Indium (a rare metal discovered in 1863 by Professors Reich and Richter of Freiberg, Saxony, by means of spectrum analysis, and so named from two indigo-colored lines in the more refrangible part of the spectrum. It was originally detected in the zinc blende of Freiberg, and has since been found by Bottger in the flue dust of the zinc furnaces at Goslar, by Winkler in the black blende (christophite) of Saxony, by Hoppe-Seyler in the wolfram of Zinnwald, by Kochler in the blende of Schon-feld, and by Cornwall in the blende of Maine. Indium is more easily prepared from crude metallic zinc than from any of the original ores. The metallic sponge remaining after treatment of the impure zinc with dilute sulphuric acid, or obtained as a residue in the cells of galvanic batteries, is acted upon by strong nitric acid, the excess of which is evaporated and the oxide of tin and sulphate of lead filtered off; the filtrate is treated with ammonia, which precipitates the oxide of indium, and after drying this oxide can be reduced to the metallic state by cyanide of potassium or by hydrogen gas. Indium is a brilliant, silver-white metal, destitute of crystalline structure, very soft and delicate, and easily scratched by the finger nail.
It is permanent in the air, and retains its metallic lustre even in moist air and carbonic acid. The specific gravity depends upon the method of its preparation, and varies from 7.11 to 7.42. The melting point is 176° C. Although more easily fused, it is less easily volatilized than zinc or cadmium. At a bright red heat the metal burns with a violet flame and brown smoke. It is slowly dissolved by dilute sulphuric and hydrochloric acid, with evolution of hydrogen; quickly-by strong hydrochloric acid. The atomic weight is given by Schrotter as 75.61, by Bunsen as 113.4. Indium monoxide has the formula InO; its sulphide is yellow, and closely resembles sulphide of cadmium. The chloride is produced by passing chlorine gas over the oxide mixed with coal, or over the metal; it can be easily sublimed to beautiful, brilliant, crystalline scales, and is highly deliquescent. Numerous salts of indium have been prepared, but none of them possess any particular value in medicine or the arts.