In 1871 the English physician Richard Leach Maddox changed photography forever by inventing the gelatin dry plate. Photo printing had always been a tricky process, with processes like the daguerreotype and collodion process requiring mercury or ether vapor to create a light-sensitive plate that could receive the image. In his medical practice, Maddox used photomicrography, or photography under a microscope, and through experimentation discovered that the sensitizing chemicals cadmium bromide and silver nitrate should be coated on a glass plate in gelatin and dried. Dry plates could be manufactured and sold wholesale, rather than made by the photographer as needed in their darkroom.
The next decade saw huge leaps forward in the sensitivity and efficiency of the gelatin silver process, and soon the silver process was transferred onto photo paper, making it even more accessible. Unfortunately, Maddox published his experiments as a gift to the world, made no money from his discovery, and died in poverty.
Reed Enger, "Gelatin Silver Print, The first step toward commercial photography," in Obelisk Art History, Published March 18, 2015; last modified May 28, 2020, http://arthistoryproject.com/mediums/gelatin-silver-print/.