ARISING FROM INEQUALITIES OF TEMPERATURE. 705 



are reflected, or, as Professor Listing might say, perverted in the reflected gas ; 

 that is to say, the properties of the incident and the reflected gas are sym- 

 metrical with respect to the tangent plane of the surface. 



The incident and reflected gas together constitute the actual gas close to 

 the surface. The actual gas, therefore, cannot exert any stress on the surface, 

 except in the direction of the normal, for the oblique components of stress in 

 the incident and reflected gas will destroy one another. 



Since gases can actually exert oblique stress against real surfaces, such 

 surfaces cannot be represented as perfectly reflecting surfaces. 



If a molecule, whose velocity is given in direction and magnitude, but 

 whose line of motion is not given in position, strikes a fixed elastic sphere, 

 its velocity after rebound may with equal probability be in any direction. 



Consider, therefore, a stratum in which fixed elastic spheres are placed so 

 far apart from one another that any one sphere is not to any sensible extent 

 protected by any other sphere from the impact of molecules, and let the 

 stratum be so deep that no molecule can pass through it without striking one 

 or more of the spheres, and let this stratum of fixed spheres be spread over 

 the surface of the solid we have been considering, then every molecule which 

 comes from the gas towards the surface must strike one or more of the spheres, 

 after which all directions of its velocity become equally probable. 



When, at last, it leaves the stratum of spheres and returns into the gas, 

 its velocity must of course be from the surface, but the probability of any 

 particular magnitude and direction of the velocity will be the same as in a 

 gas at rest with respect to the surface. 



The distribution of velocity among the molecules which are leaving the 

 surface will therefore be the same as if, instead of the solid, there were a 

 portion of gas at rest, having the temperature of the solid, and a density 

 such that the number of molecules which pass from it through the surface in 

 a given time is equal to the number of molecules of the real gas outside 

 which strike the surface. 



To distinguish the molecules, which, after being entangled in the stratum 

 of spheres, afterwards return into the surrounding gas, we shall call them, 

 collectively, the absorbed and evaporated gas. 



If the spheres are so near together that a considerable part of the surface 

 of each sphere of the outer layer is shielded from the direct impact of the 

 incident molecules by the spheres which lie next to it, then if we call that 



VOL. II. 89 



