Magnesium, Mg, 24

A silvery white metal, commercially obtainable in the form of wire, ribbon, or powder. Used in measured quantities for flashlight, as it ignites readily and gives an extremely actinic light. The powder is used in flash-lamps, or as a flash-powder, mixed with about one and a half times its bulk of chlorate of potash.

Mercury Bichloride, HgCl2, 271

Corrosive sublimate. Heavy crystalline masses, or fine white powder. Solubility, 1 in 16 of cold water. Used for intensification of negatives. Dangerous corrosive poison, 3 grains being fatal, and may be partially absorbed through the skin. Antidote-. White of egg, or in emergency flour-paste and milk.

Nitric Acid, HNO3, 63

Aqua fortis. A colourless liquid, fuming in the air; stains hands and nails yellow. Used as a preservative for pyro, for making pyroxylene, clearing negatives, etc., etc. Corrosive poison. Antidote-. Chalk, magnesia, bicarbonate of soda, etc., etc.

Oxalic Acid, H2C2O4, 126

Colourless prisms of strong acid flavour. Solubility, 1 in 10 of cold water. Used principally in dissolving the oxalates. Very poisonous. Antidotes as above.

Picric Acid, C6h2(NO2)3OH, 139

Bright yellow plates or crystals of bitter taste, which explode when heated quickly. Picric acid is sometimes added to flashlight powders. Ammonium and sodium picrate are often recommended for making yellow orthochromatic screens.

Platinum Perchloride, H2PtCl66H2O, 516.4

Small, brownish-red, deliquescent masses. Often used in platinum toning formulae. Very soluble.

Potassium Bichromate, K2Cr2O7, 294

Bright orange-red crystals, giving a very deep-coloured solution. Poisonous, and the cause of unpleasant eruptions on the skin - any stains on the fingers should be promptly washed off with the aid of a little ammonia. Solubility, about 1 in 10 parts cold water; decomposed by alcohol. Used as a sensitiser in carbon and similar processes, as a restrainer in developers, for intensifying, reversing, and toning negatives, bromide papers, etc., etc.

Potassium Bromide, PBr, 119

White cubical crystals of salt taste. Solubility, 1 in 1 1/2 parts cold water. Used as a restrainer in developers, and also in the manufacture of dry-plate emulsions.

Potassium Carbonate, K2CO3, 138

A white crystalline powder, caustic to the taste, generally containing about 15 per cent. of water of crystallisation. Deliquescent. Used as an accelerator in development. Solubility, about 1 in 09 part cold water. The corresponding sodium carbonate is generally to be preferred in developing formulae.

Potassium Chloro-Platinite, K2PtCl4,413.4

Ruby-red, four-sided prisms, very soluble in water, and somewhat deliquescent. Used for sensitising platinotype paper, and also for toning various printing-out papers.

Potassium Cyanide, KCN, 65

Irregular, hard, opaque masses, with slight smell of prussic acid. A dangerous poison, especially on account of the symptoms, which are not easily diagnosed in time to apply the antidotes, a very dilute (1/2 per cent.) solution of potassium permanganate with cold affusion over the head and neck. Fainting, difficulty of respiration, dilated pupils, and spasmodic closure of the jaws are the chief characteristics of cyanide poisoning. If absorbed through a cut or sore, sulphate of iron should be immediately applied. Used in fixing wet collodion plates, and in combination with iodine for the removal of the silver image, after the latter has been drawn on in the waterproof ink for line work.

Potassium Ferricyanide, K3Fe(CN)6, 329

Red prussiate of potash. Deep red crystals, which become covered with a yellowish powder on exposure to the air. This must be rinsed off before using. Solubility, 1 in 2 1/2 parts of cold water. Used for intensification and reduction; also in many printing and photo-mechanical processes.

Potassium Ferrocyanide, K4Fe(CN)63H2O, 422

Yellow prussiate of potash. Large yellow plates or crystals. Solubility, 1 in 4 of cold water. Occasionally added to pyro developers with the alkalis to increase brilliancy of effect.

Potassium Hydrate, KHO, 56

Caustic potash. White deliquescent sticks. Solubility, 1 in 1/2 of cold water. Used as an alkali in developers, etc., etc.

Potassium Iodide, KI, 166

White or transparent cubical crystals of sharp taste. Solubility, 1 in 07 of cold water. Is used in emulsion-making, as a solvent for iodine, etc., etc.

Potassium Metabisulphite, K2S2O5, 222

White crystals, which become powdery on exposure to air, as the substance absorbs oxygen. Solubility, 1 in 3 cold water. Used in place of sodium sulphite (or in combination with it as a preservative). Becomes strongly acid in keeping. Used also in making up acid-fixing baths.

Potassium Oxalate, K2C2O4, 164

Colourless crystals, neutral or slightly acid in solution. (The commercial oxalate, known as salts of sorrel, is a double oxalate, and unsuitable for photographic purposes.) Solubility 1 in 3 of cold water. The usual developer in the platinotype process; also an ingredient of the ferrous-oxalate developer.

Potassium Permanganate, KMnO4, 158

Dark red and purple crystals, of needle shape, from which a deep red liquid flows on immersion in water. Known in solution as Condy's Fluid. Solubility, 1 in 16. Used to intensify negatives or carbon prints, and in very minute solutions as a hypo eliminator. To test for the presence of hypo make up a solution of 2 gr. in 20 oz. of water, and add 15 gr. caustic soda. Soak the negatives or prints for a few moments in water, and then add a few drops of the test solution. The pink colour will change to green if hypo is present.

N.B. - Permanganate stains are readily removed by a solution of oxalic acid.

Potassium Persulphate, K2S208, 270

Often used as a hypo eliminator, as it converts the hypo into the harmless sodium tetrathionate. Must be employed very cautiously in solutions, not exceeding 1/2 per cent. All hypo eliminators have a tendency to bleach, by oxidising the silver image.

Silver Bromide, AgBr, 188

An insoluble, light-yellow salt, usually prepared in emulsions by double decomposition, between nitrate of silver and one of the soluble bromide salts. Soluble in solutions of certain alkaline and cyanide salts. Becomes a grey colour by prolonged exposure to light with liberation of bromine.