Toronto. Photography was a success when latent images were discovered (plus a fixer solution). Silver salts, or silver-halides as they were later called were molecules of silver and a “salt” bonded together. These molecules were light sensitive. The stronger the light, the weaker the bonding and easier the molecule broke into metallic silver (black) and the salt.
The 1839 daguerreotype process worked slightly differently. Mercury fumes could “bring out” the latent image (previously invisible to the eye) by bonding to the metallic silver to create white. The silver mirror of unexposed or under exposed silver salt molecules could reflect dark backgrounds (eg dark velvet) to create a positive image. The alternative 1839 process by Fox-Talbot used salt paper (like the called printing out paper of a later era). The light was intense enough to bring out a negative image without further development. To speed up the process, the latent image was later used plus a developer chemical.
Using latent images made the exposures needed in the camera far shorter, but meant darkroom processing before any image could be seen. Looking at the Negative-Positive processes which became dominant until digital took over, various chemicals would convert silver-halides to metallic silver and salts in proportion to the light hitting and weakening the bond. Development rendered the invisible image (latent) visible. Too long in the developer, or too intense a light and most silver-halide molecules became metallic silver and salts, ruining the image. The various developer chemicals would only work in an alkyd solution so immersion in an acid solution (stop bath) stopped the development process in its tracks.
And a fixer would eliminate any remaining light sensitive molecules so the once sensitive material no longer reacted to light and could be viewed and stored. Of course poor fixing or a lack of thorough washing after fixing (or exhausted or contaminated baths) could cause fading over time.