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DRY PLATES, FILMS AND PAPERS 181
of the exposed plate for ten minutes. When the same developer is used at 25° C, the exposed plate begins to fog after 2 J minutes, and in ten minutes is just as fogged as the unexposed plate. It is also noticed that the shorter the exposure to light, the more intense is the fog. The explanation given is, that the exposed plate forms potassium bromide in the developer, and this acts as a restrainer retarding the fog. If this amount of restrainer be added to the developer, the unexposed plates are found to fog no more rapidly than exposed plates. Old plates seem to lag in this production of potassium bromide, for both exposed and unexposed soon fog when placed in the developer.
The necessary alkali, also called the accelerator, which is used so as to help on the action of the reducer, is, as a rule, sodium carbonate. Lastly, sometimes sodium sulphite, which is a more easily oxidizable substance than pyrogallol, is added, as this salt helps to keep the developer from deteriorating.
Such a developer as the above can be varied in composition to suit the special case in hand. Thus if plates are under-exposed, the amount of bromide used should be decreased, and in exceptional cases it may even be advisable to omit it altogether.
On the other hand, if over-exposure is feared, the developer could be changed by putting in less carbonate of soda and increasing the amount of bromide.
It was a very common practice some years since to use ammonia as the accelerator with pyrogallol, because of the excellent detail which it gives, this has now been very largely discontinued owing to the tendency of this solution to bring about a general fog, especially when used for underexposed plates.
As it is an alkali which is required with pyrogallol, it might be supposed that caustic soda or caustic potash would do equally well, but it is found inadvisable to use