The Brazilian, or indirect tensile, test, in which a disk is loaded in a diametral plane by forces applied at opposite ends of a diameter, is extended by jacketing the specimen and applying an additional confining pressure. In this way failure may be studied over a range of conditions in which the least principal stress varies from tensile to compressive. The fracture is always an extension fracture in the loaded diametral plane, even if all principal stresses are compressive. The stress analysis is based on elastic strain theory. Experiments were made on three rock types, and they suggest that the value of the intermediate principal stress affects the conditions for failure. This effect is of the same order of magnitude as that shown by experiments on hollow cylinders with axial loading and external confining pressure.

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A structural phase transition, conductivity and dielectric permittivity anomalies, together with a polarization hysteresis loop which collapses above the Curie temperature (∼ 105°C) provide converging lines of evidence for ferroelectricity in the single and polycrystalline natural samples of chalcocite, Cu2S, investigated. Chalcocite is an important ore of copper, and its electrical properties are a factor both in exploration for the mineral and processing the ore.