platinocyanide

“The best suggestion of how it came about it that he was investigating cathode rays with a fluorescent screen painted with barium platinocyanide and a Crookes tube wrapped in black cardboard so visible light from the tube wouldn't interfere.”

The Guardian: Google doodle celebrates 115 years of X-rays

“In 1895, while experimenting with electric current flow in a partially evacuated glass tube (cathode-ray tube), Röntgen observed that a nearby piece of barium platinocyanide gave off light when the tube was in operation.”

Röntgen, Wilhelm Conrad

“A sheet of paper coated with barium platinocyanide was fluorescing, even though it was shut off from the tube.”

Futures Imperfect

“On the evening of November 8, 1895, he found that, if the discharge tube is enclosed in a sealed, thick black carton to exclude all light, and if he worked in a dark room, a paper plate covered on one side with barium platinocyanide placed in the path of the rays became fluorescent even when it was as far as two metres from the discharge tube.”

Wilhelm Conrad Röntgen - Biography

“With mercurous nitrate, the platinocyanide of potassium forms a thick smalt blue, and the platinidcyanide a dark blue precipitate.”

Field's Chromatography or Treatise on Colours and Pigments as Used by Artists

“A replica of the first X-ray tube Again, by experimenting with cathode rays, something new was discovered in the year 1895, the barium platinocyanide was glowing on his shelf.”

CreationWiki - Recent changes [en]

“Röntgen stumbled on X-rays by observing that when he operated a Crooke’s tube essentially a cathode-ray tube, a cardboard screen coated with barium platinocyanide several feet away subtly glowed.”

Simon & Schuster: The Nature of Technology