VOCABULARY OF PHOTOGRAPHIC CHEMICALS.

ACETIC ACID.

Symbol, C₄H₃O₃ + HO. Atomic weight, 60.

Acetic Acid is a product of the oxidation of Alcohol. Spirituous liquids, when perfectly pure, are not affected by exposure to air; but if a portion of yeast, or Nitrogenous organic matter of any kind, be added, it soon acts as a ferment, and causes the spirit to unite with oxygen derived from the atmosphere, and so to become sour from formation of Acetic Acid, or "vinegar."

Acetic Acid is also produced on a large scale by heating wood in close vessels: a substance distils over which is Acetic Acid contaminated with empyreumatic and tarry matter; it is termed Pyroligneous Acid, and is much used in commerce.

The most concentrated Acetic Acid may be obtained by neutralizing common vinegar with Carbonate of Soda, and crystallizing out the Acetate of Soda so formed; this Acetate of Soda is then distilled with Sulphuric Acid, which removes the Soda and liberates Acetic Acid: the Acetic Acid being volatile, distils over, and may be condensed.

Properties of Acetic Acid.—The strongest acid contains only a single atom of water; it is sold under the name of "Glacial Acetic Acid," so called from its property of solidifying at a moderately low temperature. At about 50° the crystals melt, and form a limpid liquid of pungent odour and a density nearly corresponding to that of water; the specific gravity of Acetic Acid however is no test of its real strength, which can only be estimated by analysis.

The commercial Glacial Acetic Acid is often diluted with water, which may be suspected if it does not solidify during the cold winter months. Sulphurous and Hydrochloric Acids are also common impurities. They are injurious in Photographic Processes, from their property of precipitating Nitrate of Silver. To detect them proceed as follows:—dissolve a small crystal of Nitrate of Silver in a few drops of water, and add to it about half a drachm of the Glacial Acid; the mixture should remain quite clear even when exposed to the light. Hydrochloric and Sulphurous Acid produce a white deposit of Chloride or Sulphite of Silver; and if Aldehyde or volatile tarry matter be present in the Acetic Acid, the mixture with Nitrate of Silver, although clear at first, becomes discoloured by the action of light.

Glacial Acetic Acid sometimes has a smell of garlic. In this state it probably contains an organic Sulphur Acid, and is unfit for use.

Many employ a cheaper form of Acetic Acid, sold by druggists as "Beaufoy's" acid; it should be of the strength of the Acetic Acid fortiss. of the London Pharmacopoeia, containing 30 per cent, real acid. It will be advisable to test it for Sulphuric Acid (see Sulphuric Acid), and other impurities, before use.

ACETATE OF SILVER. See Silver, Acetate of.

ALBUMEN.

Albumen is an organic principle found both in the animal and vegetable kingdom. Its properties are best studied in the white of egg, which is a very pure form of Albumen.

Albumen is capable of existing in two states; in one of which it is soluble, in the other insoluble, in water. The aqueous solution of the soluble variety gives a slightly alkaline reaction to test-paper; it is somewhat thick and glutinous, but becomes more fluid on the addition of a small quantity of an alkali, such as Potash or Ammonia.

Soluble Albumen may be converted into the insoluble form in the following ways:—

1. By the application of heat.—A moderately strong solution of Albumen becomes opalescent and coagulates on being heated to about 150° Fahrenheit, but a temperature of 212° is required if the liquid is very dilute. A layer of dried Albumen cannot easily be coagulated by the mere application of heat.

2. By addition of strong acids.—Nitric Acid coagulates Albumen perfectly without the aid of heat. Acetic Acid however acts differently, appearing to enter into combination with the Albumen, and forming a compound soluble in warm water acidified by Acetic Acid.

3. By the action of metallic salts.—Many of the salts of the metals coagulate Albumen completely. Nitrate of Silver does so; also the Bichloride of Mercury. Ammoniacal Oxide of Silver however does not coagulate Albumen.

The white precipitate formed on mixing Albumen with Nitrate of Silver is a chemical compound of the animal matter with Protoxide of Silver. This substance, which has been termed Albuminate of Silver, is soluble in Ammonia and Hyposulphite of Soda; but after exposure to light, or heating in a current of Hydrogen gas, it assumes a brick-red colour, being probably reduced to the condition of an organic compound of a Suboxide of Silver. It is then almost insoluble in Ammonia, but enough dissolves to tinge the liquid wine-red. The red coloration of solution of Nitrate of Silver employed in sensitizing the Albuminized photographic paper is probably produced by the same compound, although, often referred to the presence of Sulphuret of Silver.

Albumen also combines with Lime and Baryta. When Chloride of Barium is used with Albumen, a white precipitate of this kind usually forms.

Chemical composition of Albumen.—Albumen belongs to the Nitrogenous class of organic substances (see page 325). It also contains small quantities of Sulphur and Phosphorus.

ALCOHOL.

Symbol, C₄H₆O₂. Atomic weight, 46.

