308 Mr. S. T. Preston on the Kinetic Theory of Gravitation. 



naturally led to some incongruous ideas regarding the struc- 

 ture of the aether, in the effort to explain it. Thus some have 

 supposed the aether to resemble a solid, which is in direct 

 opposition to the teaching of the senses ; for we move about so 

 freely in this "solid" as to be unconscious of its existence. 

 Another supposition has been that " lines of tension," behaving 

 somewhat in analogy to stretched chords, exist in the aether. 

 Such a mechanism would be, to say the least, somewhat de- 

 ranged by the passage of a planet through the aether. Indeed 

 it is sufficiently evident that these are the hopeless attempts 

 made to surmount an impossible condition, or a difficulty for 

 whose existence there is really no warrant. If the aether be 

 not a solid, or a liquid (for liquids oppose enormous resist- 

 ances to the passage of bodies through them at high speeds), 

 then what other resource have we than to conclude that it is 

 a gas ? 



14. A gaseous constitution of the aether according to the 

 kinetic theory would perfectly satisfy the two fundamental 

 conditions of a medium highly elastic in all directions, and 

 opposing no appreciable resistance to the free movement of 

 bodies (the planets &c.) through its substance. For it is a 

 known fact that the resistance opposed by a medium consti- 

 tuted according to the kinetic theory to the passage of bodies 

 through it diminishes as the normal velocity of the particles 

 of the medium increases. The high normal velocity of the 

 particles of the aether, proved by the velocity of light, therefore 

 necessarily renders the resistance inappreciable, and the medium 

 itself impalpable and undetected by the senses. 



15. A difficulty has been raised in the way of the aether 

 being constituted as a gas on the following grounds, which, 

 being only anxious for truth, we are bound to consider *. It 

 has been argued that if the aether be constituted as a gas, the 

 specific heat of unit of volume of the aether would be the same 

 as that of any ordinary gas at the same pressure, and that 

 therefore it would appear that the presence of the aether could 

 not fail to be detected in the experiments on the specific heat 

 of ordinary gases. We have to offer the following as a means 

 of meeting this difficulty. It will be admitted that the de- 

 tection of the aether in the experiments on specific heat will 

 depend, not on the specific capacity for heat possessed by the 

 aether, but on the rate at which the heat passes from the gas 

 experimented on to the aether. The molecules of the gas are 

 moving through the aether with their normal translatory 

 motion, this motion of the molecules representing the " heat " 

 of the gas. It will be evident that the rate at which the 



* See paper u On the Dynamical Evidence of the Molecular Constitu- 

 tion of Bodies," by Prof. Maxwell (< Nature/ March 11, 1875). 



