Crookes, Sir William (1832-1919), an English physicist and chemist. In the 1870's he developed a pump that would create a much greater vacuum than had been possible before. He used this pump to withdraw air from sealed glass tubes (now called Crookes tubes) having electrical connections, or electrodes, embedded in the glass at each end. When one of these tubes was connected to an electrical circuit, Crookes observed that the negative electrode (the cathode) gave off a stream of charged particles. He discovered the properties of these particles, now called cathode rays. It was while experimenting with a Crookes tube that Wilhelm Konrad Roentgen discovered X rays in 1895.

Crookes discovered the element thallium (1861). He made small diamonds artificially and developed a chemical process for separating gold and silver from their ores. Crookes was also an authority on sanitation. He was knighted in 1897.