(For Developing Agents, see Chapter XI (Development Of The Negative))

The figures following the chemical symbol show the molecular weight.

Acetic Acid, C2H4O2, 60

A colourless liquid with strong vinegar smell. The glacial quality is that employed in photography. Solidifies at 340 Fahr. Specific gravity ro66. A corrosive poison, blistering the skin in concentrated form.

Acetone, C3H6O, 58

Also called methyl-acetyl. A volatile, highly inflammable liquid, with pleasant odour. Generally prepared by dry distillation of calcium acetate. Sp. gr. 0792. Sometimes used as a substitute for alkali in developers. A solvent of celluloid, and therefore used in repairing celluloid films and making celluloid varnish. Sometimes added to sensitiser for carbon tissue in order to promote rapid drying.

Alcohol, C2H5OH, 46

A highly volatile, colourless liquid, prepared by distillation from fermented sugars or any vinous liquid. Absolute alcohol contains up to 5 per cent. of water, rectified spirit about 10 per cent., or 58 degrees over proof. Sp. gr. .834. Used in dissolving collodion and varnishes. Methylated spirit contains, in addition to 10 per cent. of wood spirit, a certain quantity of petroleum. It is not therefore suitable for any chemical purposes, though often employed for drying negatives and carbon prints. There is always a danger of oily streaks on the film, and this must be taken into account when negatives dried with spirit have subsequently to be intensified.

Alum

A general name given to double salts of aluminium sulphate in combination with other sulphate salts, all the leading varieties having the same property of hardening gelatine, though in varying degree. All crystallise in octahedra. Potash alum, K2S04A12(S04)324H20, is the one usually sold commercially, though ammonia alum, in which (NH4)2 is substituted for the K2, is frequently supplied, and is rather more soluble (1 in 8 of cold water). In alcohol and ethers potash alum is practically insoluble. Used for clearing and hardening baths for gelatine film, generally in conjunction with an acid. Forms an insoluble precipitate in the hypo bath. Chrome alum, K2S04Cr2(S04)324H20, is a deep purple salt, crystallising in the same form, solubility varying from 1 in 6 to 1 in 10 in cold water, used chiefly for hardening dry-plate emulsions. Aluminium sulphate, Al2(S04)318H20, has also been recommended as a substitute for these variable forms for hardening films and emulsions.

Ammonia, NH3, 17

An extremely pungent gas, usually supplied in the strong aqueous solution sp. gr. .880. A highly irritant poison, the fumes of which cause serious trouble if persistently inhaled. Antidote : vinegar and water. Weak solutions are used in most photographic processes, although ammonia is now considered less suitable than potassium or sodium salts for development purposes.

The aqueous solution of ammonia, being but slightly ionised, is a much weaker base than soda and potash, which in dilute solutions undergo complete ionisation. Ammonia combines with most silver compounds, either as a gas on the dry solid, or in solution on a precipitate, to form bodies of the type AgCl(NH3)x wherein x may be either 1, 2, or 3. These compounds are mostly soluble.

Ammonium Bichromate (NH4)2Cr2O7, 252

Sometimes used in place of the potash salt for chromate printing processes. But in practice it is generally simpler to make up the solution with potassium bichromate and then add ammonia by degrees until the liquid becomes a light yellow colour.

Ammonium Bromide, NH4Br, 98

Crystals, colourless, becoming yellowish on exposure to air. Solubility 1 in 1.4 of cold water, very soluble in hot water. Frequently adopted in place of potassium bromide for emulsions.

Ammonium Carbonate

A somewhat variable substance, the normal of which is (NH4)2C03, volatile and caustic, smelling strongly of ammonia. Occasionally prescribed for developing solutions, but not to be recommended, except in minute proportions, for adding warmth to tones with hydro-quinone on bromide papers.

Ammonium Chloride, NH4C1, 53.5

Known in commercial form as Sal Ammoniac. Small white crystals, or fibrous masses. Used for salting various printing-out papers in place of sodium chloride. Solubility 1 in 3 of cold water.

Ammonium Citrate (NH4)2C6H6O7, 226

An extremely deliquescent salt, and not easy to keep in any form. It is expedient to prepare as required by neutralising 12 parts of citric acid with 11 parts strong ammonia, added gradually, and then making up to 24 parts with distilled water. Used as a restrainer in developing over-exposed plates, and in some emulsion formulae.

Ammonium Iodide, NH4I, 145

Solubility 1 in 0'6 of cold water, 1 in 4 alcohol, and very soluble in ether or mixtures of alcohol and ether.

Ammonium Persulphate (NH4)2S2O8, 228

Large colourless crystals. Strongly acid. Solubility 1 in 1 1/2 cold water. Used chiefly in solutions of about 2 per cent. as a reducer of the high lights in negative. A solvent of gelatine.

Ammonium Sulpho-Cyanide, NH4CNS, 76

Colourless deliquescent crystals. Best preserved in solution. Solubility 1 in 06 cold water. Used for toning gelatino-chloride papers, and is also a fixing agent, but its high price prevents any competition with the ordinary "hypo."

Amyl Acetate, C7H1402, 130

An amyl ester boiling at 1400, and having the odour of jargonelle. Is used in dissolving pyroxyline when preparing celluloid.

Anthion

A name under which potassium persulphate is marketed for the elimination of hypo from plates and films.

Aqua Fortis

Nitric acid.

Aqua Regia

A mixture of nitric acid I part, hydrochloric acid 3 parts, used for dissolving gold and platinum.

Barium Chloride, BaCl22H2O, 244

Tabular transparent crystals. A solvent of gelatine, used for making baryta paper and fine-grained focussing screens for photo-micrography. Sometimes also added to salting solutions for albumenised paper.

Benzole, C6H6, 78

Also known as benzine. A colourless, volatile liquid, with a smell much resembling coal-gas, obtained by distillation of coal-tar. Gives off an explosive vapour, and boils at about 1780 Fahr. Used for removing grease and oil, for making certain varnishes, and also as a vehicle for bitumen in engraving processes.