The average population of 63P1mercury atoms is calculated from the measured 2537 output and resonance radiation diffusion theory. Average populations of 63P2, 0are then found by comparing the absorptions of 5461, 4358, and 4047. Populations for a 0.42‐amp. discharge in 3.5‐mm argon of high purity plus Hg vapor at 42°C in a tube 1½ inches in diameter are for 63P2, 1, 0, respectively, 7.6×1011, 1.9×1011, and 3.5×1011cm−3; these are ⅕, 1/20, and ⅕ of the corresponding Boltzmann populations, for the electron temperature of 11,100°K.Rates of collisions of the first and second kinds involving electrons (average concentration 2.1×1011cm−3), and the 61S0and 63P2, 1, 0states are calculated using known excitation functions and detailed balancing. The results predict populations for 63P2, 1, 0within 10 percent of those observed; they further predict that ⅔ of the observed 2537, ⅔ of the visible triplet, and ¾ of the 3650 group are produced stepwise via 63P. Quenching collisions involving argon are assumed negligible.The results account fairly well for the maximum in the 2537versusHg pressure curve, for the decrease in the 2537 output efficiency with increasing current, and for the decrease in electron temperature which is associated with both these effects.