Gallium, a metallic element of atomic weight 698, which was discovered by Boisbaudran in a particular variety of zinc ore in the year 1875. He became aware of its existence by means of spectrum analysis (q.v.), and, although it exists in the ore in only very small quantities (about 1 part in 60.000), he succeeded in obtaining sufficient for examination. It is a bluish-white metal, which melts at 30° C, and when melted may remain so for several weeks, even at much lower temperatures, but instantly solidifies if touched by the solid metal. It is tough, and has the specific gravity of 5'9, is soluble in hydrochloric but not in nitric acid. It, is best detected spectroscopically, being recognised by two lines in the violet end of the spectrum. The discovery was rendered more interesting by the fact that the element was found to agree almost completely with the description of a metal whose existence was predicted by Mendeleef in 1870 on the ground of his Periodic Law, and called by him Eka-aluminium (q.v.).