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Physical Properties of Mercury

Mercury is a silvery-white metal of density 13.59535 at 0° C. It is liquid at the ordinary temperature and freezes at –38.88° C. to –38.89° C., forming regular octahedra with a silvery lustre. The effect of pressure on the melting-point has been examined. Reduction in volume takes place on freezing, the density of the liquid at freezing-point being 13.6902 and of the solid 14.193. The density at the temperature of liquid air is 14.382, and the density at absolute zero has been calculated to be 16.39. X-Ray determinations of the crystal structure at -115° C., however, indicate a density of 13.97, assuming one atom per cell. The hardness of solid mercury is 1.5 on Mohs' scale. The atomic latent heat of fusion is approximately 0.56 Cal. The specific heat of mercury is 0.03325 at 0°- 19° C., or atomic heat 6.67. Between -71.0° C. and -183.3° C. the specific heat is 0.0316. Simon finds 1.107 for the atomic heat at 9.78° abs., and 1.570 at 13.35° abs. Pollitzer has obtained values varying from 3.89 at 31.1° abs. to 6.70 at 232° abs.

The following values for the vapour pressure at different temperatures have been compiled from the experimental data of several investigators: -

Temperature, ° C.1650121.8150191.5264.16356.95433.96
Vapour pressure, mm. mercury. 0.00100.01130.8292.80213.02106.527602598.67

The vapour pressures at higher temperatures have also been determined: -

Temperature, ° C.450500600700800880
Vapour pressure, atmospheres.4.25822.350102162

The vapour pressures between the temperatures 120° C. and 435° C. may be represented by the following equation: -

log p = 9.9073436 – 3276.628/θ – 0.6519904 log θ.

The boiling-point is practically 357° C., and the rise of boiling-point per mm. of pressure is 0.0746° C. The atomic latent heat of vaporisation is 14.67 ± 0.05 Cal. At 298.1° abs., and at the freezing-point of mercury it is calculated to be 15.536 ± 0.033 Cal.

Vapour density determinations, and the ratio of the specific heats at constant pressure and constant volume respectively, indicate that the vapour is monatomic.

The approximate critical constants of mercury are: TC = 1700° abs., pC = 1040 atmospheres, dCc = 5.

The thermal conductivity is 0.0248 Cal. per cm. per sec. per degree at 0° C., and increases with rise of temperature. The ratio of the thermal conductivity of solid to that of liquid mercury is 3.

The electrical conductivity, which diminishes with rise of temperature, has been studied by numerous investigators. The unit of electrical resistance, the international ohm, is expressed in terms of mercury. The ohm is defined as the resistance offered to an unvarying electric current by a column of mercury of length 106.300 cm., of mass 14.4521 grm., and of constant cross-sectional area, at the temperature of melting ice. The ratio of the resistances of liquid and solid mercury at the melting-point is about 5.

The thermo-electric power of mercury against different metals undergoes a sudden change at the melting-point, and therefore there is a contact difference of potential between liquid and solid mercury.

The coefficient of cubical expansion by heat is given by Callendar and Moss as 0.0001820536 between 0° and 100° C., but this is criticised by Scheel and Heuse, who support Chappuis determinations, according to which the value at 100° C. is 0.000182541. Hoffman and Meissner adopt the value 0.00018252 at 100° C. Between -190° and -39° C. the coefficient is 0.0000887, and between -78° and -38° C. it is 0.000123.

The compressibility of mercury is 3.96×10-6 megabars at 20° C. It increases with temperature and decreases with pressure.

The surface tension of mercury in air is 464.9 dynes/cm., and in water 374.8. The surface tension in vacuo is practically the same as in dry air, and its dependence on temperature is expressed by the following equation: -

(σ = 467 – 0.043(t+39)-0.000386(t+39)2.

The surface tension of mercury in contact with solutions of electrolytes depends on the difference of potential established between the liquid and the mercury, a phenomenon which led to the construction of Lippmann's capillary electrometer. The most sensitive liquid is ordinary sulphuric acid solution. The effect has been studied for a number of solutions, and also for organic liquids and for air ionised by X-rays.

The physical properties of mercury at the temperature of liquid helium have been examined by Onnes. The electrical resistance at 4.3° abs. is 0.0021 of the value for solid mercury at 273° abs., and at 3° abs. it is only 0.0000001. Further lowering of the temperature to 1.5° abs. produces no measurable alteration in resistance. When a definite current strength, known as the " threshold value," is reached, the resistance practically disappears at a temperature of 4.19° abs. There is apparently a slight increase at 3.65° abs., and the specific resistance at 2.45° abs. is about 2×10-10 ohm. In the presence of impurities the resistance completely disappears at 419° abs. The mean specific heat between 4.26° and 6.48° abs. is 0.00142, and between 2.93° and 3.97° abs. 0.000534. Measurements of thermal conductivity give K = 0.27 between 4.5° and 51° abs., and 0.40 between 3.7° and 3.9° abs. The specific heat and thermal conductivity of mercury give no evidence of the discontinuity shown by the electrical conductivity at 4.19° abs.

The chief lines in the spectrum of mercury, measured in Angstrom units, are: -

Arc: Infra-red: 11288.2, 10140.6; Visible: 7729.46, 7092.46, 7082.92, 6908, 6234.6, 6123.8, 6073.0, 5790.5, 5769.6, 5461.0, 4358.6, 4078.1, 3654.9, 3650.3, 3125.8, 3021.6; Ultra-violet: 2967.4, 2847.8, 2752.9 2652.2 2536.7.

Spark: Visible: 6152.3, 5804.3, 5790.5, 5769.5, 5679.1, 5461.0, 5426.5, 4959.7, 4358.6, 4078.1, 4046.8, 3984.1, 3654.9, 3650.3, 3131.9, 3125.8; Ultra-violet: 2967.4, 2847.9, 2752.9, 2536.7.

The extreme ultra-violet between λ = 2054Å. and 1435Å. has been studied by McLennan and Lang, and, by the methods recently developed for examining the ultra-violet spectrum in the regions beyond that reached by Schumann and Lyman, it has been shown that the limiting wave-lengths of the mercury spectrum probably lie between 1000Å. and 1200Å. The mercury arc is rich in ultra-violet rays, and the quartz-mercury lamp is used as a source of ultra-violet radiation in photochemical investigations, and also for purposes of sterilisation.

The fluorescence of mercury vapour under various conditions has been studied - for example, under excitation by X-rays, ultra-violet rays, or an electrical discharge. The fluorescent spectrum cannot, apparently, be excited in mercury vapour which is quiescent, but only in vapour which is being distilled from the metal at a temperature of not less than 150° C. Hence it has been concluded that the active molecules are not the neutral monatomic molecules, but probably diatomic. The luminescence continues after the removal of the exciting influence, and is probably due to the recombination of positive and negative ions.

Two resonance potentials have been found for mercury vapour, one at 4.9 volts stimulating the line 2536.72Å., and a second at 6.7 volts giving the line 1849Å. The ionisation potential is 10.38 ± 0.05 volts, but, according to Hebb, it is a function of the temperature of the cathode, values as low as 5 volts being obtained at high temperatures. The striking voltage is V = 10.5 – 0.002T, where T is the absolute temperature.

Doubly charged mercury ions have been found at 19±2 volts.

The refractive indices of gaseous mercury, for different wave-lengths, have been determined, and also the dispersion and magnetic rotation.

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