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Broadening of the potassium resonance lines in absorption by argon and nitrogen pressures from 1 to 40 atmospheres has been observed. Half-widths and shifts of the lines, when plotted against "relative density/' yield curves that are straight lines up to about relative density 14, indicating that in this pressure region Lorentz type of broadening predominates. At about relative density 14, the curves depart from linearity. The results are in accord with theory, and partly in agreement with previous work. Theoretical contours for pressure broadened spectral lines have been fitted to those of experiment. Agreement is quite satisfactory, considering the approximations made in evaluating the theoretical curves. Three independent methods of calculation yield for the constant b (measure of van der Waals forces) the values 24(10)~3 2 and 18(10)~3 2 cm 6 sec." 1 for argon and nitrogen, respectively.