284 THE REALITIES OF MODERN SCIENCE 



ordinary air pump, connected to B, maintains the 

 pressure within the cylinder low enough to permit the 

 desired action. Molecules, due to their natural motions, 

 enter C from A and impinge on the drum D. They 

 leave D with a component motion in the tangential 

 direction of the small arrow a. Because the mean free 

 path is large few of them encounter other molecules. 

 Striking against the walls of C, they are reflected back 

 on to the drum but farther along. In this way they 

 are impelled around C from A to p. The number of 

 molecules at the exit B is therefore maintained greater 

 than at A. The action of the " rough" pump con- 

 stantly removes some of these excess molecules. 



With such a molecular pump 1 vacua corresponding 

 to 2X10~ 8 cm. of Hg have been obtained. This 

 represents a pressure of about three ten-thousandths 

 of a dyne/cm. 2 In oxygen the mean free path cor- 

 responding to this pressure is 76/2 X10~ 8 times the 

 value of page 279, or 35,400 cm. The number of mole- 

 cules per c. c. is directly as the pressure and is ap- 

 proximately 7X10 9 . The number of collisions per 

 second per molecule is the quotient of the average 

 velocity, which has not been altered by a reduction 

 of the pressure, and the mean free path, or about one 



1 In the actual pump there is a series of chambers like C. The 

 first of these connects at A to the vessel to be exhausted and the 

 last connects at B to the backing pump. The outlet B of the first 

 chamber connects at B with the intake A of the succeeding chamber, 

 and so on. The result is equivalent to that which would be pro- 

 duced by a drum of much larger diameter. The molecules which 

 are impelled around toward B have, therefore, smaller chance of 

 diffusing back to A. The actual pump runs in oil so arranged that 

 by the rotation the oil is thrown outward against the clearance 

 spaces at the end of the drums and thus seals them very effectively. 



