Chemistry and thallium
Crookes studied at the Royal College of Chemistry in London and assisted August Wilhelm Hofmann. A green spectral line led him to announce a new element in Chemical News in March 1861. Claude-Auguste Lamy independently investigated thallium and produced the metal in 1862; spectral identification and metallic isolation were different parts of the discovery.
Discharge tubes and cathode rays
In the late 1870s Crookes studied the passage of electricity through partly evacuated glass tubes. The cathode rays in these experiments were later identified as electrons. Röntgen discovered X-rays in 1895 while experimenting with such tubes, connecting this apparatus with subsequent research on electrons and X-rays.
References: [3]
Instruments and scientific publishing
Crookes also investigated instruments including the radiometer and worked in scientific publishing. He studied spiritualist phenomena as well, publishing an account of that work in 1874. These activities form part of his nineteenth-century career alongside his chemical analyses and discharge experiments.
References: [1]