While paranormal ectoplasm has no basis in fact, its popularity among scientists in the 1800s reflects “the science of its time.”
Daniel Engber at Popular Science:
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Of course, these ectoplasms were a parlor trick. Mediums used sleights of hand to present gauze and animal parts as spiritual phenomena. As silly as this now seems, many intellectuals of the time found the shows convincing, including Richet, who won a Nobel Prize for his pioneering work on anaphylaxis. “Richet was no dummy,” says Robert Brain, a historian of science at the University of British Columbia. Yet Richet was dogged in his studies of paranormal ectoplasm. “What made ectoplasm seem plausible to otherwise rational, clear-headed scientists?” Brain asks. “There had to be an underlying logic to it.”
He’s right. By the mid-1800s, scientists had discovered a gelatinous substance or “plasm” inside plant and animal cells, which they believed to be the basis for all life on Earth.