MOLECULAR MAGNITUDES 283 



spheres. But as we shall see in considering the motion 

 of the helium ions, which are shot off from radium, 

 the molecule is more like a solar system with relatively 

 large spaces between the electrons and the nucleus 

 which compose it. That matter is composed of finite 

 particles which are in constant motion, and that 

 phenomena like gaseous pressure, osmotic pressure, 

 and viscosity are due to motions and impacts of these 

 small particles is verified by this remarkable evidence. 



Two practical applications of the fundamental facts 

 of the kinetics of gases are illustrated in the pumps for 

 obtaining high vacua in the manufacture of vacuum 

 devices such as X-ray tubes and thermionic vacuum 

 tubes for radio-communication. The vacua to be 

 obtained to-day are far in excess of 

 those obtained by Torricelli. 1 



Consider for example a rotating 

 drum. Molecules of the adjacent gas, 

 which impinge upon it, have super- 

 imposed upon their natural molecular 

 motions a mass motion in a direction 

 tangential to the drum. If the mean 

 free path is small they will lose this FlG - 35 - 

 forced motion by collisions with other molecules, but 

 if it is comparatively large they will maintain it during 

 an appreciable travel. In the Gaede molecular pump 

 the drum rotates inside a hollow cylinder, as shown 

 in cross section in Fig. 35. 



The vessel to be exhausted is connected at A. An 



1 Above the mercury in his barometer there was, of course, the 

 vapor of the mercury. In accurate readings of a mercury barometer 

 an allowance is always made for this fact. 



