78 Dr. S. J. Plimpton on the Recombination of 



and negative ions which are close together giving the large 

 initial values of a. 



Comparing the curves o£ fig 5 for air and carbon dioxide 

 at 760 millimetres, 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 further, it was found, as stated above, 

 that the 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 supposition that the variations in a are due to a non- 

 uniform distribution 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 unit volume. If the ions are distributed non-uniformly 

 as indicated by these experiments, the rate of recombination 

 at any instant would not be uniform throughout the volume 

 but would vary according to the density of the ions. If the 

 distribution of the ions were known so that an average 

 density for the total volume oE the gas ionized could be 

 computed by integration, it might be proved that a does not 

 vary with the time. Since this datum is not available, the true 

 absolute value of a cannot be computed. In the following 

 table, the effect of a non-uniform distribution of the ions is 

 disregarded and a. is computed as if the ions were distributed 

 uniformly by using the equation, 



It is obvious that the values of n 2 and ?i 2 obtained in this way 

 will be too small, since when the ions are segregated they do 

 not properly occupy the entire volume between the electrodes, 

 but only a small part of that volume. The absolute values of 



