256 



SCIENCE 



[N. S. Vol. XXXIII. No. 842 



The Isolation of Ions: Professor R. A. Millibcan. 



This paper presented the methods used and the 

 results obtained thus far in the work on the 

 properties of isolated ions. It contained a more 

 complete discussion of the material which was 

 summarized in the issue of Science of September 

 30, 1910j and in addition new results obtained in 

 collaboration with Mr. Harvey Fletcher — (1) on 

 the question of valency in gaseous ionization, 

 (2) on the causes of the irregularities obtained 

 by Ehrenhaft in Vienna in his recent work on e. 

 The new work only is here briefly summarized. 

 The tabulated experimental data upon which the 

 conclusions rest are soon to be published else- 

 where. 



1. In the work previously published it was con- 

 clusively shown that the great majority of at- 

 mospheric ions carry the elementary electrical 

 charge, but there seemed to be some evidence that 

 ions of larger valency were occasionally formed. 

 This evidence was found in the fact that an oil 

 drop suspended in a strong electrical field would 

 occasionally catch, under the influence of radium, 

 two or three elementary charges at once when the 

 field was on. 



The only preceding evidence in favor of the 

 existence of valency in gaseous ionization is con- 

 tained in the experiments of Townsend' and of 

 Frank and Westphal,^ which seemed to their 

 authors to establish the production of some 

 doubly valent positive ions by X-rays, but not by 

 other ionizing agents. 



We accordingly suspended minute drops of oil 

 in an electrical field and produced a thin sheet of 

 X-ray ionization in the air just beneath the drop, 

 but no rays were allowed to fall upon the drop 

 itself. The drops experimented upon were posi- 

 tively charged and the direction of the field was 

 such that only positively charged ions could be 

 tlirown upon them. Although we used rays of 

 varying degrees of hardness we found in 125 

 catches not one which represented the advent of 

 a multiple charge upon the drop. These experi- 

 ments present therefore direct, definite, unmis- 

 takable proof that the act of ionization of air by 

 primary X-rays, even when these rays are of 

 extreme hardness, consists in the separation tf 

 one single elementary charge from a neutral 

 molecule. 



ipj-oo. Roy. Soc, 80, p. 207, 1908; 81, p. 464, 

 1908, and 82, p. 18, 1909. 



-Verh. d. D. Phys. Ges., Marz, 1909, and Juli, 

 1909. 



When /3 rays of radium were used as the ion- 

 izing agent it was occasionally found possible, 

 as in the work previously reported, to catch at 

 once upon a single drop two or three elementary 

 charges, provided the drops were of sufficient 

 size; hut when the drops were very small we 

 caught only singles. This shows we think that 

 all of the double and treble catches heretofore 

 observed were due to the simultaneous catching 

 by a large oil drop of two or three ions coming 

 from two or three different molecules rather 

 than to the separation from a single molecule of 

 two or three elementary charges. In other words 

 the capture of a number of elementary charges at 

 once upon large drops furnishes no argument for 

 the production of ions carrying two or more ele- 

 mentary charges, unless similar results can be 

 obtained with small drops, and this we never 

 found to be the case. Since it was only in the 

 case of X-rays that evidence for valency in gase- 

 ous ionization had been previously found the 

 present experiments seem to remove all ground 

 for supposing that the act of ionization of a gas 

 ever consists in the expulsion from a single mole- 

 cule of more than one elementary electrical charge. 



2. When we used oil drops of smaller radius 

 than about .00004 cm. we found that the remark- 

 able consistency in the successive values of the 

 times of fall under gravity shown in the work 

 previously published (see Science, September 30) 

 was replaced by greater and greater irregularities 

 as the drops became smaller and as the distance 

 between the cross hairs was diminished. The 

 explanation lies in the fact that the displacements 

 due to Brownian movements become in such cases 

 comparable with the displacements produced by 

 gravity during the time of observation. The cor- 

 rectness of this explanation is proved conclusively 

 by tables prepared by Mr. Fletcher which show 

 that the observed distribution of " times of fall " 

 between fixed cross hairs .07 mm. apart agrees 

 perfectly with the aistribution of these times 

 computed from the theory of probability as ap- 

 plied to Brownian movements. These considera- 

 tions offer a complete explanation of the irregu- 

 larities observed by Ehrenhaft, since he used a 

 cross hair distance of about the value mentioned 

 above and worked with particles the radii of 

 which were on the average about one fifth of 

 those of our smallest drops previously reported 

 upon. 



Ehrenhaft's failure to observe Brownian move- 

 ments with platinum and gold particles while he 



