1050 Mr. A. Fairbourne on Restricted Movements 



Molecules enter through all points along both AD and BO, 

 and at each of these points they enter in all possible 

 directions. If the walls are assumed to be ideally smooth, 

 or to behave as such with respect to impacts upon them *, 

 then every molecule entering through AD, through any 

 point, and in any direction, will necessarily pass out at BC, 

 except in the very rare case where intermolecular collision 

 occurs during its journey through the vessel. Of the 

 molecules entering at BC, some pass out at AD, and others 

 are returned through BC, two possible cases presenting 

 themselves for consideration : (1) molecules entering through 

 a point in BC under the opening AD ; and (2) molecules 

 entering through a point under either of the inclined sides, 

 BA or CD. 



Case 1 : Any point under the line AD. 

 If the point be in the centre of BC (fig. 1), then all 



Fig. 1. 



molecules entering in directions included by the angle AOD 

 will pass out through AD, while all those entering in 

 directions included by angles AOB and DOC will be returned 

 through BC, owing to impacts on walls inclined at 45°. 

 8ince AOD is a right angle, and since molecules will enter 

 equally in all directions throughout 180°, therefore half the 

 total number of those entering through this point in any 

 representative period of time will leave the vessel through the 

 top, AD, the other half being returned through BC. 



If the point be not in the centre (fig. 2), but still under 

 AD, those molecules entering in directions included by angle 

 AOD, which is less than a right angle, will escape through AD. 



* The effects of irregular surfaces, and of adsorption on the surfaces, 

 are considered later. 