Alcohol is obtained by the careful distillation of any spirituous or fermented liquor. If wine or beer be placed in a retort, and heat applied, the Alcohol, being more volatile than water, rises first, and is condensed in an appropriate receiver; a portion of the vapour of water however passes over with the Alcohol, and dilutes it to a certain extent, forming what is termed "Spirits of Wine." Much of this water may be removed by redistillation from Carbonate of Potash, in the manner described at page 196 of this work; but in order to render the Alcohol thoroughly anhydrous, it is necessary to employ Quicklime, which possesses a still greater attraction for water. An equal weight of this powdered lime is mixed with strong Alcohol of ·823, and the two are distilled together.

Properties of Alcohol.—Pure anhydrous Alcohol is a limpid liquid, of an agreeable odour and pungent taste; sp. gr. at 60°, ·794. It absorbs vapour of water, and becomes diluted by exposure to damp air; boils at 173° Fahr. It has never been frozen.

Alcohol distilled from Carbonate of Potash has a specific gravity of ·815 to ·823, and contains 90 to 93 per cent, of real spirit.

The specific gravity of ordinary rectified Spirits of Wine is usually about ·840, and it contains 80 to 83 per cent, of absolute Alcohol.

AMMONIA.

Symbol, NH₃ or NH₄O. Atomic weight, 17.

The liquid known by this name is an aqueous solution of the volatile gas Ammonia. Ammoniacal gas contains one atom of Nitrogen combined with three of Hydrogen: these two elementary bodies exhibit no affinity for each other, but they can be made to unite under certain circumstances, and the result is Ammonia.

Properties of Ammonia.—Ammoniacal gas is soluble in water to a large extent; the solution possessing those properties which are termed alkaline (see page 308). Ammonia however differs from the other alkalies in one important particular—it is volatile: hence the original colour of turmeric-paper affected by Ammonia is restored on the application of heat. Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is converted into Carbonate of Ammonia; it should therefore be preserved in stoppered bottles. Besides Carbonate, commercial Ammonia often contains Chloride of Ammonium, recognized by the white precipitate given by Nitrate of Silver after acidifying with pure Nitric Acid.

The strength of commercial Ammonia varies greatly; that sold for pharmaceutical purposes under the name of Liquor AmmoniÆ, contains about 10 per cent, of real Ammonia. The sp. gr. of aqueous Ammonia diminishes with the proportion of Ammonia present, the Liquor AmmoniÆ being usually about ·936.

Ammonia, although forming a large class of salts, appears at first sight to contrast strongly in composition with the alkalies proper, such as Potash and Soda. Mineral bases generally are protoxides of metals, as already shown at page 308, but Ammonia consists simply of Nitrogen and Hydrogen united without Oxygen. The following remarks may perhaps tend somewhat to elucidate the difficulty:—

Theory of Ammonium.—This theory supposes the existence of a substance possessing the properties of a metal, but differing from metallic bodies generally in being compound in structure: the formula assigned to it is NH₄, one atom of Nitrogen united with four of Hydrogen. This hypothetical metal is termed "Ammonium;" and Ammonia, associated with an atom of water, may be viewed as its Oxide, for NH₃ + HO plainly equals NH₄O. Thus, as Potash is the Oxide of Potassium, so Ammonia is the Oxide of Ammonium.

The composition of the salts of Ammonia is on this view assimilated to those of the alkalies proper. Thus, Sulphate of Ammonia is a Sulphate of the Oxide of Ammonium; Muriate or Hydrochlorate of Ammonia is a Chloride of Ammonium, etc.

AMMONIO-NITRATE OF SILVER. See Silver, Ammonio-Nitrate of.

AQUA-REGIA. See Nitro-Hydrochloric Acid.

BARYTA, NITRATE OF. See Nitrate of Baryta.

BICHLORIDE OF MERCURY. See Mercury, Bichloride of.

BROMINE.

Symbol, Br. Atomic weight, 78.

This elementary substance is obtained from the uncrystallizable residuum of sea-water, termed bittern. It exists in the water in very minute proportion, combined with Magnesium in the form of a soluble Bromide of Magnesium.

Properties.—Bromine is a deep reddish-brown liquid of a disagreeable odour, and fuming strongly at common temperatures; sparingly soluble in water (1 part in 23, LÖwig), but more abundantly so in Alcohol, and especially in Ether. It is very heavy, having a specific gravity of 3·0.

Bromine is closely analogous to Chlorine and Iodine in its chemical properties. It stands on the list intermediately between the two; its affinities being stronger than those of Iodine, but weaker than Chlorine (see Chlorine).

It forms a large class of salts, of which the Bromides of Potassium, Cadmium, and Silver are the most familiar to Photographers.

BROMIDE OF POTASSIUM.

Symbol, KBr. Atomic weight, 118.

Bromide of Potassium is prepared by adding Bromine to Caustic Potash, and heating the product, which is a mixture of Bromide of Potassium and Bromate of Potash, to redness, in order to drive off the Oxygen from the latter salt. It crystallizes in anhydrous cubes, like the Chloride and Iodide of Potassium; it is easily soluble in water, but more sparingly so in Alcohol; it yields red fumes of Bromine when acted upon by Sulphuric Acid.

BROMIDE OF SILVER. See Silver, Bromide of.

CARBONATE OF SODA.

Symbol, NaO CO₂ + 10 Aq.

