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Atomic Weight of Gallium History
The vapour densities, referred to air as unity, of the two chlorides of gallium have been determined to be 4.82 (at 1000° to 1100°) and 613 (from 440° to 606°), corresponding to the molecular weights 140 and 178 respectively (O_{2} = 32). Since the chlorides contain 51.0 per cent, and 40.0 per cent, of gallium respectively, the weights of metal in the preceding molecular weights are 71.4 and 71.2. Hence Avogadro's hypothesis leads to the approximate value 71 for the atomic weight of gallium, and to the molecular formula GaCl_{2} and GaCl_{3} for the chlorides.
The specific heat of gallium supports this view, since it indicates an atomic weight of about 80; and it is confirmed by the fact that gallium sulphate forms a series of double sulphates, isomorphous with ordinary alum, Al_{2}(SO_{4})_{3}.K_{2}SO_{4}.24H_{2}O, which double salts, on the assumption that the approximate atomic weight of gallium is 70, must be formulated Ga_{2}(SO_{4})_{3}.R_{2}SO_{4}.24H_{2}O, where R = K, Rb, Cs, NH_{4}, or Tl. The preceding approximate value for the atomic weight of gallium indicates that it is three times the combining weight of the metal in its oxide and highest halogen compounds. The value at present accepted for the atomic weight, Ga = 69.9, results upon a single analysis of gallium ammonium alum and a single synthesis of gallium sesquioxide. The atomic weight of gallium was calculated by Lecoq de Boisbaudran, before sufficient material was available for an experimental determination, from considerations based upon a comparison of the wavelengths of the spectrum lines of gallium and of other allied elements of known atomic weights. His method of calculation leads to the value Ga = 69.86. The molecule of gallium is monatomic in dilute solution in mercury. 
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