The sensitiveness of the emulsion is said to depend upon the fineness of sub-division of the grain of the bromide of silver in the presence of the gelatine. This extreme fineness is secured by long emulsification, or by the use of ammonia and a boiling heat.
The strength and tenacity of the emulsion depend on the quality of the gelatine, and its freedom from deterioration during the process of emulsification and the subsequent washing. A soft gelatine is suitable for the process of emulsification; afterwards a hard quality is used to give body and resistance.
The presence of free bromide or of free nitrate of silver in the finished emulsion would be fatal to its usefulness; therefore, as both of those salts are easily soluble in water, they must be got rid of by washing; consequently, before washing, the mass of jelly should p40 be induced by the use of a refrigerator to set firm and hard, so that it may be broken up into shreds for washing, and the smaller the shreds the less washing they will require.
Of the following formulæ the first two are copied from English photographic year books, and are believed to be reliable; the others are by Dr. Eder, the greatest German authority on gelatine emulsion, and they are comparatively simple.
GELATINE EMULSION. A.
PAGET PRIZE.
NO. 1.
| Hydrochloric acid | 1 drachm |
| Distilled water | 12½ oz. |
NO. 2.
| Distilled water | 3 oz. |
| Bromide Ammonium | 210 grains |
| Gelatine | 50 |