Active Deposit of Actinium in an Electric Field. Ill 



of slow- moving electrons ; further, the deposit atoms are at 

 least of ionic order o£ magnitude and perhaps larger, and 

 move with relatively small velocities. We may also assume 

 that the recoil atoms effect the process of ionization by some 

 sort of collision with or bombardment of the gas molecules, 

 expelling negative electrons from the molecules with much 

 greater initial velocity than that imparted to the main bulk 

 which forms the positive ion. We may picture to ourselves, 

 therefore, the recoil deposit atom moving all the time in and 

 at the head of a column the core of which consists of an 

 overwhelming majority of positive gas ions and the outer 

 layer is constituted of negative electrons. Now, whatever 

 the charges acquired by the deposit particles at their forma- 

 tion, it is only reasonable to expect that travelling in this 

 core, lined on all sides almost exclusively with positively 

 charged gas ions, they should possess a greater probability 

 of emerging from the recoil columns with a positive charge 

 than otherwise. If the positive charge is acquired by the 

 combination of the deposit atoms with the positive ions of the 

 core, then the resulting positively charged deposit particle 

 assumes molecular dimensions and becomes a cluster under 

 ;favourable circumstances ; this agrees very well with the 

 conclusions arrived at from the discussion of the curves of 

 fig. 3. Further, the more complete the initial exclusion of 

 the negative electrons from the columns, the less will be 

 the number of deposit atoms emerging from the columns as 

 neutral particles, and hence the higher the limiting value of 

 the percentage of the cathode deposit. It is also evident that 

 no negatively charged deposit particles, even if they existed 

 initially in great numbers, could emerge as such from the 

 recoil columns. 



The supposition here involved is that recoil atoms of 

 different initial velocity impart different amounts of energy 

 to the electrons which they expel in the act of ionization, 

 and as a consequence, for a very short time, during the 

 motion of recoil, a separation of positive and negative columns 

 of ions actually occurs. The fact that the limiting value of 

 the cathode deposit for actinium is higher than that for 

 radium is exactly what would be expected when we consider 

 that the recoil atom from actinium A is expelled with a 

 greater velocity than that from radium A. Moreover, since 

 the recoil deposit atom of thorium has a velocity intermediate 

 between that of radium and actinium, we would expect to 

 find the limiting value for the cathode percentage of the 

 thorium active deposit to lie between SS'2 and 94*9. 



Phil. Mag. S. C. Vol. 28. No. 16$. Dec. 1911. 3 E 



