Question of Valency in< Gaseous Ionization. 7G1 



column headed " # " varies slightly both because of obser- 

 vational errors and because of Brownian movements*. 

 Under the column F are recorded the various observed 

 values of the times of rise through 10 divisions of the scale 

 in the eyepiece. A star after an observation in this column 

 signifies that the drop was moving with gravity instead of 

 against it. The procedure was in general to start with the 

 drop either altogether neutral (so that it fell when the field 

 was on with the same speed as when the field was off) or 

 having one single positive charge, and then to throw on 

 positive charges until its speed came to the 6"0 second value, 

 then to make it neutral again with the aid of radium and to 

 begin over. 



It will be seen from Table I. that in 4 cases out of 44 we 

 caught negatives, although it would appear from the arrange- 

 ment shown in fig. 2 that we could catch only positives. 

 These negatives are doubtless due to secondary X rays which 

 radiate in all directions from the air molecules when these 

 are subjected to the primary X ray radiation. The smallness 

 of the number of negatives so caught shoivs conclusively that the 

 greater part of the ionization of a gas by X rays is due to the 

 direct action of the 'primary rays. 



Towards the end of Table I. is an interesting series of 

 catches. The drop was as at the beginning of this series 

 charged with 2 negatives which produced a speed in the 

 direction of gravity of 6*5 seconds. It caught in succession 

 six single positives before the field was thrown off. The 

 corresponding times were 6*5*, 10*, 20*, 100, 15*5, 8'0, 6'0. 

 The mean time during which the X rays had to be on in 

 order to produce a " catch " was in these experiments about 

 six seconds, though in some instances it was as much as a 

 minute. The majority of the times recorded in column F 

 were actually measured with a stop-watch as recorded, but 

 since there could be no possibility of mistaking the 100 

 second speed it was observed only four or five times. It 

 will be seen from Table I. that out of 44 catches of ions pro- 

 duced by very hard X rays there is not a single double. 



Table II. is even more convincing than is Table I. This 

 drop was held under observation for about 4 hours and 100 

 different catches were observed, every one of which was a 

 single. The rays were somewhat softer than those used in 

 obtaining Table I., and corresponded to a spark distance of 

 about 2 inches. 



* Millikan and Fletcher, Phys. Zcit. xii. pp. 161-3 (1911). 

 Phil Mag. S. 6. Vol. 21. No. 12G. June 1911. 3 D 



