Mr. S. T. Preston on the Kinetic Theory of Gravitation. 301 



surfaces are also mainly received in the line joining the 

 surfaces (not so much transversely) ; so that this conduces to 

 the pressure on the surfaces, or repulsion. 



4. This is precisely what we have to put forward, in its 

 application to the aether, as an explanation of the repulsion in 

 the cases referred to, such as for example the repulsion of two 

 lenses or glass surfaces placed together in such proximity as 

 to exhibit " Newton's rings," the repulsion of two molecules 

 &c. ; for if the aether be constituted according to the kinetic 

 theory, we shall inevitably have the same phenomena here, 

 though on an infinitely more energetic scale ; for the particles 

 of aether come into direct contact with the vibrating molecules 

 of matter, whose energy of vibration is known to be enormous 

 at normal temperature ; and the layer of aether is very thin, 

 and the motion of the aether particles very rapid*, so that the 

 successive increments of velocity imparted by the vibrating 

 molecules accumulate by successive reflections (backwards and 

 forwards) between the opposed surfaces, producing a forcible 

 repulsion. These results have been theoretically demonstrated 

 to follow on the basis of the kinetic theory, and have been 

 established by experimental facts. It is a point of great im- 

 portance to observe that it is specially the kinetic theory that 

 explains this otherwise most curious fact of an excess of pressure 

 in a medium in one direction (producing a repulsion), with 

 normal pressure existing in transverse directions, which other- 

 wise it would be so difficult to explain, and which must be 

 explained in order to account in a realizable manner for the 

 phenomena observed. It is difficult to conceive how any 

 other means of explaining this curious fact could be afforded 

 than that supplied by the kinetic theory. Moreover it is 

 generally admitted that heat has the property of producing 

 repulsion. The "heat" of the molecules in the cases men- 

 tioned is known to consist in their vibrations, by which they 

 generate waves of heat in the aether. We have therefore to 

 explain under what particular constitution of a medium vi- 

 brations can (within certain limits) produce repulsion. The 

 kinetic theory of the constitution of the medium solves com- 

 pletely this peculiarly difficult problem. 



creasing the repulsion (since those molecules which enter the film become 

 themselves available for producing repulsion). Possibly, from this cause, 

 these films may be actually denser than normal density. At all events 

 the above cause makes their density greater than it otherwise would be, 

 and the repulsion exerted by them greater. 



* It may be noted that, if the aether be constituted according to the 



kinetic theory, the normal velocity of its particles is X velocity of a 



Y 5 

 wave of light. See appendix to paper " On the Mode of Propagation of 

 Sound" (Phil. Mag. June 1877), added by Prof. Maxwell. 



