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to be added to it and stirred together. The effect of the mixing shows whether the solutions are properly prepared. The solutions should not at once lose transparency on being mixed together, but should change in colour in about 30 seconds, first to a pinkish yellow, then brownish, and finally to an inky black; prior to this inky black state being arrived at, the mirror should be immersed face downwards in the solution.

The above process is exceedingly effective when the strength of the ammonia is correct. But as this salt has a tendency to largely absorb moisture, its strength cannot be relied upon. If the solutions turn too quickly, or if a brown precipitate is immediately formed, the ammonia is not sufficiently strong, and the result will be a very thin coating of silver, as the deposition will be in the solution and not on the mirror. This result will also take place if B is too strong, and therefore B should not be added too rapidly, in order that if a rapid change of colour seems imminent the addition may be at once stopped. I have adopted two methods of avoiding an excess or deficit of ammonia. With the proportions just given, take 3^ ounces of solution a, and add 4 ounces of solution b, and then 3^ ounces of solution c; if there is a precipitate add some more b until it is re-dissolved, and then proceed as if there had been no precipitate in the first instance. Add a few drops alternately of solutions a and c until a precipitate is just formed. If, however, all the remaining half ounces of a and c can be added without any change taking place, a few more drops of a are sure to produce the required precipitate.

The precipitate, when formed, can be removed by filtration, straining through cotton wool, or by allowing it to settle, and pouring off the clear solution. If this plan is followed out, 4 ounces of solution d must be added to the mixed solutions when they are in the dish. The second modification of this process has been devised by Mr. Ackland, and in it, also, the strength of the Nitrate of Ammonia salt need not be exact

Solution A. —Dissolve 100 grains of nitrate of silver and 180 grains of nitrate of ammonia together in 8 ounces of distilled water, and then add 240 grains of pure potash previously dissolved in 4 ounces of distilled water. Shake or stir well together.

Solution B.—Dissolve 150 grains of sugar candy and 20 grains of tartaric acid in 12 ounces of distilled water and boil for ten minutes, and allow the solution to become quite cold, and then add one ounce of alcohol.

Solution C.—Dissolve 20 grains of nitrate of silver in one ounce of distilled water.

These three solutions should be kept in closely stoppered bottles in a cool dark place.

For use, measure out equal amounts of solutions A and B, but before mixing add solution C, drop by drop, to solution A, stirring or shaking briskly after each drop. A brown precipitate will soon be produced, which should be, if possible, re-dissolved by stirring. Directly

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