268 MR. J. S. TOWNSEND ON THE DIFFUSION OF IONS PRODUCED IN AIR 
7*^1 KZ 
accompanying curve has for co-ordinates y and yfy 1 , and from it we can deduce the 
values of K corresponding to the different pressures. These values are given in the 
following tables :— 
Table V.—Positive Ions, Dry Air. 
p. 
K. 
P x K. 
0 . 
772 
•0317 
24 - 5 
19 
550 
•0420 
23T 
13 
400 
•0578 
23T 
16 
300 
•078 
23-4 
13 
200 
T18 
23-6 
12 
Table VII.—Positive Ions, Moist Air. 
P. 
K. 
P x K. 
e. 
772 
•0364 
28-0 
18 
400 
•0668 
26-7 
11 
200 
•134 
26-8 
9-5 
Table VI.—Negative Ions, Dry Air. 
P. 
K. 
P x Iv. 
0 . 
772 
•0429 
33 
19 
550 
•0542 
29-8 
13 
400 
•078 
31-2 
16 
300 
T03 
30-9 
13 
200 
T55 
31-0 
12 
Table VIII.—Negative Ions, Moist Air. 
P. 
K. 
P x K. 
0. 
772 
•0409 
31-5 
18 
400 
•0771 
30-8 
11 
200 
T47 
29-4 
9-5 
These tables show that in each case the rate of diffusion of ions into a gas is 
inversely proportional to the pressure of the gas. 
The coefficients of diffusion at 772 millims. show a discrepancy from this law which 
is somewhat greater than the probable error of the experiments, but we should not 
expect closer agreement between the products P X K unless the temperature of the 
air was the same in each case. It will be noticed that the experiments at 772 millims. 
pressure were made when the temperature of the air was higher than the tempera¬ 
ture during the other experiments. 
Section III .—Ions produced by the Action of Ultra-violet Light on a Metal 
Surface. 
The apparatus which is described in Section II. can, with slight alterations, be 
used for ions produced by various methods. In order to distribute ions produced by 
ultra-violet light in the stream of air passing along the tube A, the changes shown 
in fig. 3 were made. The windows W : and W 3 were cut in the long tube, and two 
