ISOLATION OF AN ION MILLIKAN. 243 



The conclusion, however, that valency is exhibited in gaseous ioniza- 

 tion is not to be so easily drawn. During the observations recorded in 

 the first half of the table, a closed tube of radium, containing 500 

 milligrams of radium bromide of activity 3,000, stood about 5 feet 

 away from the testing chamber, so that its y rays and a portion also of 

 its p rays could enter this chamber. At the end of the observations 

 in the group in which G=23.14, this radium was brought up to 

 within a few inches of the testing chamber, and six elementary 

 charges were forced upon the drop. The radium was then taken 

 entirely out of the room, so that the changes recorded in the last half 

 of the table are entirely due to such ionization as exists in air under 

 normal atmospheric conditions. 



Now, so long as changes take place only when the field is off there 

 is no way of telling whether an observed change of two units is due 

 to the addition to the drop of a double ion or to the successive addi- 

 tions of two single ions. It might be possible to account, therefore, 

 for all the multiple changes which occurred when the field was off 

 on the theory of successive single changes. There is, however, one 

 single change recorded in the last part of Table I, which is not to be 

 so easily accounted for upon this hypothesis. It will be seen that the 

 drop made one particular trip up in 378 seconds, then one down 

 (recorded in the same horizontal line) in 23.6 seconds. Immediately 

 thereafter it was being pulled back again under the influence of the 

 field at the 380-second rate — a rate so slow that it could scarcely be 

 seen to be moving at all if observed for a short time. After the 

 lapse of 305 seconds, during which time the shutter had been opened 

 every 30 seconds or so to see that the star was still in view it changed 

 instantly while I was looking at it, the field being on, from the 380- 

 second to the 39-second speed skipping entirely the 71-second speed. 



This sort of a multiple change, when the field was on, has been 

 observed a dozen or more times when the ionization was so weak 

 that it seemed very improbable that two or three different molecules 

 could have been simultaneously ionized in the minute tube of force 

 having for its diameter the diameter of the drop. In fact, at tli£ 

 time at which the preliminary report upon this work was made it 

 was thought that these changes constituted pretty good evidence 

 that the ionization produced by radium does not always consist 

 in the detachment of one single elementary charge from a neutral 

 molecule, but consists in occasional instances, in the separation of 

 two or three such charges from a single molecule. The method 

 of studying ionization herewith presented is capable of furnishing 

 a definite answer to the question here raised in the case of any 

 particular ionizing agent. Recent work which will be reported in 

 detail in another paper has shown that if either radium radiations 

 or X rays of the intensities thus far used ever produce multiply- 