This salt was formerly obtained from the ashes of seaweeds, but is now more economically manufactured on a large scale from common salt. The Chloride of Sodium is first converted into Sulphate of Soda, and afterwards the Sulphate into Carbonate of Soda.

Properties.—The perfect crystals contain ten atoms of water, which are driven off by the application of heat, leaving a white powder—the anhydrous Carbonate. Common Washing Soda is a neutral Carbonate, contaminated to a certain extent with Chloride of Sodium and Sulphate of Soda. The Carbonate used for effervescing draughts is either a Bicarbonate with 1 atom of water, or a Sesquicarbonate, containing about 40 per cent, of real alkali; it is therefore nearly double as strong as the washing Carbonate, which contains about 22 per cent, of Soda. Carbonate of Soda is soluble in twice its weight of water at 60°, the solution being strongly alkaline.

CARBONATE OF POTASH. See Potash, Carbonate of.

CASEINE. See Milk.

CHARCOAL, ANIMAL.

Animal Charcoal is obtained by heating animal substances, such as bones, dried blood, horns, etc., to redness, in close vessels, until all volatile empyreumatic matters have been driven off, and a residue of Carbon remains. When prepared from bones it contains a large quantity of inorganic matter in the shape of Carbonate and Phosphate of Lime, the former of which produces alkalinity in reacting upon Nitrate of Silver (see p. 89). Animal Charcoal is freed from these earthy salts by repeated digestion in Hydrochloric Acid; but unless very carefully washed it is apt to retain an acid reaction, and so to liberate free Nitric Acid when added to solution of Nitrate of Silver.

Properties.—Animal Charcoal, when pure, consists, solely of Carbon, and burns away in the air without leaving any residue: it is remarkable for its property of decolorizing solutions; the organic colouring substance being separated, but not actually destroyed, as it is by Chlorine employed as a bleaching agent. This power of absorbing colouring matter is not possessed in an equal degree by all varieties of Charcoal, but is in great measure peculiar to those derived from the animal kingdom.

CHINA CLAY, OR KAOLIN.

This is prepared, by careful levigation, from mouldering granite and other disintegrated felspathic rocks. It consists of the Silicate of Alumina,—that is, of Silicic Acid or Flint, which is an Oxide of Silicon, united with the base Alumina (Oxide of Aluminum). Kaolin is perfectly insoluble in water and acids, and produces no decomposition in solution of Nitrate of Silver. It is employed by Photographers to decolorize solutions of Nitrate of Silver which have become brown from the action of Albumen or other organic matters.

Commercial Kaolin may contain chalk, in which state it produces alkalinity in solution of Nitrate of Silver. The impurity, detected by its effervescence with acids, is removed by washing the Kaolin in diluted vinegar and subsequently in water.

CHLORINE.

Symbol, Cl. Atomic weight, 36.

Chlorine is a chemical element found abundantly in nature, combined with metallic Sodium in the form of Chloride of Sodium, or Sea-salt.

Preparation.—By distilling common Salt with Sulphuric Acid, Sulphate of Soda and Hydrochloric Acid are formed. Hydrochloric Acid contains Chlorine combined with Hydrogen; by the action of nascent Oxygen (see Oxygen), the Hydrogen may be removed in the form of water, and the Chlorine left alone.

Properties.—Chlorine is a greenish-yellow gas, of a pungent and suffocating odour; soluble to a considerable extent in water, the solution possessing the odour and colour of the gas. It is nearly 2½ times as heavy as a corresponding bulk of atmospheric air.

Chemical properties.—Chlorine belongs to a small natural group of elements which contains also Bromine, Iodine, and Fluorine. They are characterized by having a strong affinity for Hydrogen, and also for the metals; but are comparatively indifferent to Oxygen. Many metallic substances actually undergo combustion when projected into an atmosphere of Chlorine, the union between the two taking place with extreme violence. The characteristic bleaching properties of Chlorine gas are explained in the same manner:—Hydrogen is removed from the organic substance, and in that way the structure is broken up and the colour destroyed.

Chlorine is more powerful in its affinities than either Bromine or Iodine. The salts formed by these three elements are closely analogous in composition and often in properties. Those of the Alkalies, Alkaline Earths, and many of the Metals, are soluble in water; but the Silver salts are insoluble; the Lead salts sparingly so.

The combinations of Chlorine, Bromine, Iodine, and Fluorine, with Hydrogen, are acids, and neutralize Alkalies in the usual, manner, with formation of Alkaline Chloride and water (see page 311).

The test by which the presence of Chlorine is detected, either free or in combination with bases, is Nitrate of Silver; it gives a white curdy precipitate of Chloride of Silver, insoluble in Nitric Acid, but soluble in Ammonia. The solution of Nitrate of Silver employed as the test must not contain Iodide of Silver, as this compound is precipitated by dilution.

CHLORIDE OF AMMONIUM.

Symbol, NH₄Cl. Atomic weight, 54.

This salt, also known as Muriate or Hydrochlorate of Ammonia, occurs in commerce in the form of colourless and translucent masses, which are procured by sublimation, the dry salt being volatile when strongly heated. It dissolves in an equal weight of boiling, or in three parts of cold water. It contains more Chlorine in proportion to the weight used than Chloride of Sodium, the atomic weights of the two being as 54 to 60.

