of the Excited Activity of Radium to the Cathode. 53i 



negatively charged particles, and so becomes positively 

 charged. 



To decide between these two hypotheses, Rutherford per- 

 formed experiments in which the number of ions in the vessel 

 containing the emanation was increased by external means, 

 and found that the amount of activity deposited on a cathode 

 in a given time was not increased by this means. For this 

 reason he rejected hypothesis (1) in favour of (2). 



The experiments were, however, carried out at atmospheric 

 pressure ; and it is possible that, under these conditions, the 

 number of ions produced by the emanation is sufficiently 

 great to remove the excited activity as fast as it is formed, 

 in which case any further increase of the ionization would 

 be without further effect. To test this point still further, 

 some similar experiments have been made at a low pressure, 

 using vessel III. The walls of the vessel were made the 

 cathode and the central rod the anode ; 20 volumes of the 

 capillary a (fig. 1) of emanation were introduced into the 

 vessel and allowed to stand for 3^ hours, the pressure being 

 atmospheric. The emanation was then completely pumped 

 out as quickly as possible, and a new inactive rod AB intro- 

 duced and the field reversed. One volume of the capillary a 

 was then introduced into the vessel, and the rod exposed to 

 the emanation at a pressure of 1 mm. of mercury. Although 

 the ionization in the vessel must have been increased about 

 five times by the excited activity deposited on the walls of 

 the vessel during the previous exposure to the emanation, 

 the activity collected on the rod was found to be the same as 

 when the walls were inactive. In a subsequent experiment 

 7 milligrams of pure radium bromide were brought close up 

 to the vessel during the exposure of the negatively charged 

 rod, without affecting the quantity of excited activity de- 

 posited on the cathode in 30 minutes. Supposition (2) is 

 therefore the only one capable of explaining the observed 

 phenomena. 



It is of interest to speculate as to the mechanism by which 

 a negatively charged particle is expelled from the excited 

 activity during its passage through the gas. At the moment 

 of formation of excited activity, the emanation from which it 

 is produced expels an a particle * whereby the residue must 

 itself be projected with considerable velocity. In their flight 



* The negative charge left on the excited activity by the expulsion of 

 an a particle may he neutralized by a simultaneous expulsion of slowly- 

 moving electrons similar to -those which Prof. J. J. Thomson (Proc. 

 Carnb. Phil. Soc. vol. xiii. part 1, p. 49) has shown to be given off by 

 radio-tellurium. 



20 2 



