ (yV) ^ ' of ^ pound and not more than ^ (jV) " of a pound. 

 That is, a sphere whose radius is 95,000,000 miles from the sun (the 

 distance of the earth from the sun) would contain at least 4,400,000 tons 

 of ether. By a somewhat different method. Sir W. Thomson has 

 shown that a cubic mile of ether in the space traversed by the earth 

 contains not less than lOoo xiyVi''"' of a pound. As pointed out by Prof. 

 Tait the method of Sir W. Thomson gives only the minimum density. 

 It may also be pointed out that the density of the ether is approximately 

 uniform throughout the solar system. As has been remarked by S. 

 Talan Preston in his interesting Essay on the Physics of the Ether, the 

 law established by Clausius as the result of his investigations of the 

 constitution of gases, viz., that "the mean length of path of a particle is in 



