SOME CONTEMI'OR.IRV .ADVANCES IN PHYSICS— XI 483 



amount. However, it would not be safe to infer that throughout the 

 range of these observations all of the ionizations consist in detachments 

 of valence-electrons from various atoms. Sooner or later transfers of 

 atoms into other states of ionization must commence. This is rendered 

 all the more probable by the fact that the values of /(F), determined 

 at or near the peak for each gas, show a very definite tendency to in- 

 crease steadily with the number of electrons in the atom or the molecule 

 in question. 



If a stream of electrons is projected into a sufficiently dense gas, the 

 electrons are gradually slowed down and even stopped, and the stream 

 is dispersed. Measurements of the number of ions produced per elec- 

 tron per millimetre have been made under such conditions, and meas- 

 urements also of the "total ionization" produced in a volume of gas so 

 large that the electrons lose their forward speed altogether before 

 reaching the walls; but though the intrinsic interest of such measure- 

 ments is great, it seems practically impossible to deduce /(F) from 

 them.19 The difficulties may be compared with those arising in the 

 study of alpha-particle scattering when the metal foil is too thick.-" 

 When, however, the electrons are moving with the enormous speeds 

 possessed by those ejected from radio-active substances, or when ioni- 

 zation by alpha-particles is studied, the conditions again become 

 simpler and relati\-ely intelligible. 



Ionization by Alpha-Particles and Very Fast Electrons 



Ionization by particles possessing kinetic energies amounting to 

 millions of equivalent volts, such as alpha-particles and many of the 

 electrons emerging from radioactive substances, might well be ex- 

 pected to follow other laws than ionization by particles possessing 

 little more than enough energy to detach an electron from an atom. 

 Such indeed is the case; yet it would not be justified, either by reason- 

 ing or by experiment, to suppose that even such highly energetic par- 

 ticles expel electrons of any and every class tightly-bound alike and 

 loosely-bound alike, with equal ease and abundance from the atoms 

 which they strike. 



It is not particularly difficult to measure the total number of ions 

 produced by an alpha-particle in its course through a gas from the mo- 

 ment it enters, with a measurable initial speed, to the moment when it 

 goes into retirement (so to speak) as an ordinary helium atom; nor to 



i»G. A. Anslovv, Fhys. Rev. (2) 25, pp. 484-500 (1925) and literature there cited. 



20 Anyone desiring to learn how complicated the circumstances mav become when 

 electrons arc shot into a dense gas should read P. Lenard's brochure '"Ouantitatives 

 uber Kathodenstrahlen," published by the Heidelberg Academy in 1918. 



