Silver Chloride, AgCl, 143.5

A white insoluble salt, prepared in a similar manner to the bromide, by direct admixture of nitrate of silver with one of the soluble chloride salts. Soluble in ammonia, and the same solutions as silver bromide. On exposure to light remains unchanged in the pure dry state; but in the presence of organic matter becomes gradually dark in colour with liberation of chloride, and is partially converted into metallic silver, together with, probably, a subchloride and oxychloride. Forms the principal sensitive salt in many emulsions and printing-out papers.

Silver Iodide, AgI, 235

A white or light-yellow insoluble salt, prepared by precipitation in emulsions in a similar way to the silver salts mentioned above. Less important in gelatine dry-plates than in wet-plate collodion, but in small quantities adds sensitivity and density-giving properties to bromide emulsions.

Silver Nitrate, AgNO3, 170

White tabular crystals which, when pure, are neutral or very slightly acid. Darkens on exposure to light in contact with organic matter. Solubility, 1 in 044 distilled cold water. Precipitates in common water. Used in preparing nearly all emulsions, sensitive silver papers, etc., etc. An irritant poison. Antidote : common salt, followed by emetics. Produces black stains on the skin, removable only with potassium cyanide.

Sodium Acetate, NaC2H3023H2O, 136

White crystals, slightly hygroscopic. Solubility, 1 in 3 cold water. A favourite alkali in gold toning baths.

Sodium Carbonate, Na2CO3, 106

Solubility (anhydrous), 1 in 6. Usually met with in the monohydrated form of large white crystals, which whiten on exposure to air, losing part of their water of crystallisation. Some formulae suggest the commercial form of common washing soda, but this varies much in quality, often contains sulphate, and is liable to give stains. Care must be taken when purchasing from ordinary chemists not to mistake this salt for the bicarbonate, of little value to the compounder of developers. Sodium carbonate is the most valuable alkali, being of moderate energy and reliable action. When substituting for potassium carbonate, the following are the relative quantities required according to Bolton's table:

Sodium Carbonate (anhydrous)..... .76

„ ,, (monohydrate) ..... .85

„ „ (crystallised)..... 2.07

Potassium Carbonate (anhydrous) 1 ,, „ (crystallised) . . . . 1.26

Ammonium Sesquicarbonate . . . . . 2.1

Caustic Soda ........ .29

Caustic Potash ........ .40

Sodium Chloride, NaCl,58.5

Common salt. The ordinary block quality often contains sulphates and magnesia salts. Solubility, 1 in 3 cold water. Used for chloride emulsions and papers; also in first washing water of prints to convert free nitrate, which might otherwise cause an insoluble yellow stain after fixing.

Sodium Thiosulphate, Na2S2O3.5H2O, 248

Large, irregular crystals, or, in purer form, small white crystals. The yellow efflorescent crystals should be rejected. Known commercially as hyposulphite of soda, or in photography as "hypo." Manufactured by passing sulphurous acid gas through sodium sulphide. Solubility, 1 in 06. Its discovery as a fixing agent, dissolving out the superfluous silver salts, was discovered by Sir William Herschel.

Sodium Phosphate, Na2HPo4.12H2O, 358

Large, bright, colourless crystals. Efflorescent on exposure to air. Solubility, about 1 in 6. Used as an alkali in gold toning baths.

Sodium Phosphate (Tribasic), Na3PO412H20, 380

Recommended by Lumiere as a substitute for ordinary alkalies in development {see Developing Formulae), and is said to prevent pyro stains. In substitution of salts, 100 parts of sodium carbonate should have the chemical equivalent value of 133 parts tribasic phosphate.

Sodium Sulphite, Na2SO3, 126

Large, clear crystals, efflorescent by absorption of oxygen when exposed to the air, or anhydrous as a white powder; the latter need only be used in half the quantity prescribed for the more common crystals. Used as a preservative for developing agents, and also to prevent stains on the film in development. Solubility, 1 in 2 (crystals), 1 in 4 (dry).

Thiocarbamide, Cs(NH2)2, 76

Small prismatic crystals. Solubility, I in II. A weak solution, acidified with citric acid, is recommended by some makers for brightening bromide prints, but must not be used until all traces of hypo have been removed from the paper; it is also added to developers for lantern (bromide) plates, and gives blue and violet tones. As a developer it often causes reversal.

Sulphuric Acid, H2SO4, 98

A heavy liquid. The commercial qualities of oil of vitriol contain impurities. For most photographic purposes it is expedient to substitute the less dangerous nitric or hydrochloric acid. It is highly corrosive, and great care must be taken in mixing it with water. The acid should be poured into the water drop by drop; water must never be poured into the acid. Antidotes : Chalk or bicarbonates. Water must not be administered in the early stages of vitriol poisoning.

Uranium Nitrate, UO2(N03)26H20, 504

Yellowish green deliquescent crystals. Prepared from pitchblende. Solubility, about 1 in 0.5 of water. Decomposed by light in contact with organic matter. As used for toning and intensifying, there seems some doubt as to the stability of the results. As a moderate reducer of overprinted P.O.P. paper, when added to the ordinary fixing bath it possesses a much higher value.

Sodium Borate, Na2B4O7.10H2O, 382

White crystals or powder, of sweet taste and slightly alkaline. Solubility, about I in 12. Used as a restrainer in pyro developers, but acts as an accelerator with eikonogen and hydroquinone. A favourite alkali for gold toning formulae, but the bath does not keep well.

Sodium Hydrate, NaOH, 40

Caustic soda. Very soluble. Sometimes specified in developing formulae, and for other purposes when a strong alkali is required.