Cao Dai, a then 30-year-old Vietnamese religion and anticolonialist movement lampooned in Graham Greene's The Quiet American (1955), offers some insight into the cultural context of Radium-Age SF.
Notes that follow are based on an essay in Cabinet #30.
In 1848, two sisters, Kate and Margaret Fox, ages 10 and 8, began communicating with spirits — via rapping sounds — in Hydesville, NY. By midcentury, they were a sensation; New Yorkers visited them to hear the ghosts rapping. A craze for seances spread from the US to Scotland, then England, then the continent. From 1853-55, Victor Hugo — then in exile, on the English island of Jersey — conducted scores of seances, with the spirits of Jesus, Dante, Shakespeare, Moses, Moliere, Racine, even the "living phantasm" of Napoleon III. Not long after that, the French began their rule of l'Indochine française. Communicating with spirits spread to Vietnam; in 1923, when a long-lost seance notebook of Hugo's came to light, the news inspired a group of Vietnamese civil servants to conduct a seance.
The members of the Pho loan group, as they were later called, encountered a spirit later identified as "Jade Emperor, Supreme Deity, alias Cao-Dai, religious teacher of the Southern Quarter." Five years earlier, a spirit called Cao Dai — the term used to designate God in Chinese transations of Protestant scripture — had also appeared to Ngo Minh Chieu, the governor of an island in the Gulf of Siam. Cao Dai had revealed a sign to him: an eye surrounded by rays. (A Masonic symbol that would have been familiar to French-educated Vietnamese officials.)
In 1924, Chieu shared his revelations with the Pho loan group, and together they communicated with Lao-Tzu, Li Po, Joan of Arc, Pasteur, Descartes, Lenin, and Victor Hugo. Western figures, in other words, associated with the ongoing Enlightenment movement, but also Eastern sages and poets.
Writing from the afterlife, Victor Hugo sent a message (in alexandrines) about chemistry: "Yes, it's this kind of gas, which they call hydrogen./More or less dense, which makes the healthiest part,/When they say that the Spirit of God swam above the waters,/It's in that sense that the word must be understood." Hugo, it seems, was rationalizing Biblical passages in terms of modern chemistry; the Spirit of God was a lighter-than-air gas! Note that another spiritist work also concerns itself with chemistry: Occult Chemistry: Investigations by Clairvoyant Magnification into the Structure of the Atoms of the Periodic Table and Some Compounds (1908), by the Theosophists Annie Besant (a Promethean) and C.W. Leadbeater.
Joan of Arc was France's best-loved resistance fighter; Hugo was a famous Frenchman vehemently opposed to Napoleon III's empire, which had grown to encompass Vietnam.
Caodaism was officially founded in 1926. The spirits appointed Victor Hugo head of Caodaism's Overseas Mission. By 1928, there were some 200,000 members. By the early 1930s, there were between 500,000 and 1 million Caodaists, as much as a fourth of the population of South Vietnam. Today, there are about 2 million Caodaists in Vietnam, and perhaps 20,000 in the US.