480 Scientific Intelligence. 



difficult. In substance, Gaede says : " If we compare the mechan- 

 ical rugosities with the hills and furrows of a ploughed field, then 

 the molecular unevennesses correspond to the grains of sand and 

 small stones, and the gaseous skin corresponds to a thin layer of 

 hoarfrost or to a light coating of snow upon the hills." If a differ- 

 ence in gas pressure exists at the ends of a comparatively highly 

 exhausted tube, the processes favorable to equalization of pressure 

 will be hindered by the gaseous skin and the mechanical inequal- 

 ities. For the molecules which have a component of velocity 

 towards the more remote end of the tube, and which also impinge 

 against the wall, will strike the mechanical inequalities of surface 

 which are turned towards them so that, as stated above, the gas- 

 eous skin will, in the majority of cases, send the molecules back 

 along their lines of incidence towards the end of the tube from 

 which they had come. On the other hand, when the degree of 

 exhaustion has been carried so far that no gaseous film remains 

 on the walls, the molecules will be reflected by the molecular un- 

 evennesses of the walls in a perfectly regular manner, that is, with- 

 out any selective relation to the angle of incidence. Therefore, 

 just as many molecules will be thrown back towards one end of the 

 tube as will be sent forward towards the other end. In this case, 

 the equalization of pressure will proceed more rapidly than when 

 a gaseous skin adheres to the tube. At about - 001 mm this film 

 practically vanishes, so that for lower pressures the assumption 

 of regular, non-selective reflection holds and the Kinetic Theory 

 of Gases applies in its more elementary form. 



An obvious extension of the preceding considerations to the 

 case of the friction between a gas and a cylinder rotating in the 

 gas has been made by Gaede. The result is that he has designed 

 and tested a pump which bids fair to revolutionize the production 

 of extremely high vacua. It is so difficult and wasteful of space 

 to clearly describe the construction of a piece of apparatus with- 

 out the aid of diagrams that the attempt will not be made to go 

 into full details here. Suffice it to say that the " Molecular Air- 

 pump " consists essentially of a cylinder which can be revolved 

 at very high speed inside of a coaxial cylinder or housing. The 

 housing is grooved or recessed over a certain arc and in a plane 

 perpendicular to the common axis of the cylinders. The " leading" 

 and "trailing" ends of the recess communicate respectively with 

 the auxiliary pump and with the vessel to be exhausted. In prac- 

 tice, a number of grooves and inlet and outlet tubes are placed in 

 series or tandem. . At pressures below O^OOl" 31 " the molecules of 

 the gas are diffusively reflected at the metallic surfaces and fly 

 from one wall to the other without, in general, colliding with other 

 molecules of their kind. " The reflection of the molecules can be 

 pictured as if the surface of the cylinder were covered with a 

 great number of small cannon out of which the molecules are 

 shot in all possible directions with a certain speed, the molecular 

 speed." If the surface of the cylinder moves with a linear speed 

 greater than the molecular speed, then the molecules inside the 



