Actinium in an Electric Field. 553 



selves, i. e., volume recombination takes place at a widely dif- 

 ferent rate for the two cases. 



The same remarks hold true in the case of columnar recom- 

 bination also. The fact that the central electrode receives, 

 even for the higher potentials, a smaller proportion of charged 

 deposit particles than of positive ions shows that the deposit 

 particles are liable to lose their charge by recombination in the 

 columns more readily than the positive ions. The difference 

 in this particular phenomenon may be more strikingly shown 

 by considering the curves of ionization and activity correspond- 

 ing to an infinitesimal amount of emanation 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 are 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 favorable 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 recombination and initial 

 or columnar recombination have to be taken into consideration 

 for a complete explanation of the experimental results. 



4. It has been shown that both volume and columnar recom- 

 bination 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 



