INDIUM, a metal discovered with the aid of the spectroscope in 1863 by Reich and Richter when testing certain specimens of Freiberg zinc-blende for thallium. Instead of the brilliant green line characteristic of this latter metal, they observed an intense indigo-blue line occupying a position different from that of any known line, and were thus at once led to suspect the presence of a previously unknown element. The name indium was chosen for the metal, when they succeeded in isolating it, on account of this circumstance. It has since been detected in blendes from various sources, but always in extremely minute amount, and still remains one of the rarest of the elements. Indium is best prepared from crude zinc made from indium-containing blendes. As it is less positive than zinc, if the crude zinc is treated with insufficient hydrochloric acid to dissolve it completely, a residue is obtained containing all the indium together with several other metals also present in small quantity in the zinc. The properties of indium have already been partially described (vol. v. p. 533). Its flame spectrum exhibits, besides the indigo-blue line (w. 1. 4509), a violet line of w. 1. 4101. Lockyer has stated {Royal Society Proceedings, 1878, xxviii. p. 177) that the strongest line in photographs of the spectrum of indium in the electric arc is, as already recorded by Thalén, the h line of hydrogen, the hydrogen line near G being, however, absent. He argues that this is not to be explained by the supposition that the indium contains occluded hydrogen, since none of the hydrogen lines become impressed on the plate when palladium-hydrogen is volatilized in the arc. Indium is commonly regarded as closely allied to aluminium, on account of the general resemblance of corresponding compounds ...