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



path of the sether particles was contained within compact 

 limits, or was, at any rate, shorter than the length of the wave 

 itself. It has been proved recently, in investigations by Mr. 

 Johnstone Stoney in connexion with the radiometer*, that a 

 medium constituted according to the ki?ietic theory has a 

 special power of propagating a pressure unequal in various 

 directions, or that, when a layer of the medium (such as a layer 

 of air) is intercepted between two surfaces whose distance 

 apart is a small multiple of the length of free path of the 

 particles of air, the layer can then transmit a pressure in the 

 line perpendicular to the surfaces which is in excess of the 

 transverse pressure ; and thus a repulsion is produced, account- 

 ing for the spheroidal state, the motion of the radiometer, &c. 

 In fact it is evident (as pointed out) that, since in a medium 

 constituted according to the kinetic theory the particles move 

 in straight lines, the particles (when the distance of the opposed 

 surfaces approximates to the range of free path) get reflected 

 backwards and forwards repeatedly between the opposed sur- 

 faces, the increments of energy received by the particles 

 accumulating by successive reflections, so that the particles 

 produce a bombardment tending to separate the two opposed 

 surfaces f. The increments of velocity imparted by the heated 



* Philosophical Magazine, December 1877. 



t There is another point in connexion with the motion of the particles, 

 which no doubt, however, has been already noticed. Under normal con- 

 ditions, a body vibrating opposite to another tends (as is known) to pro- 

 duce rarefaction in the intervening medium ; but in the case of a tilm 

 whose thickness is near the range of mean path of the particles, there 

 would appear to be a special cause tending greatly to reduce this effect, 

 and even perhaps to produce the contrary effect, viz. a condensation 

 (which would greatly increase the repulsion). Thus, under the increments 

 of velocity received, there is a tendency for the molecules of the gaseous 

 film to be turned round so as to move more normal to the film. Suppose, 

 for instance, an elastic sphere to be rebounding obliquely between two 

 planes. Suppose increments of velocity to be given to the sphere by vi- 

 brating one of the planes. Then these increments of velocity given to the 

 sphere will evidently make it rebound more normal to the surfaces. So 

 in the case of the molecules of a gaseous film, rebounding backwards and 

 forwards between two surfaces (such as the air film which supports a 

 drop in the so-called " spheroidal " state, the air film which supports a 

 grain of powder in some experiments of Professor Barrett, referred to by 

 Mr. Johnstone Stoney), the molecules of the film will tend, by the incre- 

 ments of velocity given them, to turn round so as to move in a direction 

 more normal to the surfaces. This evidently makes the lateral pressure 

 exerted by the film less, and consequently its lateral expansion (or rare- 

 faction) less. If we imagine the extreme case where the molecules of the 

 film are all turned round so as to move exactly normal to the surface of the 

 film, then whatever the velocities of the molecules of the film (i. e. what- 

 ever the longitudinal pressure, or repulsion, exerted by them), the film 

 would exert no lateral pressure at all. There would consequently be a 

 lateral inrush of air, increasing the density of the film, and therefore in- 



