148 Dr. S. Tolver Preston on some 



accuracy for large bodies and small bodies, and for bodies of 

 different kinds of substance " (Phil Mag. May 1873, p. 323). 



Le Sage's Theory. 

 Le Sage based bis theory on perfectly arbitrary assump- 

 tions. He assumed {Deux Traites de Physique mScanique, 

 Paris 1818, edition Pierre Prevost) : — 



(1) That a number of streams of atoms, equally distributed 

 in space, exist ; of which each stream moves continually in 

 one and the same direction. 



(2) The length of these streams (at the centre of which the 

 universe known to us is placed) is finite, but very great ; 

 therefore gravitation must have a correspondingly limited 

 period for existence. 



(3) That the streams must be everywhere equally dense. 



(4) That the mean velocity of the streams is everywhere 

 the same. 



The conditions above set forth depend manifestly on per- 

 fectly arbitrary assumptions, and it is not easy to see by 

 what mechanism such streams should either originate or be 

 kept up. As regards the behaviour of these streams of atoms 

 towards gross matter, Le Saoe assumes the folio wins;. Gross 

 matter is chiefly freely penetrated by the streams of atoms, 

 only a small part of their energy is absorbed by gross matter, 

 which implies a continuous annihilation of energy. Whence 

 it arises that every portion of gross matter opposes a certain 

 shelter to every other neighbouring portion from the en- 

 counters of the streams of atoms ; and from this the apparent 

 attraction of the gross matter according to the Newtonian law 

 of gravity is easily explained. 



Lord Kelvin presupposes exactly the same streams of atoms 

 as Le Sage ; the mechanism which regulates or maintains 

 these atom-streams therefore remains with him as obscure as 

 with Le Sage. An important progress in Lord Kelvin's case 

 consists, however, in the fact that he regards the atoms as 

 elastic. In order to explain the elasticity, he proposes to 

 regard the atoms as vortex rings in a perfect liquid. The 

 elasticity of these is then explained by the laws which Helm- 

 holtz found to apply to the motions of such vortex rings. 



The aether atoms then rebound from gross matter in 

 accordance with the laws of elastic collision : instead of the 

 absorption (annihilation) of energy assumed by Le Sage, 

 Lord Kelvin supposes that the aether atoms, in addition to 

 their translator^ energy, also possess an energy of internal 

 motion, just as Clausius assumes for the molecules of ordinary 

 gases. 