CHLORIDE OF BARIUM.

Symbol, BaCl + 2 HO. Atomic weight, 123.

Barium is a metallic element very closely allied to Calcium, the elementary basis of Lime. The Chloride of Barium is commonly employed as a test for Sulphuric Acid, with which it forms an insoluble precipitate of Sulphate of Baryta. It is also said to affect the colour of the Photographic image when used in preparing Positive paper, which may possibly be due to a chemical combination of Baryta with Albumen; but it must be remembered that this Chloride, from its high atomic weight, contains less Chlorine than the alkaline Chlorides (see page 124).

Properties of Chloride of Barium.—Chloride of Barium occurs in the form of white crystals, soluble in about two parts of water, at common temperature. These crystals contain two atoms of water of crystallization, which are expelled at 212°, leaving the anhydrous Chloride.

CHLORIDE OF GOLD. See Gold, Chloride of.

CHLORIDE OF SODIUM.

Symbol, NaCl. Atomic weight, 60.

Common Salt exists abundantly in nature, both in the form of solid rock-salt and dissolved in the waters of the ocean.

Properties of the pure Salt.—Fusible without decomposition at low redness, but sublimes at higher temperatures; the melted salt concretes into a hard white mass on cooling. Nearly insoluble in absolute alcohol, but dissolves in minute quantity in rectified spirit. Soluble in three parts of water, both hot and cold. Crystallizes in cubes, which are anhydrous.

Impurities of Common Salt.—Table Salt often contains large quantities of the Chlorides of Magnesium and Calcium, which, being deliquescent, produce a dampness by absorption of atmospheric moisture: Sulphate of Soda is also commonly present. The salt may be purified by repeated re-crystallization, but it is more simple to prepare the pure compound directly, by neutralizing Hydrochloric Acid with Carbonate of Soda.

CHLORIDE OF SILVER. See Silver, Chloride of.

CITRIC ACID.

This acid is found abundantly in lemon-juice and in lime-juice. It occurs in commerce in the form of large crystals, which are soluble in less than their own weight of water at 60°.

Commercial Citric Acid is sometimes mixed with Tartaric Acid. The adulteration may be discovered by making a concentrated solution of the acid and adding Acetate of Potash; crystals of Bitartrate of Potash will separate if Tartaric Acid be present.

Citric Acid is tribasic. It forms with Silver a white insoluble salt, containing 3 atoms of Oxide of Silver to 1 atom of Citric Acid. When the Citrate of Silver is heated in a current of Hydrogen gas, a part of the acid is liberated and the salt is reduced to a Citrate of Suboxide of Silver; which is of a red colour. The action of white light in reddening Citrate of Silver is shown by the Author to be of a similar nature.

CYANIDE OF POTASSIUM.

Symbol, KC₂N, or KCy. Atomic weight, 66.

This salt is a compound of Cyanogen gas with the metal Potassium. Cyanogen is not an elementary body, like Chlorine or Iodine, but consists of Carbon and Nitrogen united in a peculiar manner. Although a compound substance, it reacts in the manner of an element, and is therefore (like Ammonium, previously described) an exception to the usual laws of chemistry. Many other bodies of a similar character are known.

Properties of Cyanide of Potassium.—These have been sufficiently described at page 44, to which the reader is referred.

ETHER.

Symbol, C₄H₅O. Atomic weight, 37.

Ether is obtained by distilling a mixture of Sulphuric Acid and Alcohol. If the formula of Alcohol (C₄H₆O₂) be compared with that of Ether, it will be seen to differ from it in the possession of an additional atom of Hydrogen and of Oxygen: in the reaction the Sulphuric Acid removes these elements in the form of water, and by so doing converts one atom of Alcohol into an atom of Ether. The term Sulphuric applied to the commercial Ether has reference only to the manner of its formation.

Properties of Ether.—The properties of Ether have been described to some extent at pages 85 and 195. The following particulars however may be added. It is neither acid nor alkaline to test-paper. Specific gravity, at 60°, about ·720. Boils at 98° Fahrenheit. The vapour is exceedingly dense, and may be seen passing off from the liquid and falling to the ground: hence the danger of pouring Ether from one bottle to another if a flame be near at hand.

Ether does not mix with water in all proportions; if the two are shaken together, after a short time the former rises and floats upon the surface. In this way a mixture of Ether and Alcohol may be purified to some extent, as in the common process of washing Ether. The water employed however always retains a certain portion of Ether (about a tenth part of its bulk), and acquires a strong ethereal odour; washed Ether also contains water in small quantity.

Bromine and Iodine are both soluble in Ether, and gradually react upon and decompose it.

The strong alkalies, such as Potash and Soda, also decompose Ether slightly after a time, but not immediately. Exposed to air and light. Ether is oxidized and acquires a peculiar odour (page 85).

Ether dissolves fatty and resinous substances readily, but inorganic salts are mostly insoluble in this fluid. Hence it is that Iodide of Potassium and other substances dissolved in Alcohol are precipitated to a certain extent by the addition of Ether.

