At first photographers used glass plates they had to coat with an emulsion. But they had to be used within minutes of being coated. They also had to be processed into a picture right away. In 1878, Eastman successfully created dry plates that could be coated with a gelatin solution. These could be used at the photographer's convenience. They did not require all the heavy equipment wet plates did. Eastman decided to go into business selling them. He invented a machine to mass-produce dry plates. In 1888, the company sold the first Kodak box cameras. The slogan was "you press the button, we do the rest". For $24 USD the camera came already loaded with film capable of taking 100 pictures. The camera was sent back to the factory where for $10 the film was developed into pictures. These cameras first used paper film but later turned to Nitrocellulose film. An indexing roller to hold film was invented by David Houston. It had square holes in the edge of the film to advance the film with no waste or Multiple exposures. But Eastman did not want to pay to license his patent. So they bought his patent in 1899. Kodak kept making dry plates until that market dried up. Their main profit was in film and film developing papers and chemicals. They made film for the movie industry, 16mm film for hobby filmmaking and cameras of all kinds. In the mid- 1920s they came out with color film. In 1929 Kodak created motion picture film for adding sound to motion pictures.