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developer, oxalate, ferrous, action, acid, development, silver and plate

DETECTIVE CAMERA.—See ' DETERGENT—Any substance possessing the property of cleansing. Alkalies have the power of dissolving both soap and grease, hence both are used as detergents. For cleansing glass the agents used are generally Tripoli powder, ammonia, alcohol, caustic soda, potash and diluted acids. A solution of potassium dichromate d sulphuric acid is an effective cleanser, although the use of acids is always open to objection. Cyanide is also a very good detergent, although highly dangerous to use. A glass can usually be rendered perfectly clean by rubbing well with a rag dipped in a cold solution of caustic potash, and then immersing it into a dilute solution of nitric acid, and giving it a thorough washing afterwards.

DEVELOPER.—The agent which builds up, or otherwise renders visible, the latent image. When the plate is exposed on the camera no visible change takes place, but some of the particles have been affected by the light, whilst others remain as they were. The action of the developer may be merely to change the color of these light-affected particles, and render them visible to the eye ; or it may be to build up an image upon those which have been altered. In general terms, the developer may be described as the agent which exerts an action upon those portions of a sensitive compound which have been exposed to the light in a different manner to that wIlich it has upon those parts which have not been so exposed. The two principal developers are the pyrogallic acid and the ferrous oxalate. In the first the reducing action of the pyrogallic acid is assisted with an alkali, such as ammonia, and in the second we have the ferrous oxalate as the reducer.

Alkaline pyrogallic development is, perhaps, the one chiefly used for developing gelatino bromide dry- plates. If we take a dry plate and expose it to the light, we have a certain quantity of silver sub-bromide. If the plate be then immersed in a solution of plain pyrogallic acid, scarcely any perceptible change will take place, but if a small quantity of an alkali, such as ammonia, be added, the plate will instantly blacken, forming metallic silver. The'action that is supposed to take place is this : The silver bromide is split up into silver and bromine, which is immediately laid hold of by the ammonia to form ammonium bromide, and possibly other complex compounds, and the oxygen of the ammonia combines with the pyrogallic acid, causing some immediate actions to take place. The whole theory of the latent image and development

is wrapped up in apparently unfathomable obscurity. Many theories have been put forward by able scientists, differing considerably from one another.

With the ferrous-oxalate developer the action is somewhat similar to the alkaline. This method of development was introduced almost simultaneously by Carey Lea and W. Willis, Jun. The oxalate of potash should be neutral, and the iron sulphate slightly acid. When saturated solutions of these two chemicals are added together in the proportions of one of iron to three of potash the following change takes = The ferrous oxalate is the developing agent; the potassium sulphate is useless, and has apparently no action whatever upon the plate. If much more iron be added than the proportion given a larger quantity of ferrous oxalate will be formed than the solution will hold, and it will fall as a yellow precipi tate, rendering the developer useless.

The action of this developer upon the silver bromide is a little clearer if the following accepted theory be correct:-3(Fe, + =- + FeBr, 4Ag; or, in plainer language, ferrous oxalate and silver sub-bromide give ferric oxalate, ferrous bromide, and silver. With the development it will be seen that ferric salts are produced. These have a directly opposite action to the ferrous salts, so that the developer rapidly loses its strength.

If an old developer be exposed to light the ferric oxalate will become decomposed into ferrous oxalate and carbonic anhydride, thus, + Owing to the non-actinic color of the solution the action of the light is slow; the bottle should be occasionally shaken up. This old developer can then be used to start the development of an exposed plate, adding fresh developer afterwards to give strength to the image. Old developers may also be regenerated by the addition of zinc, which abstracts from the iron the bromine that it has taken during development. ), The non-actinic color of the ferrous oxalate developer allows of a little brighter light being used in the dark room during development.

Many other kinds of developers, such as hydrokinone, hydroxylamine, eikonogen, acetate, phosphate, lactate of iron, etc., etc., are used. Some thousands of different formulx have been given. The most important are the following:—