FLUORIDE OF POTASSIUM.

Symbol, KF. Atomic weight, 59.

Preparation.—Fluoride of Potassium is formed by saturating Hydrofluoric Acid with Potash, and evaporating to dryness in a platinum vessel. Hydrofluoric Acid contains Fluorine combined with Hydrogen; it is a powerfully acid and corrosive liquid, formed by decomposing Fluor Spar, which is a Fluoride of Calcium, with strong Sulphuric Acid; the action which takes place being precisely analogous to that involved in the preparation of Hydrochloric Acid.

Properties.—A deliquescent salt, occurring in small and imperfect crystals. Very soluble in water: the solution acting upon glass in the same manner as Hydrofluoric Acid.

FORMIC ACID.

Symbol, C₂HO₃. Atomic weight, 37.

This substance was originally discovered in the red ant (Formica rufa), but it is prepared on a large scale by distilling Starch with Binoxide of Manganese and Sulphuric Acid.

Properties.—The strength of commercial Formic Acid is uncertain, but it is always more or less dilute. The strongest acid, as obtained by distilling Formiate of Soda with Sulphuric Acid, is a fuming liquid with a pungent odour, and containing only one atom of water. It inflames the skin in the same manner as the sting of the ant.

Formic Acid reduces the Oxides of Gold, Silver, and Mercury to the metallic state, and is itself oxidized into Carbonic Acid. The alkaline formiates also possess the same properties.

GALLIC ACID.

Symbol, C₇H₃O₅ + H₃O. Atomic weight, 94.

The chemistry of Gallic Acid is sufficiently described at page 27, to which the reader is referred.

GELATINE.

Symbol, C₁₃H₁₀O₅N₂. Atomic weight, 156.

This is an organic substance somewhat analogous to Albumen, but differing from it in properties. It is obtained by subjecting bones, hoofs, horns, calves' feet, etc., to the action of boiling water. The jelly formed on cooling is termed size, or, when dried and cut into slices, glue. Gelatine, as it is sold in the shops, is a pure form of Glue. Isinglass is gelatine prepared, chiefly in Russia, from the air-bladders of certain species of sturgeon.

Properties of Gelatine.—Gelatine softens and swells up in cold water, but does not dissolve until heated: the hot solution, on cooling, forms a tremulous jelly. One ounce of cold water will retain about three grains of Isinglass without gelatinizing; but much depends upon the temperature, a few degrees greatly affecting the result.

When long boiled in water, and especially in presence of an acid, such as the Sulphuric, Gelatine undergoes a peculiar modification, and the Solution loses either partially or entirely its property of solidifying to a jelly.

GLYCERINE.

Fatty bodies are resolved by treatment with an alkali into an Acid—which combines with the alkali, forming a soap,—and Glycerine, remaining in solution.

Pure Glycerine, as obtained by Price's patent process of distillation, is a viscid liquid of sp. gr. about 1·23; miscible in all proportions with water and Alcohol. It is peculiarly a neutral substance, exhibiting no tendency to combine with acids or bases. It has little or no action upon Nitrate of Silver in the dark, and reduces it very slowly even when exposed to light.

GLYCYRRHIZINE.

Glycyrrhizine, obtained from the fresh root of Liquorice, is a substance intermediate in properties between a sugar and a resin. Sparingly soluble in water but very soluble in Alcohol. It precipitates strong solution of Nitrate of Silver white, but the deposit becomes reddened by exposure to light. Its preparation is described in the larger works on organic chemistry.

GOLD, CHLORIDE OF.

Symbol, AuCl₃. Atomic weight, 303.

This salt is formed by dissolving pure metallic Gold in Nitro-hydrochloric Acid, and evaporating at a gentle heat. The solution affords deliquescent crystals of a deep orange colour.

Chloride of Gold, in a state fit for Photographic use, may easily be obtained by the following process:—Place a half-sovereign in any convenient vessel, and pour on it half a drachm of Nitric Acid mixed with two and a half drachms of Hydrochloric Acid and three drachms of water; digest by a gentle heat, but do not boil the acid, or much of the Chlorine will be driven off in the form of gas. At the expiration of a few hours add fresh Aqua-Regia in quantity the same as at first, which will probably complete the solution, but if not, repeat the process a third time.

Lastly, neutralize the liquid by adding Carbonate of Soda until all effervescence ceases, and a green precipitate forms; this is Carbonate of Copper, which must be allowed several hours to separate thoroughly. The Chloride of Gold is thus freed from Copper and Silver, with which the metallic Gold is alloyed in the standard coin of the realm. The solution so prepared will be alkaline, and consequently prone to a reduction of metallic Gold: a slight extra quantity of Hydrochloric acid should therefore be added, sufficient to redden a piece of immersed litmus-paper.

The weight of a half-sovereign is about 61 grains, of which 56 grains are pure Gold. This is equivalent to 86 grains of Chloride of Gold, which will be the quantity contained in the solution.

