Since, as we have seen, the recombination is due to the coming together of the positive and negative ions under the influence of the electrical attraction between them, it follows that a large electric force sufficient to overcome this attraction would keep the ions apart and hence diminish the coefficient of recombination. Simple considerations, however, will show that it would require exceedingly strong electric fields to produce an appreciable effect. The value of [alpha] indicates that for two oppositely charged ions to unite they must come within a distance of about 1.5 × 10^-6 centimetres; at this distance the attraction between them is e² × 10^12/2.25, and if X is the external electric force, the force tending to pull them apart cannot be greater than Xe; if this is to be comparable with the attraction, X must be comparable with e × 10^12/2.25, or putting e = 4 × 10^-10, with 1.8 × 10²; this is 54,000 volts per centimetre, a force which could not be applied to gas at atmospheric pressure without producing a spark. _Diffusion of the Ions._--The ionized gas acts like a mixture of gases, the ions corresponding to two different gases, the non-ionized gas to a third. If the concentration of the ions is not uniform, they will diffuse through the non-ionized gas in such a way as to 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.