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Gadolinium sulfate octahydrate is the hydrated salt $\mathrm{G}{\mathrm{d}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3\cdot 8{\mathrm{H}}_2\mathrm{O}$. Gadolinium(III) ions provide localized magnetic moments, while water of crystallization is part of the crystal composition.

The material was used in the Giauque-MacDougall magnetic cooling experiment. Changing the magnetic field under suitable thermal isolation changes the spin entropy and the temperature, connecting the salt's microscopic magnetic states with refrigeration.

William Francis Giauque's 1949 lecture, Some Consequences of Low Temperature Research in Chemical Thermodynamics, credits Peter Debye with an independent proposal. It describes gadolinium sulfate octahydrate in a jacket immersed in liquid helium, with an external solenoid. Exchange gas conducts away heat during magnetization; evacuation isolates the sample before field reduction. A liquid-nitrogen shield reduces radiative heat transfer. Its photograph identifies the 1933 apparatus operated with Duncan Peck MacDougall. An inductance bridge reads the sample's magnetic susceptibility; measuring coils remain outside the evacuated jacket in liquid helium. Increasing magnetic susceptibility indicates cooling, but converting it to temperature using Curie's law becomes approximate as that law fails.^[ref]

The measurement logic deserves a separate check. A changed inductance alone is not an absolute temperature calibration: the sample geometry, applied measuring magnetic field, and constitutive response enter the inference. A modern control would compare the susceptibility-based thermometer with an independently calibrated thermometer over their common range, check recovery after repeated sweeps, and measure subsequent warming to bound incoming heat. These are proposed controls derived from the readout principle, rather than additional procedures attributed to the 1933 experiment. The historical working salt has a richer magnetic level structure than the illustrative two-level paramagnetic cooling model.

William Giauque investigated chemical thermodynamics at very low temperatures. Calorimetry and magnetic cooling connect accessible microscopic states with entropy and thermal response.

A reduction in temperature makes previously negligible level spacings relevant to state populations. Low-temperature measurements therefore test how molecular rotation, spin, and crystal structure contribute to thermodynamic quantities.^[ref]

A chemist at Berkeley whose research and teaching focused on chemical thermodynamics.^[ref]