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The refractive index, for [gamma][subscript Na]=5893 [angstroms], of dense fluid argon was measured by the determination of the angle of minimum deviation. The study covered states from 133 to 173[degrees]K for pressures 20 to 100 atm. The density data of J. Levelt were used to calculate values of the Lorentz-Lorenz function for these states.

A prism cell with 1/4" diameter sapphire windows in a cryostat was used to contain the sample. The cell temperature was measured to ?0.015[degrees]K and controlled to ?0.001[degrees]K. The pressure was measured with a gage calibrated against a dead weight tester to ?0.06%.

In a detailed study of the critical region the Lorentz-Lorenz function was used to find the difference between coexisting gas-liquid densities, and the critical coefficient resulting from this work is [Beta]=0.361. The critical temperature was varied to find the best fit of the experimental data to a linear equation for the critical coefficient; the value, T[subscript C]=150.704[degrees]K, is significantly different from the accepted value, 150.86[degrees]K. Values of n[subscript C]=1.0859 and P[subscript C]=48.18 atm. were also determined. The measurements on the coexistence curve outside the critical region yielded an average gas value of (L-L)[subscript G]=4.152 cc/mole. The average liquid value is 4.213 cc/mole. There are no indications of an anomalous value at the critical point. The values of L-L on the eight isotherms between 133 and 173[degrees]K show a sharp peak near the critical point, 2.1% maximum, but this is within the experimental uncertainty.

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