OUR knowledge of the nature of gas ions is derived mainly from measurements of their mobility in an electric field. Experimental evidence has shown that as the pressure (p) is altered the mobility (fe) of the ion alters generally in accordance with the law, pk constant; when this law holds good it is safe to. conclude that the ion remains unaltered at the different pressures. One of the the most interesting results of experiment is that in the case of the negative ion formed in air the product pk increases as the pressure is diminished below about 10 cm.; this result indicates a simplification in the nature of the negative ion at the lower pressures. Prof. Townsend and his students have examined this phenomenon in great detail, and have come to the conclusion that the velocity (v) of the ion should be expressed as a function of the field (X) and the pressure (p) in the form v=f(X/p), indicating that the nature of the ion depends on the field and the pressure. In his recently published treatise Prof. Townsend concludes that the negative ion in a dry gas is in general a cluster of molecules which for a certain range of electric forces and pressures passes through a transition stage until finally, when X/p exceeds a certain value, the negative carriers are practically all electrons.