Dry Plates are glass plates coated with gelatine-based suspension of photosensitive silver dried before being taken out into the field. The plate is loaded into a plate holder in the dark and exposed in camera. A latent image is formed and the plate is then chemically developed just like any modern film. They where invented way back in 1871 as a way more practical substitute to wet plates. But with the arrival of modern cellulose based films and development in emulsion speed they unfortunately got lost in time.