SOME CONTEMPORARY .IDVANCES IN I'lIYSICS—XI 4S9 



experimental work. It furthermore indicates that the total ionization 

 effected by a beam of particles in traversing a given thickness of 

 matter should, beyond a certain speed, diminish with increasing speed; 

 which for alpha-particles is true for the entire available speed-range, 

 and for electrons is true l)eyond the speed of optimum ioni/ing- 

 efiiciency mentioned in a previous section.'^ 



Multiple Ionization 



The analyses of positive rays issuing from gases sustaining electrical 

 discharges show that under such conditions some atoms are deprived 

 of two, three, or even so many as eight electrons. The recently- 

 developed methods of interpreting spectra make it practically certain 

 that some of the spectrum-lines emitted from gases bombarded by 

 electrons or sustaining discharges, and particularly from the excep- 

 tionally violent discharges known as "sparks," are due to atoms 

 lacking one, two, or so many as six of their normal complement of 

 electrons. ^^ 



Such ions might conceivably be produced either in one operation 

 or in several; that is, the two (or more) missing electrons might have 

 been removed by a single agency at a single moment, or they might 

 have been detached one after the other by separately and successively 

 acting agents. Measurements by the method of H. D. Smyth, such 

 as those upon argon already cited, are capable of showing the minimum 

 amount of energy which bombarding electrons must possess, in order 

 that doubly-ionized atoms may appear in a bombarded gas; but thev 

 do not show, at least not directly, whether this minimum amount is 

 what is required to effect double ionization in a single operation, or 

 merely what is required to effect the most difficult among two or 

 several steps leading cumulatively to the result. The same holds 

 true about the experiments in which the least bombarding-voltage 

 sufficient to bring out the spectrum associated with the doubly-ionized 

 atom is measured. -"^ Granted that the energy-difference between the 

 once-ionized and the normal argon atom is 15 equivalent volts, and 



^-i The attempts to account for "straggling" of alpha-particles — that is, for the fact 

 that different particles of the same initial speed are slowed down at somewhat 

 different rates in progressing through the same gas — by ascribing it to mere statistical 

 fluctuations in the number of electrons close to which they passed seem to have been 

 unsuccessful; the observed straggling is much too great for this explanation. See 

 G. H. Henderson, Phil. Mag. 44. 



-^ Multiply-ionized atoms are regularly observed in electrolytic solutions of com- 

 pounds of other-than-monovalent elements; strangely enough they are rarelv if ever 

 found among the ions issuing spontaneously from hot metals and salts. 



-« P. D. Foote et al., Phil. Mag. 42, pp. 1002-lOLS (1921); Astroph. Jour. 55, pp. 

 145-161 (1922); Origin oj Spectra, 1922. 