The following process for preparing Chloride of Gold is more perfect than the last:—Dissolve the Gold coin in Aqua-Regia as before; then boil with excess of Hydrochloric Acid, to destroy the Nitric Acid,—dilute largely with distilled water, and add a filtered aqueous solution of common Sulphate of Iron (6 parts to 1 of Gold); collect the precipitated Gold, which is now free from copper; redissolve in Aqua-Regia, and evaporate to dryness on a water bath.

Avoid using Ammonia to neutralize Chloride of Gold, as it would occasion a deposit of "Fulminating Gold," the properties of which are described in the next page.

Properties of Chloride of Gold.—As sold in commerce it usually contains excess of Hydrochloric Acid, and is then of a bright yellow colour; but when neutral and somewhat concentrated, it is dark red (Leo ruber of the alchemists). It gives no precipitate with Carbonate of Soda unless heat be applied; the free Hydrochloric Acid present forms, with the alkali. Chloride of Sodium, which unites with the Chloride of Gold, and produces a double salt, Chloride of Gold and Sodium, soluble in water.

Chloride of Gold is decomposed with precipitation of metallic Gold by Charcoal, Sulphurous Acid, and many of the vegetable acids; also by Protosulphate and Protonitrate of Iron. It tinges the cuticle of an indelible purple tint. It is soluble in Alcohol and in Ether.

GOLD, FULMINATING.

This is a yellowish-brown substance, precipitated on adding Ammonia to a strong solution of Chloride of Gold.

It may be dried carefully at 212°, but explodes violently on being heated suddenly to about 290°. Friction also causes it to explode when dry; but the moist powder may be rubbed or handled without danger. It is decomposed by Sulphuretted Hydrogen.

Fulminating Gold is probably an Aurate of Ammonia, containing 2 atoms of Ammonia to 1 atom of Peroxide of Gold.

GOLD, HYPOSULPHITE OF.

Symbol, AuO S₂O₂. Atomic weight, 253.

Hyposulphite of Gold is produced by the reaction of Chloride of Gold upon Hyposulphite of Soda (see page 133).

The salt sold in commerce as Sel d'or is a double Hyposulphite of Gold and Soda, containing one atom of the former salt to three of the latter, with four atoms of water of crystallization. It is formed by adding one part of Chloride of Gold, in solution, to three parts of Hyposulphite of Soda, and precipitating the resulting salt by Alcohol: the Chloride of Gold must be added to the Hyposulphite of Soda, and not the Soda salt to the Gold (see page 250).

Properties.—Hyposulphite of Gold is unstable and cannot exist in an isolated state, quickly passing into Sulphur, Sulphuric Acid, and metallic Gold. When combined with excess of Hyposulphite of Soda in the form of Sel d'or, it is more permanent.

Sel d'or occurs crystallized in fine needles, which are very soluble in water. The commercial article is often impure, containing little else than Hyposulphite of Soda, with a trace of Gold. It may be analyzed by adding a few drops of strong Nitric Acid (free from Chlorine), diluting with water, and afterwards collecting and igniting the yellow powder, which is metallic Gold.

GRAPE SUGAR.

Symbol, C₂₄H₂₈O₂₈. Atomic weight, 396.

This modification of Sugar, often termed Granular Sugar, or Glucose, exists abundantly in the juice of grapes and in many other varieties of fruit. It forms the saccharine concretion found in honey, raisins, dried figs, etc. It may be produced artificially by the action of fermenting principles and of dilute mineral acids, upon Starch.

Properties.—Grape Sugar crystallizes slowly and with difficulty from a concentrated aqueous solution, in small hemispherical nodules, which are hard, and feel gritty between the teeth. It is much less sweet to the taste than Cane Sugar, and not so soluble in water (1 part dissolves in 1½ of cold water).

Grape Sugar tends to absorb Oxygen, and hence it possesses the property of decomposing the salts of the noble metals, and reducing them by degrees to the metallic state, even without the aid of light. Cane Sugar does not possess these properties to an equal extent, and hence it is readily distinguished from the other variety. The product of the action of Grape Sugar upon Nitrate of Silver appears to be a very low form of Oxide of Silver combined with organic matter.

HONEY.

This substance contains two distinct kinds of Sugar, Grape Sugar, and an uncrystallizable substance analogous to, or identical with, the Treacle found associated with common Sugar in the cane-juice. The agreeable taste of Honey probably depends upon the latter, but its reducing power on metallic oxides is due to the former. Pure Grape Sugar can readily be obtained from inspissated Honey, by treating it with Alcohol, which dissolves out the syrup, but leaves the crystalline portion.

Much of the commercial article is adulterated, and, for Photographic use, the Virgin Honey should be obtained direct from the comb.

HYDROCHLORIC ACID.

Symbol, HCl. Atomic weight, 37.

Hydrochloric Acid is a volatile gas, which may be liberated from most of the salts termed Chlorides by the action of Sulphuric Acid. The acid, by its superior affinities, removes the base; thus,—

NaCl + HO SO₃ = NaO SO₃ + HCl.

Properties.—Abundantly soluble in water, forming the liquid Hydrochloric or Muriatic Acid of commerce. The most concentrated solution of Hydrochloric Acid has a sp. gr. 1·2, and contains about 40 per cent, of gas; that commonly sold is somewhat weaker, sp. gr. 1·14 = 28 per cent, real acid.

