Florence Cook, Katie King, and William Crookes

Materialization was the most theatrically ambitious claim of Victorian physical mediumship: rather than a knock, a moved object, or a disembodied voice, sitters were told they would see, and sometimes touch, a complete spirit form. The claim's most famous test case is Florence Cook, a young English medium who from 1872 held séances in which a fully formed female spirit calling herself Katie King would emerge, veiled, from a curtained cabinet while Cook herself remained bound and unseen inside.

Chemist William Crookes, already a Fellow of the Royal Society and one of the most respected experimental scientists in Britain, began a formal, extended investigation of Cook's séances in 1874, imposing conditions he described as strict, his own choice of venue and observers, and reported he was unable to explain Katie King's appearances as anything other than a genuine materialized form distinct from Cook; he took a series of photographs of Katie King, forty four in one account, which became widely circulated evidence for Cook's supporters and drew ridicule and protest from much of the scientific establishment, who felt Crookes had staked his considerable scientific reputation on an unworthy and poorly controlled claim.