Gold Chloride, 303.—The commercial forms usually obtained are either (I) bright yellow crystals, consisting of the chloride in combination with one equivalent of hydro chloric acid, made by dissolving gold in aqua regia and evaporation ; (2) potassio-chloride, yellow hexagonal needles ; (3) sodio-chloride, yellowish-brown deliquescent needles, the latter being the variety most commonly sold. The anhydrous chloride is of a yellowish brown colour. The comparative values are shown by the table below : Gold chloride solutions are sensitive to light, and must• therefore be kept in the dark.
Hydrochloric Acid, HCI, 36.3.—The commercial impure forms, known as muriatic acid or spirits of salt, are unsuit able for most photographic purposes. Colourless, fuming liquid, with strong smell of chlorine. A dangerous corrosive poison. Antidotes : Milk, gruel, etc., with magnesia, chalk, or other carbonates.
Hydrogen Peroxide, H202, 34.—A colourless solution of sharp taste. Powerful oxidiser and bleacher. Used in ex tremely weak solutions as a hypo eliminator (t in woo parts of water).
Hydrofluoric Acid, HF, 34.—A colourless gas, generally supplied in hydrated form, obtained by heating fluorspar with sulphuric acid in a leaden or platinum retort. Even when dilute, it causes very painful ulcers on skin and nails. Must be stored in rubber bottles, as it dissolves glass, hence its use in detaching the gelatine film from glass plates.
Iron Ammonia-Citrate (Brown), 4 2030. (Green), 5 H, 1956.—Small, shiny, reddish-brown scales of hygroscopic character. Prepared by dissolving ferric hydrate in citric acid, and adding ammonia till neutral. Solubility, in cY5 parts water. Chiefly used for the blue prussiate printing-paper.
Iron Ammonia-Oxalate (Ferric), Greenish-white crystals, decomposed by light. Prepared by dissolving ferric hydrate in acid oxalate of ammonium solution. Employed in some platinotype processes.
Iron Oxalate (Ferric), Fe2(C204)3, 376.—Obtained by precipitation, or by mixing ferric hydrate in the syrupy state with finely crystallised oxalic acid, and keeping for a few days in a dark, cool place. Not very soluble in water, but soluble in oxalic acid or alkaline oxalates. The sensitising agent in most platinotype processes.
Iron Oxalate (Ferrous), 180.—A light-yellow crystalline powder. Almost insoluble in water. Rarely used dry, and is usually created as a developing agent by the admixture of neutral potassium oxalate and acid ferrous sulphate.
Iron Perchloride (Ferric), 325.—Yellowish-red opaque masses ; very deliquescent. Solubility, 16o per cent. in cold water ; also soluble in alcohol and ether. Used as a reducing agent in copper etching, and also occasionally as a sensitiser. From its property of hardening gelatine, which becomes again soluble when, on exposure to light, the salt is converted into a ferrous chloride, perchloride of iron was formerly recommended for the making of negative transparencies by the carbon process.
Iron Sulphate (Ferrous), 278.—Green vitriol or copperas. Green crystals, which absorb oxygen from the air, when they become covered with a brownish rust. This must be rejected. Solubility, I in r5 of water. Used for the developer in the wet-plate process, and also as a constituent of the ferrous-oxalate developer.
Lead Acetate, 379.—White prismatic crystals of sweet taste ; poisonous.
Lead Nitrate, 331.—White opaque octahedral crystals ; poisonous.
Both of the above are sometimes added to combined toning baths, but are of doubtful value. They are also added to the fixing-bath (i oz. to the pint) for the purpose of direct toning by the deposition of sulphide of lead on the image. A few drops of nitric or acetic acid are required with these salts, to dissolve the certain amount of basic salt present in the solid crystals.
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, 271.—Corrosive sublimate. Heavy crystalline masses, or fine white powder. Solubility, I 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.