The psychical research literature has many examples of séance room materialization phenomena. This article consists of a reprint of, and commentary about, Eric J. Dingwall’s paper “The Plasma Theory”, published in the Journal of the American Society for Psychical Research in 1921. Dingwall discussed some of the previously published ideas on the topic, and emphasized those related to mediums Eva C. and Kathleen Goligher. The purpose of the current article is not to provide evidence for the phenomena, but to present (...) relevant contextual information relevant to the article, additional bibliography, and theoretical concepts, some of which are forgotten today. (shrink)