By means of a superposed alternating current the negative resistance of hot cathode neon and mercury direct current arcs was measured and found to follow closely the slope of the static characteristic of the neon arc and to be widely different in the case of the mercury arc. At the same time the reactance for the ripple current in the arc was determined. This reactance is explained on the basis of a time lag in the concentration of metastable atoms; from the phase shift the order of magnitude of the lifetime of the metastable atoms is computed and the result is in agreement with calculations made by others.