Solubility in silver amalgams and deviations from Raoult's law

The validity of direct determination of the solubility of silver in mercury is established from theoretical and practical considerations, and error due to using a sealed vessel is shown to be negligible. Values of solubility from room temperature up to 450°C., i.e., well above the boiling point of mercury at normal pressure, are reported. From liquidus data it is shown that solubility right up to the melting point of silver can be represented very closely by three straight lines: log₁₀N = 0.67035 - 1134.7T^-1. . . .up to 330°C. log₁₀ N = 2.9065 - 2481.8T^-1. . . .330° to 450°C. log₁₀ N = 0.7441 - 918.2T^-1. . . .450°C. to the melting point of silver Their intersections, however, have no relation to the peritectic formation temperatures of Ag₅Hg₄ at 276°C. and Ag₅Hg₈ at 127°C., and these points do not appear at all on the logarithmic curve. At low temperatures the solubility is about one-fortieth of that calculated from the elementary equation established by Schröder in 1893: log₁₀ N = H_f/R(1/T_m - 1/T) or about one-hundredth if H_f is corrected for temperature variation. Possible explanations for this are unconvincing. The approximate expression derived from consideration of unlike atoms RT log₁₀ (a₁/N₁) = KN₂² (a being the Schröder solubility) fits the experimental curve very well up to 340°C. if K = 465, as calculated from solubility at the steam point; and even after its departure gives an analogous reversed-S curve. A roughly parallel curve holds for the solubility of silver in liquid tin. In the more exact expression RT log₁₀ (a₁/N₁) = V₁ (N₂V₂/N₂V₂ + N₁V₁)² [P^1/2₁ - P^1/2₂]² the observed solubilities lead to a figure of 14 for [P^1/2₁ - P^1/2₂] compared with approximate values of 23.8, 40.9, 47.3, 18.2, 39.1, derived from diverse physical properties. The deviation from theoretical solubility is thus only a half or a third of what might be expected from consideration of the internal pressures of the two metals. It is concluded that the mutual attraction within a pair of unlike atoms is greater than the geometric mean of corresponding attractions within pairs of like atoms, in sharp contrast to the behaviour of most organic molecules and to previous conjectures, on theoretical grounds. A convenient method is described for determining strain in glass vessels after blowpipe manipulation, by use of polarized light.