This second slower group was resolved only in the third order maxima and was present in such small numbers that its mobility could not be accurately determined. Nevertheless, when the two groups of ion are resolved in this way the correct value is obtained for the mobility of the majority group. The lower values of mobility, sometimes found when the first and second order maxima were used, were due to the perturbation of the peaks of the majority group by the unresolved minority group. This minority group had a lower drift velocity than the other group and was seen to become more predominant as the conditions were altered so that the number of collisions between the faster ions and the gas molecules was increased.