Some time ago dry-plates were placed on the market which would develop, apparently, with water and a little ammonia only. The secret of the method was that the backs of the plates were coated with a soluble gum, containing the developing agents, and, of course, when the plate was immersed in the water, they instantly dissolved and formed the developer. Plates thus prepared are useful in traveling where it is not always possible to get the necessary developing solutions. To prepare them the backs are coated with the following mixture:

This is allowed to dry at an ordinary temperature. After exposure, all that is necessary to develop is to immerse the plates in water containing a small quantity of ammonia.