URANIUM (symbol U, atomic number 92, atomic weight 238.2). Pure uranium is a lustrous white metal although it may be obtained as a brown or black powder. The metal is malleable, softer than steel and slightly paramagnetic. At 13°C. it has a density of 18.685, and although its exact melting point is not known it is stated to be of the order of 1,85o°C. Uranium exhibits marked chemical activity. It burns readily in oxygen at 170°C. and reacts vigorously with the halogens. With nitrogen at I,000°C. a yellow nitride is formed whilst with carbon it yields a crystalline carbide, LT,. Uranium is soluble in dilute hydro chloric and sulphuric acids with evolution of hydrogen, whilst with nitric acid a nitrate is formed. Aqueous alkalis are apparently without action on the element. So far as is known, uranium is the element of highest atomic weight and highest atomic number. Its chief use at the present time is in the ceramic industry, the compounds usually employed being the sodium or ammonium diuranates and the green oxide By varying the composition of the glaze and the firing conditions, colours such as yellow, orange, shades of brown and even dark green may be obtained. In the manufacture of glass, uranium compounds produce an opalescent yellow, green by reflected light. It is claimed that uranium will replace satisfactorily tungsten in high-speed steels. The uranium is added to the steel as an alloy, ferro-uranium, made by heating oxides of uranium and iron with coke in the electric furnace. Uranium salts have been used in photography and are useful in the laboratory for volumetric determinations of phos phates and arsenates.
Prior to 1789 varying views were held as to the composition of the mineral pitchblende. In that year Klaproth advanced the opinion that the ore contained a new element definitely distinct from zinc, iron or tungsten, and by reduction of the mineral at a high temperature, isolated a metallic-looking product which he believed to be the new metal. To this new substance he gave the name uranium in honour of Herschel's discovery of the planet Uranus in 1781. Not till 1841 was it discovered that the so-called element was in reality an oxide, now written as Peligot prepared, in that year, a new chloride (now identified as of which the composition indicated that Klaproth's uranium was not an element, and in the next year, by reduction of the new chloride with potassium, Peligot isolated the element itself. Uranium is not widely distributed in nature but is found in a num ber of rare minerals. The two principal commercial ores are pitch blende and carnotite. Pitchblende or uranitite is an impure uranium oxide of the formula in which the uranium may be considered to be present partly as and partly as The ore may contain 4o-9o% of associated with such impurities as silica, thoria, the rare earths, and compounds of lead, iron, calcium, magnesium, manganese, bismuth, etc. Radium and helium are
always present. Pitchblende is a dark bluish-black massive mineral, found in igneous rocks, and possessing a pitch-like lustre from which fact its name is derived. The most celebrated deposits are those of St. Joachimsthal in Austria where the mines have been worked since 1517. Other deposits are known in Cornwall, in the United States, Russia, Sweden and Norway. A high-grade uranium ore containing pitchblende is found at Katanga in the Belgian Con go. Carnotite, a canary-yellow mineral of secondary origin, occurs principally in sandstone deposits, especially in the United States. Other deposits are in South Australia and in Portugal. Carnotite is essentially a complex vanadate of potassium and uranium of the approximate composition Of the less important uranium ores, the yellow autunite, a hydrated phosphate of calcium and uranium, Ca(U002(PO4)2.8H20, is generally as sociated with pitchblende. Uranium occupies an important place in the history of scientific discovery. In 1896, Henri Becquerel exposed a photographic plate, wrapped in black paper to the action of the beautiful fluorescent salt, potassium uranyl sulphate, and made the momentous observation that a distinct impression had been produced on the plate. Further ex periments indicated that metallic uranium, all uranium salts and uranium minerals (especially pitchblende) gave rise to the same phenomenon. This epoch-making discovery, which initiated the study of radioactivity, was closely followed by the discovery of radium in pitchblende. Increasing knowledge of radioactive pro cesses clearly showed that uranium was continuously undergoing atomic disintegration whereby products of less atomic weight were produced. Amongst these products was radium, and its presence in all uranium minerals was found to be a direct outcome of the degradation of uranium atoms. Moreover, in uranium ores, where the conditions have been such that the radium has not been re moved mechanically, it has been found 3.4 X gram of radium is in equilibrium with one gram of uranium. So well is this relationship established that in the sale of radium-bearing ores it is the common practice to estimate the radium, not by anal ysis, but by calculation from the determined uranium content.