of Molecules at very low pressures. 1055 



would those near the bottom *, thus suffering a greater total 

 downward impulse than those in the lower layers ; and as 

 there is no balancing effect in their particular neighbour- 

 hood a downward flow must be produced, this conclusion 

 being identical with that already obtained. The cause for 

 such an effect has been explained in terms of: an extra 

 restriction selectively applied to molecules with particular 

 movements, and would appear to be in strict accordance 

 with the statements of Maxwell to which reference 

 has been made. If, however, perfect reflexion were not in 

 operation, and absolute independence of angles of approach 

 and departure existed, then any point on the wall would 

 discharge molecules haphazardly on either side of the normal 

 to an equal extent, this being true, whatever theory of ad- 

 sorption be adopted (compare Langmuir, J. Amer. Chem. 

 Soo. 1916, xxxviii. p. 2221). Since the normal to either 

 wall (inside the vessel) points downwards, and since this 

 must be the mean direction of all discharge, and also of all 

 approach so long as the molecules are moving equally in all 

 directions, therefore hits upon the walls must still produce, 

 in the main, downward impulses on the molecules concerned ; 

 and, for the reason given, these impulses are inflicted to a 

 greater extent in the higher and narrower portions of the 

 vessel than in the lower layers, whence the ultimate effect 

 must be the same, qualitatively, as with perfect reflexion, 

 although quantitatively it may not be so great. On the 

 other hand it may be greater, for, if the theory of Langmuir 

 (loc. cit.) on the repulsion of adsorbed particles be accepted, 

 then these will probably be discharged from the sides mainly 

 in directions substantially normal to the surFace and will 

 consequently leave the vessel through the base, thus giving 

 an exalted effect above that to be expected in the case of 

 ideally smooth walls and perfect reflexion. 



A vane mounted with sufficiently minute friction inside or 

 below one of the vessels described, while the vessel itself be 

 suspended in a gas at a suitably low pressure, should there- 

 fore be capable in all cases of realizing experimentally the 



* Compare note p. 1049, which shows that although the pressure may 

 be the same inside and outside the walls of a vessel, yet more impacts per 

 molecule can occur iuside than outside the vessel in the same period of 

 time. So, in a vessel with converging- walls, more impacts per molecule 

 may occur in-one portion of the vessel than in another. Assuming the 

 concentration to be the same throughout the vessel, and the molecules to 

 be moving equally in all directions, then the more constricted any portion 

 of the vessel may be, the more frequently will molecules in that portion 

 collide with the walls. 



