Nitric Acid, HNC), 63.—Aqua fortis. A colourless liquid, fuming in the air ; stains hands and 'nails yellow. Used as a preservative for gyro, for making pyroxylene, clearing negatives, etc., etc. Corrosive poison. Antidote: Chalk, magnesia, bicarbonate of soda, etc., etc.
Oxalic Acid, 126.—Colourless prisms of strong acid flavour. Solubility, 1 in to of cold water. Used principally in dissolving the oxalates. Very poisonous. Antidotes as above.
Picric Acid, C6H2(NO2)30H, 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, 516'4. — Small, brownish-red, deliquescent masses. Often used in platinum toning formula:. Very soluble.
Potassium Bichromate, 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 I in io 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, RBr, II9.—White cubical crystals of salt taste. Solubility, i in 11 parts cold water. Used as a restrainer in developers, and also in the manufacture of dry plate emulsions.
Potassium Carbonate, 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 I in part cold water. The corresponding sodium carbonate is generally to be preferred in developing formul.
Potassium Chloro-platinite, 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 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 com bination 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, K,Fe(CN),,, 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 2k parts of cold water. Used for intensification and reduction ; also in many printing and photo-mechanical processes.
Potassium Ferrocyanide, 422.—Yellow prussiate of potash. Large yellow plates or crystals. Solu bility, i 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, r in of cold water. Used as an alkali in developers, etc., etc.
Potassium Iodide, KI, i66.—White or transparent cubical crystals of sharp taste. Solubility, i in c•7 of cold water. Is used in emulsion-making, as a solvent for iodine, etc., etc.
Potassium Illetabisulpkite, 222.—White crystals, which become powdery on exposure to air, as the substance absorbs oxygen. Solubility, r 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, 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 r in 3 of cold water. The usual developer in the platinotype process ; also an ingredient of the ferrous-oxalate developer.