Note 41 — Chapter XVII · BORON, ALUMINIUM, AND THE ANALOGOUS METALS OF THE THIRD GROUP

Thallium was afterwards found in certain micas and in the rare mineral crookesite, containing lead, silver, thallium, and selenium. Its isolation depends on the fact that in the presence of acids thallium forms thallous compounds, TlX. Among these compounds the chloride and sulphate are only slightly soluble, and give with hydrogen sulphide a black precipitate of the sulphide Tl2S, which is soluble in an excess of acid, but insoluble in ammonium sulphide.