Heinrich Johann Wilhelm Geissler (26.05.1815 — 24.1 1879) was a German physicist and inventor. He was born in Igelshieb. In 1854 he founded a workshop for scientific instruments in Bonn and was a mechanic at the University of Bonn, where he collaborated with J. Plücker, supplying him with instruments. In 1855 he invented the mercury vacuum pump, and in 1858 a tube filled with rarefied gas and with two electrodes fused into the glass (the Geissler tube), convenient for studying the spectrum of a gas. He demonstrated the phenomenon of the glow of a gas when an electric current passes through it. He showed (1868) that the colour of the glow depends on the nature of the gas (in 1858 he had suggested that the chemical nature of a gas could be determined from the colour of the glow)