I emulsified the two together in the usual way, allowed the whole to cool, and then poured the thin emulsion into about ten ounces of alcohol, stirring the while. As I had anticipated, a flocculent precipitate was formed, which settled to the bottom of the vessel in a few minutes. This was, in fact, sensitive bromide of silver mixed with a very small quantity of gelatine (about five per cent.), and could, I found, be treated in the same manner as a bromide precipitate from an aqueous solution; it might be washed, either by decantation or by filtration, easily dried, and doubtless could, when dry, be kept for an indefinite time, and be at any time used by mixing with gelatine and water in any proportion thought fit.
I found that a less amount of gelatine than four grains to the ounce was sufficient to carry the bromide down, while five grains to the ounce carried it down in something which I considered too near an approach to a plastic mass.
It will be noticed that in the experiments which I have described the emulsion had not been boiled, so that the sensitiveness of the bromide was probably not great. As the experiment was done in daylight it was of no practical use for making emulsion; but I have since made several batches in this manner and have found them most satisfactory.
When sensitiveness is sought by boiling I rind it necessary to add a small quantity of gelatine after boiling and before precipitating, as that which has been kept for some time at a high temperature seems to have lost the viscosity necessary to carry down the silver bromide in such a form that it can he easily separated from the alcohol and water.
The practical manner of making an emulsion by this method may be as follows. Make up the following mixtures:
I.
Silver nitrate...........................................400 grains.
Water..................................................... 3 ounces.
II.
Ammonia bromide..........................................240 grains.
Gelatine..................................................24 grains
Water..................................................... 3 ounces.
Hydrochloric acid enough to slightly acidify the solution.
III.
Gelatine................................................. 20 grains.
Water.................................................... ½ ounce.
IV.
Hard gelatine (say Nelson's X opaque,
or Mr. A. L. Henderson's)................................240 grains.
Soft gelatine (Nelson's No.1)........................... 240 grains.
Water.....................................................24 ounces.
Nos. II., III., and IV. are allowed to stand until the gelatine is softened. No. I is then warmed in a hock bottle until the gelatine is just melted, when No. II. is poured into it, a little at a time, with vigorous shaking, until the whole is emulsified. It is then transferred to an ordinary jelly can, which is placed in a saucepan half full of water over a ring Bunsen burner in the dark room, and boiled for half an hour. It is then allowed to cool to about 100° Fahr., when No. III. is added. The whole is then allowed to get quite cool, when it is poured, with stirring, into about one pint of methylated spirit. If it be wished the precipitate may now be filtered out and washed at once like an ordinary filtrate, but I prefer to allow it to settle, which it will do in about five minutes. The supernatant fluid is then gently poured off.
This fluid will have the appearance of still containing a considerable amount of the silver bromide; but if it be kept and filtered it will be seen that the quantity is really so small that it may be disregarded. We all know what an alarming quantity of silver seems to be going down the sink when we wash vessels to which a very small quantity of emulsion is adhering. If filtering be resorted to the liquid which comes through will be quite clear. This was somewhat unexpected by me, as, if an emulsion containing the whole of the gelatine be precipitated into alcohol in the usual way, the alcohol becomes milky with a substance which could not, I imagine, be filtered from it.
Two or three ounces of methylated spirit are now added to the vessel containing the silver bromide, and the latter well mixed with it. This makes the precipitate "firmer"--if such an expression be allowable--and this time it will sink to the bottom almost immediately after the stirring has ceased, and the alcohol may be poured off.
I consider that the bromide in this state is practically free from soluble salts, but it may be washed with one or two changes of water if desired.