produce a
more uniform distribution. A very valuable series of determinations of
the coefficient of diffusion of ions through various gases has been
made by Townsend (_Phil. Trans._, 1900, A, 193, p. 129). The method
used was to suck the ionized gas through narrow tubes; by measuring
the loss of both the positive and negative ions after the gases had
passed through a known length of tube, and allowing for the loss by
recombination, the loss by diffusion and hence the coefficient of
diffusion could be determined. The following tables give the values of
the coefficients of diffusion D on the C.G.S. system of units as
determined by Townsend:--

Table I.--_Coefficients of Diffusion (D) in Dry Gases._

+-----+------------+------------+----------+------------------+
|Gas. |D for +ions.|D for -ions.|Mean Value| Ratio of D for |
| | | | of D. | - to D for +ions.|
+-----+------------+------------+----------+------------------+
| Air | .028 | .043 | .0347 | 1.54 |
| O2 | .025 | .0396 | .0323 | 1.58 |
| CO2 | .023 | .026 | .0245 | 1.13 |
| H2 | .123 | .190 | .156 | 1.54 |
+-----+------------+------------+----------+------------------+

Table II.--Coefficients of Diffusion in Moist Gases.

+-----+------------+------------+----------+------------------+
|Gas. |D for +ions.|D for -ions.|Mean Value| Ratio of D for |
| | | | of D. | - to D for +ions.|
+-----+------------+------------+----------+------------------+
| Air | .032 | .037 | .0335 | 1.09 |
| O2 | .0288 | .0358 | .0323 | 1.24 |
| CO2 | .0245 | .0255 | .025 | 1.04 |
| H2 | .128 | .142 | .135 | 1.11 |
+-----+------------+------------+----------+------------------+

It is interesting to compare with these coefficients the values of D
when various gases diffuse through each other. D for hydrogen through
air is .634, for oxygen through air .177, for the vapour of isobutyl
amide through air .042. We thus see that the velocity of diffusion of
ions through air is much less than that of the simple gas, but that it
is quite comparable with that of the vapours of some complex organic
compounds