ACCELERATORS.
At one time ammonia was the accelera tor almost universally employed with the pyro. developer. It has many good points, being rapid in action and giving excellent detail. It possesses, however, the disad vantage of tending to produce fog, especi ally in cases of under-exposure ; besides which its strong, pungent smell is very objectionable. Modern dry plates appear to be more susceptible to fogging by the use of ammonia than the older makes, and, consequently, sodium carbonate has prac tically superseded ammonia in general favour. Sodium carbonate is identical with common washing soda ; but as the latter is frequently adulterated, it is ad visable to obtain the pure salt at the photographic dealers'. It is also procur able in the anhydrous form, without water of crystallisation. This variety is much stronger than the crystalline form, and must be used in smaller proportions. In
the majority of formulae the term sodium carbonate is intended to mean the crystals. Potassium carbonate is employed as the alkali in some cases : this must be kept in Nvell-stoppered bottles, and dried in an oven before using, as it absorbs moisture from the atmosphere with avidity. Am monium carbonate is occasionally used in stead of ammonia, but is less effective fot ordinary work than the former. All the accelerators mentioned may be used with pyro., as well as with some other devel opers. Caustic soda and caustic potash (sodium and potassium hydrate) cannot be used with pyro., but are excellent in con junction with hydroquinone, pyrocatechin, kachin, and other reducers. They should be purchased in sticks, and should be per fectly dry. Being very absorbent, they must be kept in well-stoppered bottles.