silver chloride

It had already been proved that paper soaked with silver chloride would blacken when exposed to sunlight; this discovery was one of the earliest stepping-stones toward the field of photography. [Johann] Ritter wondered whether all sunlight’s colors would create this reaction with equal speed. He exposed silver chloride–soaked paper to various parts of the prismatic spectrum, cast onto the paper by sunshine striking cut glass. Red light had only a negligible effect in darkening, or reducing, the compound to silver, while green light did it much faster and violet did it fastest of all. Ritter then placed the chemically soaked paper in the dark spot beyond the violet end of the spectrum, and voilà—the paper darkened even more rapidly than it had in violet light. Obviously some invisible rays that lay beyond the violet end of the spectrum had a dramatic, repeatable chemical effect. Ritter had done it—he had discovered an entirely new form of invisible light [ultraviolet].

2017, Bob Berman, Zapped: From Infrared to X-rays, the Curious History of Invisible Light, Little, Brown and Company, →ISBN, pages 50-51