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amidol, solution, sulphite, water, sodium, developers and action

AMBER (Synonym, fossil resin from an extinct pine, Pinites succinifer. A hard brittle substance, generally yellow, but sometimes reddish-brown or whitish and clouded. The chief supplies come from the Baltic. It is used for preparing a varnish, being soluble in chloroform or ether. See Varnish.

AMBROTYPE..—An old term for a picture on glass made by the collodion positive process with another piece of glass as a protector cemented to it with Canada balsam.

AMIDOL.—The trade name for diamidophenol (Formula, molecular weight, 124). —A white crystalline powder, easily soluble in water, giving a colorless solution having an acid reaction. It was first discovered by Gauche as early as 1869. He prepared it by reducing dini trophenol by means of iodophosphorous.* Its power as a developing agent for bromide plates was discovered by Dr. Andresen in 1890. Solutions of amidol quickly discolor by absorption of oxygen from the air, and, therefore, require the addition of a sulphite as a preservative. Amidol is also slightly soluble in alcohol. Ferric chloride turns its solution to a deep red color, and bichromate of potash changes it to a brownish red, a precipitate being formed. According to Colonel Waterhouse, alkalis turn the solution green. With carbonate of lithia and ammonia, the green color darkens and then gradually becomes a bright yellow, which afterward darkens.

Amidol, mixed with water and neutral sodium sulphite, acts as a powerful developing agent, superior in some respects to pyro and other developers. It is particularly remarkable in the fact that it has a slight developing action for gelatino-bromide plates even without the addition of an alkali, the solution, in fact, giving an acid reaction. It is for this reason especially suitable for plates having a tendency to frill.

Amidol gives negatives of a grayish-black color and very transparent in the clear parts. Its action, however, is very powerful, and for this reason it has been stated that the exposure can be somewhat less than that required with other developers. This possibility has not, however, been absolutely decided. The image flashes up almost immediately the plate is immersed in the solution, and, unless accustomed to its use, one is likely to imagine the plate to have been hope lessly exposed. Its continued action, however, is to strengthen the parts necessary, and a brilliant

negative is the final result. As there is less loss of density in fixing after development with ami dol than with other developers, it is not necessary to carry it so far.

For use with amidol, it is advisable to employ the purest form of sodium sulphite. The common kinds contain a very large percentage of carbonates.

Bromides have a restraining action upon amidol developers, but much larger quantities must be added to effect this than with other developers. A small quantity will, however, prevent fog. According to Eder, acids, like sulphuric or citric, and acid bisulphite of sodium, restrain development. Small quantities, however, have but little effect, and large ones stop the develop ing action altogether.

The addition of a weak alkali accelerates development.

Amidol mixed with sodium sulphite will remain good for several weeks. A convenient form of preparing it is to keep on hand a solution of neutral sulphite and add the amidol to it as required. One ounce of developer should contain from 2 to 3 grains of amidol.

Dissolve 10 ounces of neutral sodium sulphite in crystals (Andresen's) in 50 ounces of distilled water, and add Z ounce of amidol. Dissolve, filter quickly, and fill up in small bottles. For use, dilute with from 2 to 4 volumes of water. Do not spare bromide of potassium.

To Zoo C.c. of water add Zoo C.c. of A, and of B from 5 to 5o C.c., according to time of exposure. For normal exposures, 20 C.c. are sufficient. Do not apply the solution till effervescence has ceased. For other formulae see under Developer.

AMIDOL CARTR1DOES.—Small packages made in the form of cartridges, and contain ing the necessary ingredients for the amidol developer. The different substances are separated from each other by means of tufts of cotton wool. When the whole contents are removed and dissolved in a certain quantity of water, the developer is found ready for use. They are very convenient in traveling.