He has studied in a very ingenious manner, not only the mobilities, but also the law of recombination which regulates the spontaneous return of the gas to its normal state.
He has determined experimentally the relation of the number of recombinations to the number of collisions between two ions of contrary sign, by studying the variation produced by a change in the value of the field, in the quantity of electricity which can be collected in the gas separating two parallel metallic plates, after the passage through it for a very short time of the Roentgen rays emitted during one discharge of a Crookes tube.

If the image of the ions is indeed conformable to reality, this relation must evidently always be smaller than unity, and must tend towards this value when the mobility of the ions diminishes, that is to say, when the pressure of the gas increases.

The results obtained are in perfect accord with this anticipation.
On the other hand, M.Langevin has succeeded, by following the displacement of the ions between the parallel plates after the ionisation produced by the radiation, in determining the absolute values of the mobilities with great precision, and has thus clearly placed in evidence the irregularity of the mobilities of the positive and negative ions respectively.

Their mass can be calculated when we know, through experiments of this kind, the speed of the ions in a given field, and on the other hand--as we can now estimate their electric charge--the force which moves them.

They evidently progress more slowly the larger they are; and in the viscous medium constituted by the gas, the displacement is effected at a speed sensibly proportional to the motive power.
At the ordinary temperature these masses are relatively considerable, and are greater for the positive than for the negative ions, that is to say, they are about the order of some ten molecules.