238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



absorbed by the vaseline. The existence of these polyvalent ions has 

 been confirmed by Franck and Westphal/ who returned to the older 

 method, using a gaseous current and devised by Townsend, in which 

 K and D are separately measured. With X rays the proportion of 

 polyvalent ions is about 1/10; with the a rays of polonium of 

 the P rays of radium there seem to be no polyvalent ions. Millikan 

 and Fletscher - do not agree with these conclusions, basing their ob- 

 jections upon the method of drops earlier described. But the earlier 

 physicists maintain their interpretation, which also seems to be in 

 good accord with the results from other methods (multiple charges 

 of the a rays from radium, of the canal rays, the positive rays of 

 vacuum tubes, according to J. J. Thomson, Gehrke, and Reichenheim 

 and others). 



However, the question must seem at present unsolved. Very re- 

 cently, Langevin and Salles,^ measuring the ratio K/D by a new 

 direct method, have concluded against the existence of polyvalent ions 

 in the ionization by X rays. We must therefore still leave the ques- 

 tion open. 



(5) Finally, we must note the remarkable experiment by which 

 C. T. K. Wilson * has enlightened us as to the mechanism of ioniza- 

 tion. Continuing his celebrated experiments on the condensation 

 of water vapor on ions, he succeeded in seeing and photographing 

 the trail of ions, produced in a gas by an angle a or ^ particle from 

 radium or a very narrow pencil of X rays. 



His admirable photographs themselves alone can give an idea of 

 all of which we can learn from them. Upon them we see the a and ^ 

 particles following their rectilinear trajectories; we learn that the 

 X rays do not ionize directly but by the secondary rays which they 

 tear from the molecules encountered in the gas, etc. We find also 

 a direct verification of the hypothesis advanced by Langevin and put 

 to experimental test by Moulin ^ in order to explain the " initial re- 

 combination" discovered by Bragg. According to the latter, the 

 saturation current of a gas ionized by a rays is much more difficult 

 to obtain than when X rays are used. This is due, not to an " initial 

 recombination" between the positive atom ions and electrons just 

 liberated, but to a localization of the ions along the path of the 

 a particles ; a saturation current is indeed much easier to obtain when 

 the field is perpendicular to the radiation than when parallel. 



1 Franck and Westphal, Verb, der Deutsch. Phys. Ges., vol, 11, pp. 146 and 276, 1909. 



2 Millikan and Fletscher, Phys. Rev., vol. 32, p. 239, 1911, and Phil. Mag., vol. 21, 

 p. 753, 1911. See also Townsend, Phil. Mag., vol. 22, p. 204, 1911 ; Franck and West- 

 phal, Phil. Mag., vol. 22, p. 547, 1911. 



3 Langevin and Salles, Soci6te de chimie physique, February, 1913. 



* Wilson, Proc. Roy. Soc, vol. 85, p. 285, 1911 ; Radium, January, 1913. 

 E Moulin, Radium, p. 350, 1910. 



