Active Deposit of Actinium in an Electric Field. 775 



difference in this particular phenomenon may be more 

 strikingly shown by considering the curves of ionization and 

 activity corresponding to an infinitesimal amount of emana- 

 tion in the vessel. These were obtained by the method used 

 by Wellisch for the case of radium by producing the curves 

 of fig. 2 and of: the corresponding figure for ionization 

 currents, so as to intersect the axis of ordinates, and plotting 

 these points of intersection against the potentials, and ara 

 marked o in fig. 3 ; they may be regarded as limiting curves, 

 corresponding to the absence of volume recombination. They 

 show clearly that any given potential is able to prevent 

 columnar recombination of ions much more easily than of 

 active deposit particles. The two curves approach at about 

 600 volts. 



IV. Summary and Discussion of Results. 



1. When actinium emanation is mixed with dust-free dry 

 air and allowed to come into equilibrium with its active 

 deposit, the percentage of the deposit which is collected by 

 the cathode increases with increasing potentials, but even 

 under the most favourable conditions and at the highest 

 potentials applied there seems to be a definite limit to 

 the percentage of the active deposit which settles on the 

 cathode. This limit is 94*9 per cent., or 95 per cent, 

 roughly. 



2. The remaining five per cent, of the active deposit consists 

 of neutral particles which reach the electrodes by diffusion. 

 It was shown also that no negatively charged deposit particles 

 take part in the transfer of activity. 



3. For values of the activity distribution which are less 

 than this limiting value, the formation of the neutral particles 

 is explained on the view that the deposit atoms recombine 

 with the negative ions in the volume of the vessel for small 

 applied potentials, and with negative ions formed in the 

 columns for larger potentials. Thus both volume recombina- 

 tion and initial or columnar recombination have to be taken 

 into consideration for a complete explanation of the experi- 

 mental results. 



4. It has been shown that both volume and columnar 

 recombination take place at a greater rate between the 

 deposit particles and ions than for the ions among themselves. 

 This was shown by a comparison of the two sets of curves in 

 fig. 3, one for equilibrium ionization current and the other 

 for the cathode percentage of the equilibrium active deposit. 

 This behaviour of the deposit atoms leads one to the- 



