Deposit of Radium in an Electric Field. 497 



the ionization current, so that in general the percentage case 

 activity is equal to the percentage lack of saturation of the 

 current. The values given in section 5 for the percentage 

 cathode activities at a pressure of 260 mm are smaller for the 

 higher potentials than the corresponding values for atmospheric 

 pressure ; we are consequently constrained to regard the per- 

 centage lack of saturation as being greater for the ionization 

 currents at the lower pressure than at the higher pressure. 

 The horizontality of the curves would thus appear to furnish 

 no evidence as to the degree of saturation. The curves both 

 for activity and ionization current do not appear to have hori- 

 zontal asymptotes such as belong to the ordinary saturation 

 curves for Rontgen-ray ionization. We must rather look upon 

 the curves as having a continued upward slope, even when we 

 are considering the ionization curves corresponding to low 

 pressures. This upward slope suggests that extra ionization 

 is produced by the electric field after the a-particle has ceased 

 to ionize. 



On this view there must be present in the gas certain mole- 

 cules, or neutrons, which are in a condition allowing of rela- 

 tively easy ionization. It is highly probable that the molecules 

 have already been put in this unstable condition by the action 

 of the a-particle ; we may look upon this condition as the result 

 of ineffectual attempts by the a-particle at ionization, possibly 

 as the result of actual ionization immediately succeeded by 

 initial recombination. However, these molecules are left by 

 the a-particle in an electrically neutral although unstable con- 

 dition, and a certain number of them are afterwards resolved 

 into ions, probably as a result of collision with the ions already 

 established in the columns. 



These neutrons are in all probability formed most numer- 

 ously during the early part of the range of the a-particle, when 

 it is moving with its largest velocity. The approximately hori- 

 zontal parts of the ionization curves such as are obtained at 

 low pressures, or when the a-particle moves perpendicularly to 

 the lines of electric force, would therefore only represent satu- 

 ration in the sense that all the free ions have been brought over 

 to the electrodes because in these cases it is unlikely that an 

 appreciable number of neutrons would be resolved into ions. 



As far as the active deposit particles are concerned we may 

 regard them as neutral restatoms which have been exposed to 

 the action of the a-particle at the moment of disintegration of 

 the emanation atom ; we would thus expect them to be for the 

 most part radio-active neutrons which have in general to be sub- 

 jected to further influence before acquiring a positive charge. 



Experiments which are now in progress suggest also that the 

 shape of the Bragg-Kleeman ionization curve can be accounted 



