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Debye rings of several samples of HgS were taken at various temperatures between 20° and 370°C. Impure samples transformed from α-HgS (trigonal) to β-HgS (cubic) at about 300°C in heating and from β to α at about 200°C in cooling. The transformation temperature in heating or cooling shifts upwards for samples of higher purity. For a sample of 5N purity, the former was 362°C and the latter was 295°C. The thermal expansion coefficients of α-HgS along the a and the c axes were found to be αa = (1.81±0.05) x 10−5 and ac = (1.88±0.08) x 10−5 for 20–200°C, respectively, and that of β-HgS, α = (4.3±1.4) x 10−6 for 211–348°C.