[en] The microcalorimeter vessel is constructed of tantalum metal, with a nominal volume of 5 cm³. Its energy equivalent is 24 J K^-1 when containing 5 cm³ H₂O. The thermal leakage modulus is 0.010 min^-1. A thermistor is employed as the temperature sensor. The operating sensitivity is about 1 x 10^-5 K (300 μJ). The performance of the calorimetry system was tested using tris(hydroxymethyl)aminomethane (TRIS) and magnesium metal. The results of the TRIS experiments, at a concentration of 1 g dm^-3 in 0.1 N HCl at 298 K, yielded a heat of solution of -29.606 +- 0.063 kJ mol^-1. The magnesium experiments, in 1 N HCl at 298 K, gave a heat of solution of -465.965 +- 1.136 kJ mol^-1. The heat of solution of curium-248 metal in 1 N HCl at 298 K was measured. The experiments, which should not be considered definitive, yielded a heat of solution of -606.4 +- 1.8 kJ mol^-1. A single measurement in 6 N HCl gave a heat of solution of -602.3 kJ mol^-1. From these results the heat of formation of Cm³⁺ /sub (aq)/ is calculated to be -607.2 +- 2.5 kJ mol^-1