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Joined 2026-09-21 Message
Incoming links: Mercury (element)
Andrews's carbon dioxide experiment 2026-10-14
The 1869 The Bakerian Lecture: On the Continuity of the Gaseous and Liquid States of Matter by Thomas Andrews describes dried carbon dioxide trapped above mercury in a sealed capillary tube. A steel screw pressurized water in a strong copper tube; movable mercury transmitted the load. A second capillary tube held air as a pressure gauge. Flowing water around the visible capillary tubes controlled temperature; a thermometer read the bath. A cathetometer read column positions, converted to volume readings using weighed-mercury calibration.[ref]
The Bakerian Lecture: On the Continuity of the Gaseous and Liquid States of Matter by Thomas Andrews. Andrews, pp. 576–578 and Plate LXIII, figures 3 and 5–7: sample preparation, screw, coupled capillaries, water jackets and optical readout
Below the critical temperature, compression produced a visible liquid–gas boundary; above it, large density changes occurred without that boundary. Three trials gave as his critical temperature. Purity checks detected residual air; mercury height corrections accounted for unequal hydrostatic pressures. He explicitly left air nonideality and pressure-induced tube-capacity corrections unresolved, so his air-gauge pressures were approximate. These readouts support a liquid-gas critical point; they do not measure a homogeneous unstable loop or establish universal quantitative accuracy of the later Van der Waals equation.[ref]
The Bakerian Lecture: On the Continuity of the Gaseous and Liquid States of Matter by Thomas Andrews. Andrews, pp. 576, 578–580 and 583: residual air, approximate air-gauge pressure, hydrostatic corrections and three critical-temperature trials
Saturation-pressure equilibrium measurement 2026-10-14
A concrete experiment turns the phase coexistence curve into pairs of temperature and saturation pressure. A 1969 paper reports water measurements made in 1942 and 1947–1949 using a boiler and precision mercury manometer. Platinum resistance thermometers read the boiler temperature through a resistance bridge. The pressure-transfer line passed through an ice-water condenser and a dry-ice trap; helium filled connecting sections. Corrected mercury column height gave the pressure at the thermometer coils, including hydrostatic pressure contributions from water vapor, helium, mercury, and mercury vapor.[ref]
Some Precise Measurements of the Vapor Pressure of Water in the Range From 25 to 100 °C by H. F. Stimson. Stimson, §§1–2, pp. 493–495: boiler, bridge thermometry, and fluid-head corrections
Controls included normal/reversed-lead readings, readings at two thermometer electric currents extrapolated to zero current to remove self-heating, repeated bridge calibration, and daily triple point thermometer calibration checks. Cellar temperature measurements corrected fluid-column and gauge-block thermal expansion. One measurement series was excluded because a trapped gas bubble was suspected of corrupting the mercury head. The paper adjusted measurements to round temperatures using previously tabulated slopes; those adjusted values therefore are not independent measurements of those slopes.[ref]
Some Precise Measurements of the Vapor Pressure of Water in the Range From 25 to 100 °C by H. F. Stimson. Stimson, §2, pp. 494–495, and §3, p. 495: corrections and suspected gas-bubble bias
The thermodynamic inference requires liquid and gas in phase coexistence, a well-defined common temperature, and a pressure corrected to that same location. Leaks, contamination, or a temperature gradient can spoil that interpretation. A separate calorimetry measurement of latent heat, together with the two specific volumes, allows a test of the full Clapeyron equation. Converting a measured phase coexistence curve slope directly into latent heat instead assumes that equation; it is not an independent test.