Until recently far more attention has been given to the problem of the mobility of the negative ion than to that of the positive since the phenomena relating to the latter appeared to be simpler and already well defined. The position has, however, been modified by the interesting discovery by Erikson of the existence of two types of positive ions in a given gas, namely initial ions of short age and final ions of lower mobility formed by their later transformation. Evidence of the existence of these two types has also been obtained since by Wahlin. The method adopted by Erikson involved the use of a blast of gas of high velocity, of the order of 2000 cm. per sec., in order to obtain the shortage necessary. He found that the initial positive ions had a mobility which could not be distinguished from that of negatives, and that in the case of air this held whether he used air saturated with water vapour or air previously dried by passage over calcium chloride.