CONTEMPORARY ADVANCES IN PHYSICS 693 



"area" o- by the formula R = ttct^; then to divide R into the "radius 

 ro of the molecule" and the "radius rj of the ion," according to some 

 more or less plausible guess; then to compute the value of -wr^, and call 

 it the cross-section of the ion. But all such procedures are more or less 

 dubious, whereas the quantity a always retains its meaning as a meas- 

 ure of the likelihood of interception. 



There is a particular mode of interception for positive ions, different 

 from any to which electrons are subject. If an ion passes close to an 

 atom, it may steal an electron to neutralize itself. The former atom 

 becomes of course an ion, but in general it does not acquire the velocity 

 of the former ion, while the latter being neutralized is not perceived 

 even if it continues on its course to the collector. Therefore, the net 

 result of the electron-transfer is the vanishing of an ion from the beam. 

 It follows from quantum-mechanics that the cross-section of the atom 

 for this special sort of interception is greater, the more nearly the 

 energy required to ionize it agrees with the energy required to re-ionize 

 the neutral atom into which the former ion is transformed by the 

 electron-transfer. Consequently, the greatest values occur when the 

 ion-stream consists of ionized atoms of the very gas through which it is 

 being sent.^° 



One would wish, other things being equal, to duplicate with positive 

 ions the apparatus already used with electrons. The best example of a 

 duplication is that presented by Ramsauer and O. Beeck, who used 

 the device of Ramsauer's earlier work depicted in Fig. 2, substituting 

 for the metal plate at Z a metal ribbon painted with an amalgam of 

 mercury with any of the five alkali metals; heating the ribbon, they 

 got a stream of the ions Li+, K+, Na+, Rb+, or Cs+. The gases which 

 they used were A, He, Ne, Ho, N2, and O2, though of these argon was the 

 only one of which they measured the interception for all five kinds of 

 ions. The range of energy-values extended from 1 to 30 equivalent 

 volts, and over it the value of a in every case diminished steadily with 

 increasing energy. When different ions were driven through the same 

 gas, the value of a was found to be greater, the greater the atomic num- 

 ber of the ion ; when ions of the same kind were tried in different gases, 

 it was found to be greater, the greater the molecular weight of the gas. 

 All this is as one would expect. The value of R (as defined in the 

 paragraph above) is of the same order of magnitude as the sum of the 

 gas-kinetic radius of the molecule or atom of the gas, and that of the 

 ion. 



^^ For the theorj^, and for a bibliography of the experimental work, see H. Kall- 

 mann & B. Rosen, ZS. J. Phys. 61, 61-86 (1930). 1 shall treat the subject more 

 extensi\ely elsewhere. 



