Discovered in 1863 by F. Reich and H. Richter in Freiberg, Germany. Indium derives its name from the characteristic indigo line in its spectrum. It is a soft, malleable and ductile metal which is generally unaffected by air or water but is soluble in acids. It is found only in the form of minor components of various minerals (as are gallium and thallium, other members of the boron group of elements in the periodic table) and the pure element is produced by electrolytic reduction in aqueous solution. It has an abundance in the earth's crust of 0.049 ppm. Indium has a large cross-section for slow neutrons and is, therefore, readily activated. Indium is used in the forms of InAs and InSb within the semiconductor industry in thermistors and transistors. As a result of its physical properties, it is particularly suited to being used as a sealing material in vacuum systems and also as bonding material in acoustic transducers. Indium is also widely used in the manufacture of "fusible" materials, a range of alloys which have low melting points and can be used as thermal fuses and solders.