Pure Hydrochloric Acid is colourless, and fumes in the air. The yellow colour of the commercial acid depends upon the presence of traces of Perchloride of Iron, or of organic matter; commercial Muriatic Acid also often contains a portion of free Chlorine and of Sulphuric Acid.

HYDRIODIC ACID.

Symbol, HI. Atomic weight, 127.

This is a gaseous compound of Hydrogen and Iodine, corresponding in composition to the Hydrochloric Acid. It cannot however, from its instability, be obtained in the same manner, since, on distilling an Iodide with Sulphuric Acid, the Hydriodic Acid first formed is subsequently decomposed into Iodine and Hydrogen. An aqueous solution of Hydriodic Acid is easily prepared by adding Iodine to water containing Sulphuretted Hydrogen gas; a decomposition takes place, and Sulphur is set free: thus, HS + I = HI + s.

Properties.—Hydriodic Acid is very soluble in water, yielding a strongly acid liquid. The solution, colourless at first, soon becomes brown from decomposition, and liberation of free Iodine. It may be restored to its original condition by adding solution of Sulphuretted Hydrogen.

HYDROSULPHURIC ACID.

Symbol, HS. Atomic weight, 17.

This substance, also known as Sulphuretted Hydrogen, is a gaseous compound of Sulphur and Hydrogen, analogous in composition to the Hydrochloric and Hydriodic Acid. It is usually prepared by the action of dilute Sulphuric Acid upon Sulphuret of Iron, as described at page 373; the decomposition being similar to that involved in the preparation of the Hydrogen acids generally:—

FeS + HO SO₃ = FeO SO₃ + HS.

Properties.—Cold water absorbs three times its bulk of Hydrosulphuric Acid, and acquires the peculiar putrid odour and poisonous qualities of the gas. The solution is faintly acid to test-paper, and becomes opalescent on keeping, from gradual separation of Sulphur. It is decomposed by Nitric Acid, and also by Chlorine and Iodine. It precipitates Silver from its solutions in the form of black Sulphuret of Silver; also Copper, Mercury, Lead, etc.; but Iron and other metals of that class are not affected, if the liquid contains free acid. Hydrosulphuric Acid is constantly employed in the chemical laboratory for these and other purposes.

HYDROSULPHATE OF AMMONIA.

Symbol, NH₄S HS. Atomic weight, 51.

The liquid known by this name, and formed on passing Sulphuretted Hydrogen gas into Ammonia, is a double Sulphuret of Hydrogen and Ammonium. In the preparation, the passage of the gas is to be continued until the solution gives no precipitate with Sulphate of Magnesia, and smells strongly of Hydrosulphuric Acid.

Properties.—Colourless at first, but afterwards changes to yellow, from liberation and subsequent solution of Sulphur. Becomes milky on the addition of any acid. Precipitates, in the form of Sulphuret, all the metals which are affected by Sulphuretted Hydrogen, and, in addition, those of the class to which Iron, Zinc, and Manganese belong.

Hydrosulphate of Ammonia is employed in Photography to darken the Negative image, and also in the preparation of Iodide of Ammonium, the separation of Silver from Hyposulphite solutions, etc.

HYPOSULPHITE OF SODA.

Symbol, NaO S₂O₂ + 5 HO. Atomic weight, 125.

The chemistry of Hyposulphurous Acid and the Hyposulphite of Soda has been sufficiently described at pages 43, 129, and 137 of the present Work. The crystallized salt includes five atoms of water of crystallization.

HYPOSULPHITE OF GOLD. See Gold, Hyposulphite of.

HYPOSULPHITE OF SILVER. See Silver, Hyposulphite of.

ICELAND MOSS.

Cetraria Islandica.—A species of Lichen found in Iceland and the mountainous parts of Europe; when boiled in water, it first swells up, and then yields a substance which gelatinizes on cooling.

It contains Lichen Starch, a bitter principle soluble in Alcohol, termed "Cetrarine," and common Starch; traces of Gallic Acid and Bitartrate of Potash are also present.

IODINE.

Symbol, I. Atomic weight, 126.

Iodine is chiefly prepared at Glasgow, from kelp, which is the fused ash obtained on burning seaweeds. The waters of the ocean contain minute quantities of the Iodides of Sodium and Magnesium, which are separated and stored up by the growing tissues of the marine plant.

In the preparation, the mother-liquor of kelp is evaporated to dryness and distilled with Sulphuric Acid; the Hydriodic Acid first liberated is decomposed by the high temperature, and fumes of Iodine condense in the form of opaque crystals.

Properties.—Iodine has a bluish-black colour and metallic lustre; it stains the skin yellow, and has a pungent smell, like diluted Chlorine. It is extremely volatile when moist, boils at 350°, and produces dense violet-coloured fumes, which condense in brilliant plates. Specific gravity 4·946. Iodine is very sparingly soluble in water, 1 part requiring 7000 parts for perfect solution; even this minute quantity however tinges the liquid of a brown colour. Alcohol and Ether dissolve it more abundantly, forming dark-brown solutions. Iodine also dissolves freely in solutions of the alkaline Iodides, such as the Iodide of Potassium, of Sodium, and of Ammonium.

