 The psychical researcher Gustav Geley defined ectoplasm as being "very variable in appearance, being sometimes vaporous, sometimes a plastic paste, sometimes a bundle of fine threads, or a membrane with swellings or fringes, or a fine fabric-like tissue".Arthur Conan Doyle described ectoplasm as "a viscous, gelatinous substance which appeared to differ from every known form of matter in that it could solidify and be used for material purposes".The physical existence of ectoplasm has not been scientifically demonstrated, and tested samples purported to be ectoplasm have been found to be various non-paranormal substances.