of Molecules at very low pressures, 1053 



wherever may be situated, so that at least half of the total 

 number of molecules entering through BC at all points, and 

 in all directions equally, are returned through BC. More- 

 over, many molecules travelling at greater angles than 45° to 

 BO are returned also (for example along OM, where angle 



Fis:. 4. 



C 



B O 



3JOB = G0" and angle YOG = 45°), and therefore the 

 resultant effect is as has already been proved for a vessel 

 w ith an angle of 90°. 



It can be shown further by similar reasoning that these 

 results are true for vessels in which the height is greater 

 than in those which have been dealt with, that is in which 

 BC is greater than twice the length of AD. 



In a consideration of three dimensional molecular move- 

 ment, it will readily be seen that truncated pyramids and 

 truncated cones must necessarily behave in the same manner, 

 the additional velocity component producing its corre- 

 sponding downward movement. 



Only ideally smooth walls have so far been considered. 

 Since polished surfaces are known to reflect even light, in 

 the main, according to the usual laws of reflexion, although 

 they are always very imperfectly smooth as seen under the 

 microscope, it would consequently appear not unreasonable to 

 suppose that this may be the general effect also where gaseous 

 molecules are concerned, especially if the so-called molecular 

 impacts be merely the bottoms of parabolic curves caused by 

 the repulsion of the wall, but, in view o£ adsorption 

 phenomena and other differences, such similarity of regular 

 reflexion, even as an average effect, perhaps ought not to be 

 premised, and it becomes desirable therefore to consider the 

 effects which would be caused if the angles of reflexion 

 were different from those of approach. 



