The value of radium in medicine has centred attention on radium-bearing ores, with the result that pitchblende and carnotite are treated, primarily, for their radium content, whilst the ex traction of uranium is of secondary industrial importance. To extract uranium from pitchblende the broken-up ore may be treated with sulphuric acid, to which some nitric acid has been added, the concentrations being so adjusted that whereas the uranium goes completely into solution the radium remains behind quantitatively with the insoluble sulphates of barium, lead, cal cium, etc. From the acid filtrate containing uranyl sulphate, the uranium may be obtained as follows. Addition of excess of sodium carbonate solution precipitates iron, aluminium, etc., whilst the uranium forms a double carbonate soluble in the alkali carbonate solution. The addition of sodium hydroxide pre cipitates the uranium as insoluble sodium diuranate in which form uranium comes on to the market and is sold by the trade as yellow oxide of uranium. The extraction of uranium from carnotite is based on the same fundamental reactions as outlined above. It must be noted, however, that many alternative methods of extraction are available. The preparation of the pure element is a matter of difficulty. By reduction of with carbon in the electric furnace, Moissan obtained a product always containing carbon, but Rideal obtained uranium as a powder containing 99.6% of the element by reduction of the oxide with magnesium. From the reaction of uranous chloride, with sodium, potas sium or calcium, uranium may be isolated. Electrolysis of an aqueous solution of a uranium salt does not produce uranium ; instead, hydrogen is obtained at the cathode, often accompanied by a deposit of hydrated uranium oxides. By using a mercury
cathode in an aqueous solution of uranium tetrachloride, uranium amalgam is said to be formed, from which by distillation in vacuo the mercury is volatilized, leaving pyrophoric uranium.
Elemental uranium exhibits a valency of either 2, 3, 4, 5, or 6 in its compounds, but the most stable derivatives are those con taining sexavalent uranium. The oxide exists in yellow and red modifications and has both basic and acidic properties. It dissolves in acids yielding green-yellow solutions from which uranyl salts, such as the nitrate, chloride.
and sulphate, can be isolated. The only true normal salt of sexavalent uranium is the fluoride, The addition of alkali hydroxides to uranyl salts produces insoluble diuranates such as The highest chloride is UCL. Uranous compounds corresponding to the basic oxide are usually green or blue but their solutions are unstable and revert in the air to the sexavalent state. Solid uranium salts when shaken, crushed, or rubbed, emit light, i.e., they have the property of triboluminescence. This is readily observed in a darkened room. Moreover, uranyl salts in solution exhibit a bril liant greenish-yellow fluorescence, destroyed by ferric or uranous salts. In the presence of organic matter uranyl salts are decom posed by light, undergoing reduction to a lower-valency stage. This latter phenomenon is known as photolysis and has been widely in vestigated. Photolysis and fluorescence seem to be connected, for substances which destroy fluorescence accelerate photolysis.
(W. WA.)