Antimony discontinued. Antimony is resistant to attack by acids. Four allotropes of antimony are known: a stable metallic form and three metastable forms (explosive, black and yellow). Elemental antimony is a brittle, silver-white shiny metalloid. When slowly cooled, molten antimony crystallizes in a trigonal cell, isomorphic with the gray allotrope of arsenic. A rare explosive form of antimony can be formed from the electrolysis of antimony trichloride. When scratched with a sharp implement, an exothermic reaction occurs and white fumes are given off as metallic antimony forms; when rubbed with a pestle in a mortar, a strong detonation occurs. Black antimony
Crystal detector voltage of around 0.2V from a battery to make it more sensitive. Although the zincite-chalcopyrite "Perikon" was the most widely used crystal-to-crystal detector, other crystal pairs were also used. Zincite was also used with carbon, galena, and tellurium. Silicon was used with arsenic, antimony and tellurium crystals. During the first three decades of radio, from 1888 to 1918, called the "wireless telegraphy" or "spark" era, primitive radio transmitters called spark gap transmitters were used, which generated radio waves by an electric spark. These transmitters were unable to produce the continuous sinusoidal waves which are used to transmit audio (sound) in