MOLECULAR MAGNITUDES 



285 



collision per second. This gives an interesting picture 

 of the spatial relations of the molecules in a c. c. of 

 highly rarefied gas. Although there are about seven 

 thousand million, they are so widely separated, rela- 

 tive to their size, that each molecule travels on the 

 average about a third of a kilometer between col- 

 lisions and makes but one collision a second with an- 

 other gas molecule. 



The principle of the other pump is somewhat similar, 

 but the mass motion of the molecules which are to be 

 exhausted is due to 

 impacts with directed 

 molecules. The sys- 

 tem is shown in Fig. 

 36. The rough pump 

 is connected at B and 

 the vessel, which is 

 to be evacuated, at A. 

 The reservoir R con- 

 tains mercury which 

 is heated and boils. 

 The mercury molecules rise in a dense stream and shoot 

 through the tube c into the chamber C. Molecules of 

 the gas entering at A get in the way of this stream, 

 and receive a mass motion toward the outlet at B. 

 The chamber C is cooled, usually by a water jacket 

 (not shown in the sketch), and hence the vapor mole- 

 cules condense on its sides and trickle down again into 

 the reservoir R. 



The kinetic energy of the mercury vapor molecules is 

 in part transferred to the gas molecules with which 

 they happen to come into collision, but in greater part 



FIG. 36. 



