The quantum theory

tion for contact effects and of the small difference between NH3 and air mobilities. It points, however, in the same direction as the NH3-air work; namely, to a preferential change in concentration of NH3 molecules in the neighborhood of both ions. This effect is noticeable even at high concentrations of NH3. One thus sees in the negative ion the effect of the dielectric constant of NH3 on the mobility without clustering while in the positive ion one has both effects superimposed, the clustering, in this case increasing the mobility by its protective action, the dielectric constant causing a lowering as is to be expected. The results again indicate the importance of minute traces of gases in ion mobility work and thus indicate the necessity of extreme care in controlling the purity of the gases.