Working on spectroscopy, in 1861 he discovered a previously unknown element with a bright green emission line in its spectrum. He named the element thallium, from the Greek `thallos' a green shoot.
He was the inventor of the Crookes radiometer, which today is made and sold as a novelty item.
In his investigations of the conduction of electricity in low pressure gases, he discovered as the pressure was lowered, that the negative electrode appeared to emit rays (the so-called `cathode rays', now known to be a stream of free electrons, and used in cathode ray display devices). He was one of the first scientists to investigate what are now called plasmas
In his later years he became involved with the Society for Psychical Research, though his association with a young spirtualist medium, Florence Cook, caused considerable detriment to his scientific reputation - it was rumoured at the time that he was in love with her and helping fake evidence.