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:
Pyrogallic acid 154 grains
Salicylic acid 15 grains
Gum or dextrine 154 grains
Alcohol 1 fluid dr.
Water 5 fluid dr.
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.