W. R. Barss — Measurements of Radio-activity. 547 



separating the positive and negative ions. Hence the lack of 

 saturation should be more apparent in the former case than in 

 the latter. These facts were experimentally determined by 

 Moulin. He obtained saturation for the parallel field at 1200 

 to 1500 volts per centimeter while for the perpendicular field 

 only about 200 volts per centimeter were necessary. 



Moulin concludes that general recombination within the 

 columns (proportional to the square of the density of ioniza- 

 tion within the columns) is so much greater than "Initial Re- 

 combination" that the latter is negligible in comparison. 



Ionization by <x-particles was further investigated by Whee- 

 lock;* among other results, he obtained the following. When 

 an electric field is applied parallel to the path of the a-parti- 

 cle and therefore parallel to the axis of the column of ions, the 

 column would not be broken up and the recombination occur- 

 ring would be between ions belonging to the same column. 

 Since each particle makes the same number of ions along its 

 path, the density of ionization would be the same in any one 

 column and therefore it would be expected that the ratio of 

 currents obtained with sources of different intensities would be 

 constant for different potential gradients applied. When the 

 field is applied perpendicular to the column and when the 

 source of ionization is small, very few columns would exist in 

 the ionization vessel during the time required for the ions to 

 be carried over to their respective electrodes. Hence there 

 would be little chance for recombination between the columns, 

 so that it would be expected that the ratio of currents obtained 

 with sources of different intensities would be constant as in the 

 case of the parallel field. When the field is perpendicular 

 and the source of ionization is stronger, enough columns might 

 exist between the electrodes at one time to make recombina- 

 tion possible, not only between ions of the same column but 

 between those of different columns. In this case the ratio of 

 currents obtained with different source intensities might not 

 be constant because of the added recombination of ions of dif- 

 ferent columns. 



Wheelock found that the ratio of the current produced by a 

 more intense source of rays to that produced by a weaker 

 source is constant for the parallel field ; that it is approxi- 

 mately so for the perpendicular field when the sources are 

 both weak, and that it increases slightly with the potential 

 gradient applied when the sources are stronger. This is as 

 would be expected if the ions formed by a-particles are arranged 

 in columns. 



When the gas is ionized by /3- or X-rays it would not be 

 expected that the ratio of currents obtained for different source 



* This Journal, xxx, 233, 1910. 



