﻿728 Profs. AYellisch and Bronson on the Distribution of 



was reduced to about one third of an atmosphere saturation 

 set in fairly read.lv. There can be little doubt as to the 

 reality of this columnar effect and the intense recombination 

 resulting from the local distribution of the ions; this is 

 clearly brought out by the slowness with which the ioniza- 

 tion current increases with the potential in the early stage 

 of any curve for a. particle ionization. The experimental 

 results obtained in the present research appear to lead to a 

 radically different explanation of the shape of the ionization 

 curves at the higher potentials. It has been shown that for 

 potentials which are not too low the ratio of the percentage 

 cathode activities for two different potentials is equal to the 

 ratio of the corresponding ionization currents due to the 

 a particles, and over a wide range is independent of the amount 

 of emanation employed, that is, of the intensity of the in- 

 trinsic ionization. This experimental result has already led 

 to the suggestion that the fraction of the total activity which 

 is deposited on the walls of the testing vessel is a measure of 

 the lack of saturation of 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 con- 

 sequently constrained to regard the percentage lack of 

 saturation as being- greater for the ionization currents at the 

 lower pressure than at the higher pressure. The horizon- 

 tality 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 horizontal 

 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 wdien 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 

 molecules, or neutrons, which are in a condition allowing of 

 relatively 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 con- 

 dition as the result of ineffectual attempts by the a particle 

 at ionization, possibly as the result of actual ionization im- 

 mediately succeeded by initial recombination. However, 

 these molecules are left by the « particle in an electrically 



