GALLIUM, a metallic element of steel-grey colour melting at 30.15° C and boiling at about C. Owing to these proper ties it has been recommended as a thermometer liquid for high temperatures (S. Boyer, 1926). Gallium (symbol Ga, atomic number 31, atomic weight 69.9), contracts on melting—the spe cific gravity in the solid state is 5.885 and in the liquid 6•o81. It was discovered in 1875 through its spectrum (violet lines X 4172 and 4033) in a specimen of zinc blende of Pieerefitte (Haute Pyrenees) by Lecoq de Boisbaudran. Its chief physical and chem ical properties had been predicted six years previously by D. I. Mendeleeff, who attributed certain qualities to an undiscovered element, eka-aluminium, which coincided with those actually pos sessed by gallium. It is one of the scarcest of elements although widely distributed through nature in minute quantities, occurring especially in many zinc blendes and bauxites, the black blende of Bensberg containing 0.0016% gallium.
Gallium accumulates in the final residues of the distillation of zinc spelters from Bartlesville, Oklahoma. The alloy containing gallium, indium and zinc is converted into anhydrous chlorides, and gallium trichloride, being by far the most volatile chloride present, is purified from the others by distillation (L. M. Dennis and J. A. Bridgman, 1918). From its ores gallium is extracted by dissolving in aqua regia, and after removal of acid other metals are precipitated by metallic zinc, whereas only after prolonged boiling with this metal is a basic gallium salt deposited. This process is repeated, and finally metallic gallium is isolated by electroylsis of an alkaline solution of its salts (de Boisbaudran, 1877).
Metallic gallium dissolves only slowly in nitric acid but more readily in hydrochloric acid, aqua regia or aqueous caustic potash. In these respects it shows its relationship to aluminium. It forms a well-defined oxide, infusible at a red heat; the corresponding hydroxide, Ga(OH)3 is amphoteric, like aluminium hydroxide, dissolving either in acids or in aqueous alkalis. De Boisbaudran recorded two chlorides, and the latter is a crystalline, hygroscopic solid melting at C and boiling at 2I0-215° C.
Gallium sulphate, Ga2(SO4)3, forms well-defined alums with the sulphates of ammonium, potassium, rubidium and caesium having the general formula R' Ga (SO4) 2, I where or an alkali metal. (See ALUM.) Gallium acetylacetone, prepared by the inter action of acetylacetone and freshly precipitated gallium hydroxide, separates from chloroform or acetone in acicular or tabular crystals and melts at 194°-195° C. It sublimes under reduced pressure with partial decomposition and exhibits isomorphous relationships with the acetylacetones of indium, aluminium and iron (G. T. Morgan, H. D. K. Drew and T. V. Barker, 1921).
Gallium aluminium alloys are suggested for optical mirrors and as cathodes in metal vapour lamps. In a monochromatic lamp a gallium-cadmium alloy produces a pure red light.