parts of alcohol (92 per cent.) and 9 parts of solution of ammonia (sp. gr. =0-91). Dissolve 30 parts of nitrate of silver in 150 parts of water, and now, in the dark-room, add the silver solution in small portions and with frequent shaking to the alcoholic bromide solution. The mixture should now be shaken frequently for two hours, the flask being closed by means of a cork, and the whole allowed to stand ten hours, or if made in the evening, till the next morning. Forty parts of Winterthur gelatine must now be covered with distilled water, allowed to soak for half an hour, and then, after pouring off the surplus water, it should be melted and added to the emulsion, which should be heated to 350 C. (= 950 F.). The whole must now be well shaken and poured out into a flat dish to set (and this should take place in an hour or two), and then broken up and washed. In the winter it will be difficult by this process to obtain a sensitiveness greater than about 150 W. after the emulsion has stood ten hours. In the summer, however, as high as 220 W. may be obtained, but there is a danger, if the emulsion is allowed to stand more than eight hours, of fog setting in, or of obtaining thin emulsions of no value in practical work. Henderson's original method was to pour the liquid emulsion into three or four times its quantity of alcohol, and stir with a glass rod ; when the emulsion adhered to the rod, it was removed, cut up, and well washed. Henderson has since suggested another process of making emulsion, which is some-what similar to one proposed by Obernetter in 1882.
There are several methods of washing emulsions but to the amateur emulsion-maker the simplest is to place the set emulsion in the piece of canvas netting previously mentioned, to gather up the ends, and then twist the same and force the emulsion through the meshes, thus breaking it up into little nodules, or shreds, which, 1 presenting a greater surface to the washing water, allow of a quicker extraction of the inert salts. The squeezing of the emulsion should always be done in distilled water, care being taken to wash both the hands and canvas well first, and rinsing the same in distilled water. In breaking up the emulsion in this way there is always some lost by adherence to the canvas, but not much ; and where experimental batches of, say, 4 or 5 ounces only are made, the simplest plan is to cut the emulsion up, with a silver fruit-knife, into little dice, and then place these in a