Crookes, William (1832–1919)

English scientist whose many chemical and physical discoveries include the metallic element thallium (1861), the radiometer (1875), and the Crookes high-vacuum tube used in X-ray techniques. He was knighted in 1897.

The radiometer consists of a four-bladed paddle wheel mounted horizontally on a pinpoint bearing inside an evacuated glass globe. Each vane of the wheel is black on one side (making it a good absorber of heat) and silvered on the other side (making it a good reflector). When the radiometer is put in strong sunlight, the paddle wheel spins round.

Crookes was born in London and studied at the Royal College of Chemistry. Financially independent, he carried out most of his researches in his own laboratory. In 1859 he founded the weekly Chemical News, which he edited until 1906.

During the 1870s Crookes's studies concerned the passage of an electric current through glass ‘vacuum’ tubes containing rarified gases; such discharge tubes became known as Crookes tubes. The ionized gas in a Crookes tube gives out light – as in a neon sign – and Crookes observed near the cathode a light-free gap in the discharge, now called the Crookes dark space. He named the ion stream ‘molecular rays’ and demonstrated how they are deflected in a magnetic field and how they can cast shadows, proving that they travel in straight lines.

Topics covered by his publications included chemical analysis; the manufacture of sugar from sugar beet; dyeing and printing of textiles; oxidation of platinum, iridium, and rhodium; use of carbolic acid (phenol) as an antiseptic in the treatment of diseases in cattle; the origin and formation of diamonds in South Africa; and the use of artificial fertilizers and their manufacture from atmospheric nitrogen. He also published papers on spiritualism.