694. Metol

Metol. A whitish powder, soluble in water. Is a developing agent, producing detail even to flatness.

695. Sulphite Of Soda

Sulphite Of Soda. Transparent crystals, also granular and dried (anhydrous). Soluble in water. Two parts of crystals are equivalent to one part dried or anhydrous. Chemical action - neutral, or slightly alkali. Controls tone of the print. Do not confuse sulphite with sulphate of soda.

696. Hydroquinone

Hydroquinone. Nearly white, or pale yellowish needle-like crystals. Must be protected from light and dampness; otherwise it will discolor. Soluble in water. Is a developing agent and produces contrast.

697. Bromide Of Potassium

Bromide Of Potassium. Colorless crystals. Dissolves readily in water and is a restraining agent.

698. Carbonate Of Soda

Carbonate Of Soda. Dried, anhydrous crystals. (See Glossary.) Is extremely soluble in water. Action, alkali. Its action in developing - opens pores of the emulsion in the paper so that the developing agent may act.

699. Hyposulphite Of Soda

Hyposulphite Of Soda. Put up in crystals and granular form. Colorless. Chemical action, found both neutral and acid.

700. Alum

Alum. Colorless crystals; also put up in powder form. Chemical action, acid. Dissolves slowly in water. Possesses qualities that harden the film.

701. Acetic Acid

Acetic Acid. Colorless liquid. Full strength. Will blister the skin. Chemical action, acid. Hardens film and prevents staining. The strength of this acid increases with the temperature; therefore, care should be taken that the fixing bath is kept cool, or the acid will gain too much strength and a strong sediment will be formed in the bath, causing it to become milky and liberate an excessive amount of sulphur from the hypo, causing sulphurization. Prints fixed in a bath of this condition will turn yellow when exposed to the air and light.

702. The Use Of Bromide Of Potassium

The Use Of Bromide Of Potassium. All developers require the addition of a certain amount of bromide of potassium to keep the whites cleared and it is most conveniently used in the form of a 10% solution, which is made by dissolving one ounce of bromide in ten ounces of water. As the amount of bromide necessary varies with the age, degree of dryness of the paper, the purity of the water and chemicals used, it can only be given approximately. It is easily determined, however, by first adding the amount given in the above formula to the solution and then making a trial test by laying an inch strip of Velox over an important portion of the negative and printing and developing it in the regular way. If the whites appear fogged, add a few more drops of the bromide solution. If on the contrary, the whites are clear and the blacks have a greenish tinge, there is too much bromide in the developer and it will then be necessary to add a little of a stock solution of developer that contains no bromide. To avoid this latter necessity, in adding the amount of bromide do so very slowly and drop by drop, making tests as directed.

703. Effect of Bromide on Tone of Velox Print-Note that when just enough bromide has been added to keep the whites clear, the blacks may have a bluish tinge. Then, if more bromide is added, little by little, and a test print made after each addition, the tone of the blacks will be seen to change gradually from bluish-black to pure black, and if still more bromide is added, to greenish or brownish blacks.