The battery in which the reduction is effected is constructed on precisely the same principles as the silver chloride battery of Warren de la Rue, and one form of this, as is well known, consists of a rod of amalgamated zinc immersed in acidulated water, and opposed to a similar rod of fused silver chloride, a platinum wire being embedded in this latter to serve as a conducting terminal. When the plates of the battery are connected by a conducting circuit, the silver chloride becomes reduced to the con-dition of metallic silver, while the chlorine unites with some of the zinc to form zinc 'chloride.

If the negative plate of the battery is made of fused silver bromide, reduction takes place quite readily when the terminals are united; and when the battery is exhausted, it is merely necessary to fuse the resulting spongy silver in order to obtain it in a convenient condition for use in making a fresh supply of nitrate, while the whole of the bromine takes the form of zinc bromide, and remains in solution.

(u) Those who are familiar with gelatine dry plates must be aware of the large amount of undeveloped silver bromide film there is left on the plate after the development of the exposed image. For the purpose of dissolving the unused silver bromide and clearing the negative, it is immersed in a bath of soda hyposulphite. The bath, in the course of time, naturally becomes highly charged with the dissolved silver bromide, and many processes have been devised for recovering the waste silver therefrom.

One of the most simple is to pour into the hypo lath a saturated solution of potash sulphide, which immediately throws down the silver in the form of a black, flaky precipitate.

The principal objection to this process is the disagreeable smell of sulphuretted hydrogen gas given off, which is especially annoying when large quantities are used. The gas also does damage by injuring chemicals, sensitive plates, and paper within its reach. Dr. Legrange, a German chemist, recommended the use of a solution of ferrous oxalate, but as this is expensive its use has not become general. Quite recently Dr. F. Stolz, editor of the Wochenblatt, has followed out Legrange's idea by suggesting the use of the waste ferrous oxalate or pyro developer for precipitating the silver; a plan which is of practical use to the photographer and amateur. The precipitation of the silver is slower by this process than with the potash sulphide, but it possesses the advantage of utilising the waste developer, which is usually thrown down the sink.

After the old hypo bath has been ' poured into a stone jar or any old vessel the waste developer is mixed with it from time to time after it is used. Repeated pourings of the developer will, after a time, precipitate all the silver.

When all the silver has been apparently thrown down, a little of the clear liquid at the top is taken out in a test tube and tested with a solution of potash sulphide; if no precipitate occurs, all the clear liquid above the precipitate in the bottom of the jar may be decanted off, and the next saturated solution of old hypo may be poured in and treated with the waste developer.

(v) The following is the course we advise for the adoption of the user of dry plates on a moderate scale, who is not averse to the reduction of his expenses by saving his silver waste: - Obtain a two-gallon glass jar having a loosely fitting cover, and about 4 in. from the bottom drill a hole in the side. This hole is then fitted with a wooden plug, or, what is better, a small tap, which is retained in place by screwed nut, aided by rubber washers. This jar is placed on a shelf in a garden or outside of a window; at any rate, in some place with which there is direct open-air communication. This is rendered necessary on account of the offensive smell emitted at a subsequent stage. Into this the waste fixing solutions are poured, and when the jar is nearly full, a small portion of a solution of potassium sulphide (sulphuret of potash or liver of sulphur) is poured in and mixed with the solution by stirring with a wooden or glass slab. Instantly there is a dense deposition of silver sulphide in the form of a dark mass. Precisely how much of the sulphuret ought to be added can be determined only by experiment.

After the precipitate descends a little, if the addition of two or three drops of the sulphuret is not seen to produce a further precipitate, then has enough been added; but if this addition causes further blackness, it must be continued until all the silver is found to be converted into sulphide. It is advisable that no more be added than suffices to effect this end. Owing to the slowness with which the precipitation takes place, it may, in many cases, be expedient to have in use two such jars as that described. After the ' precipitate has settled down to below the level of the hole or tap in the side of the jar, the supernatant liquid must be run off through this aperture. Upon replacing the plug, water may be added to wash the silver sulphide which is insoluble in water. In this case the deposition will take place more quickly than before. Having obtained the silver sulphide as a dark pasty mass, it can either be placed in the hands of a professional metallurgist or reducer, who will most willingly either purchase it at full value, or fuse it into a button of metallic silver; or, secondly, it can be converted into silver nitrate by the addition of nitric acid, in which it readily dissolves; or, thirdly, it may be mixed with a flux composed of soda carbonate, 7 oz.; potash carbonate, 7oz.; potash nitrate, 2 oz.

Mixing this with the silver in equal proportions, placing in a crucible, and then fusing in a furnace, this gives a button of pure silver. The silver nitrate obtained by the second operation is not sufficiently pure for some purposes, but, of course, it may be easily purified. The following is a pretty experiment, and one which is also useful, inasmuch as it reduces the sulphide very quickly. Having out a hollow in a , block of wood, place in it a mixture composed as follows: - Sulphur, 2 parts; nitre, 4 parts; fine sawdust, 2 parts; with an equal volume of silver sulphide. Apply a lighted match to this, and deflagration will take place with great rapidity, and at the close, in a few seconds, the silver will be found at the bottom of the cavity as a beautiful white lump ox shining metal.-r(Photo. Times.)