Note 12 — Chapter XXI · CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, AND MANGANESE

Uranium dioxide, or uranyl, UO2, which is contained in the salts UO2X2, has the appearance and many of the properties of a metal. Uranic oxide may be regarded as uranyl oxide, (UO2)O, its salts as salts of this uranyl; its hydroxide, (UO2)H2O2, is constituted like CaH2O2. The green oxide of uranium, uranoso-uranic oxide (easily formed from uranic salts by the loss of oxygen), U3O8 = UO2,2UO3, when ignited with charcoal or hydrogen (dry) gives a brilliant crystalline substance of sp. gr. about 11·0 (Urlaub), whose appearance resembles that of metals, and decomposes steam at a red heat with the evolution of hydrogen; it does not, however, decompose hydrochloric or sulphuric acid, but is oxidised by nitric acid. The same substance (i.e. uranium dioxide UO2) is also obtained by igniting the compound (UO2)K2Cl4 in a stream of hydrogen, according to the equation UO2K4Cl4 + H2 = UO2 + 2HCl + 2KCl. It was at first regarded as the metal. In 1841 Peligot found that it contained oxygen, because carbonic oxide and anhydride were evolved when it was ignited with charcoal in a stream of chlorine, and from 272 parts of the substance which was considered to be metal he obtained 382 parts of a volatile product containing 142 parts of chlorine. From this it was concluded that the substance taken contained an equivalent amount of oxygen. As 142 parts of chlorine correspond with 32 parts of oxygen, it followed that 272 - 32 = 240 parts of metal were combined in the substance with 32 parts of oxygen, and also in the chlorine compound obtained with 142 parts of chlorine. These calculations have been made for the now accepted atomic weight of uranium (U = 240, see Note 14). Peligot took another atomic weight, but this does not alter the principle of the argument.