PREPARING GELATINE EMULSION.
To compound such an emulsion, take 100 grains of any hard pure gelatine, place it in a jar or beaker, and cover with distilled water. If the sheet gelatine is used, it is more convenient to cut it into narrow strips. After allowing it to stand for a few moments, rinse it round and throw away the water. Then pour upon it a second quantity of water, and allow the gelatine to swell. The object of the first watering is to ensure its being perfectly clean. About 2 oz. of water may be allowed to 100 grains of gelatine. _Next take 30 grains of citrate of soda, and dissolve it in 1- oz. of warm distilled water. Place the gelatine and water in a water bath, as described on p. 65, until com pletely dissolved. Test the mixture by drawing a glass rod through it, as the solution, while appearing perfect, may contain thick slimy veins of gelatine not completely dissolved, which if present will attach themselves to the rod. When the dissolving action is complete, the citrate solution is added little by little with much stirring. Now take 45 grains of silver nitrate and 80 grains of citric acid, and dissolve in 6 drains of distilled water.
Warm the solution slightly, and then spray it into the gelatine solution as described o'h p. 63. The emulsification being ac complished, it may be dealt with accord ing to the instruction given in the section on Plates and Films, pages 61 to 67. The temperature of the solution must not be too high during the emulsification, or decomposition is liable to set in ; the solution should be only just warm enough to ensure its being thoroughly dis solved. Keeping gelatines in solution at a high temperature destroys their power of setting. The actual temperature will, of course, depend upon the kind of gelatine used. Barker's formula for printing-out emulsion is : Gelatine No. 1 and Coignet's equal parts), 175 grains ; chloride of ammonium, 18 grains ; Rochelle salts, 50 grains ; nitrate of silver, 75 grains ; alcohol, 4 drachms ; water, 5 oz. Heat to 100° F., and allow to remain at this tem perature, after all is dissolved, for ten minutes.