50 S. J. Plimpton — Recombination of Ions 



Discussion of Results. 



The high initial values of a and the subsequent falling off in 

 its value, first rapidly and then more slowly, may be explained 

 as follows : 



If we suppose the action of Kontgen rays to be the liberation 

 of rapidly moving electrons from the gas molecule, which in turn 

 liberate slow-moving electrons from molecules in their path, thus 

 ionizing them, one would expect the ions to be segregated in 

 the path of the rapidly moving electrons as shown by the 

 C. T. R. Wilson cloud experiment. This would make the 

 value of a greater than for a uniform distribution of ionization, 

 and, as a result of diffusion, the value of a would slowly dimin- 

 ish. This would probably not account for the very high initial 

 values of a. It has been found* that the /3-particle produces, 

 on the average only about 67 ions per centimeter of path. As 

 a rule, therefore, there must be a very large number of 

 molecules between them, and the distance between the two 

 ions constituting an initial pair would be, in general, less than 

 the average distance between the ions strewn along the path 

 of the /3-particle. 



If a similar condition exists in the case of ionization by the 

 electrons expelled under the action of X-rays, there would be 

 a relatively great probability of collision between the positive 

 and negative ions which are close together, giving the large 

 initial values of a. 



Comparing the curves of fig. 5 for air and carbon dioxide at 

 760 millimeters, it will be seen that for both gases, the high 

 initial values of a are indicated by the steepness of the curves 

 for short intervals of time, but that the air curve becomes 

 linear at about one-third of a second, while for carbon dioxide, 

 the curve is very steep at this point, suggesting that it has 

 probably not reached its final value. When the two curves 

 were extended farther, it was found, as stated above, that these 

 air curve remained almost linear, but that the carbon dioxide 

 curve became almost parallel with the air curve after about 

 two-thirds of a second. This result is in accord with the sup- 

 position that the variations in a are due to a non-uniform dis- 

 tribution of the ions produced by Rontgen rays. Since carbon 

 dioxide ions diffuse only about one-half as quickly as the ions 

 of, air, the final effectively uniform distribution in which a 

 becomes constant would be reached in carbon dioxide after 

 about twice the time required in the case of air. 



The absolute value of a. 



In order to ascertain the absolute value of a at any time, it 

 is necessary to know the number of ions present in the gas per 



*Geiger and Kovarik, Phil. Mag. (6), xxii, p. 604, 1911. 