Chemical Properties.—Iodine belongs to the Chlorine group of elements, characterized by forming acids with Hydrogen, and combining extensively with the metals (see Chlorine). They are however comparatively indifferent to Oxygen, and also to each other. The Iodides of the alkalies and alkaline earths are soluble in water; also those of Iron, Zinc, Cadmium, etc. The Iodides of Lead, Silver, and Mercury are nearly or quite insoluble.

Iodine possesses the property of forming a compound of a deep blue colour with Starch. In using this as a test, it is necessary first to liberate the Iodine (if in combination) by means of Chlorine, or Nitric Acids saturated with Peroxide of Nitrogen. The presence of Alcohol or Ether interferes to a certain extent with the result.

IODIDE OF AMMONIUM.

Symbol, NH₄I. Atomic weight, 144.

The preparation and properties of this salt are described at page 198, to which the reader is referred.

IODIDE OF CADMIUM.

Symbol, CdI. Atomic weight, 182.

See page 199, for the preparation and properties of this salt.

IODIDE OF IRON.

Symbol, FeI. Atomic weight, 154.

Iodide of Iron is prepared by digesting an excess of Iron filings with solution of Iodine in Alcohol. It is very soluble in water and Alcohol, but the solution rapidly absorbs Oxygen and deposits Peroxide of Iron; hence the importance of preserving it in contact with metallic Iron, with which the separated Iodine may recombine. By very careful evaporation, hydrated crystals of Proto-iodide may be obtained, but the composition of the solid salt usually sold under that name cannot be depended on.

The Periodide of Iron, corresponding to the Perchloride, has not been examined, and it is doubtful if any such compound exists.

IODIDE OF POTASSIUM.

Symbol, KI. Atomic weight, 166.

This salt is usually formed by dissolving Iodine in solution of Potash until it begins to acquire a brown colour; a mixture of Iodide of Potassium and Iodate of Potash (KO IO₅) is thus formed; but by evaporation and heating to redness, the latter salt parts with its Oxygen, and is converted into Iodide of Potassium.

Properties.—It forms cubic and prismatic crystals, which should be hard, and very slightly or not at all deliquescent. Soluble in less than an equal weight of water at 6O°; it is also soluble in Alcohol, but not in Ether. The proportion of Iodide of Potassium contained in a saturated alcoholic solution, varies with the strength of the spirit:—with common Spirits of Wine, sp. gr. ·836, it would be about 8 grains to the drachm; with Alcohol rectified from Carbonate of Potash, sp. gr. ·823, 4 or 5 grains; with absolute Alcohol, 1 to 2 grains. The solution of Iodide of Potassium is instantly coloured brown by free Chlorine; also very rapidly by Peroxide of Nitrogen (page 86); ordinary acids however act less quickly, Hydriodic Acid being first formed, and subsequently decomposing spontaneously.

The impurities of commercial Iodide of Potassium, with the means to be adopted for their removal, are fully given at page 197.

IODIDE OF SILVER. See Silver, Iodide of.

IODOFORM.

The composition of this substance is analogous to that of Chloroform, Iodine being substituted for Chlorine. It is obtained on boiling together Iodine, Carbonate of Potash, and Alcohol.

Iodoform occurs in yellow nacrous crystals, which have a saffron-like odour. It is insoluble in water, but soluble in spirit.

IRON, PROTOSULPHATE OF.

Symbol, FeO SO₃ + 7 HO. Atomic weight, 139.

The properties of this salt, and of the two salifiable Oxides of Iron, are described at page 29. It dissolves in rather more than an equal weight of cold water, or in less of boiling water.

Aqueous solution of Sulphate of Iron absorbs the Binoxide of Nitrogen, acquiring a deep olive-brown colour: as this gaseous Binoxide is itself a reducing agent, the liquid so formed has been proposed as a more energetic developer than the Sulphate of Iron alone (?).

IRON, PROTONITRATE OF.

Symbol, FeO NO₃ + 7 HO. Atomic weight, 153.

This salt, by careful evaporation in vacuo over Sulphuric Acid, forms transparent crystals, of a light green colour, and containing 7 atoms of water, like the Protosulphate. It is exceedingly unstable, and soon becomes red from decomposition, unless preserved from contact with air. The preparation of solution of Protonitrate of Iron for developing Collodion Positives, is given at page 206.

IRON, PERCHLORIDE OF.

Symbol, Fe₂Cl₃. Atomic weight, 164.

There are two Chlorides of Iron, corresponding in composition to the Protoxide and the Sesquioxide respectively. The Protochloride is very soluble in water, forming a green solution, which precipitates a dirty white Protoxide on the addition of an alkali. The Perchloride, on the other hand, is dark brown, and gives a foxy-red precipitate with alkalies.

Properties.—Perchloride of Iron may be obtained in the solid form by heating Iron wire in excess of Chlorine; it condenses in the shape of brilliant and iridescent brown crystals, which are volatile, and dissolve in water, the solution being acid to test-paper. It is also soluble in Alcohol, forming the Tinctura Ferri Sesquichloridi of the Pharmacopoeia. Commercial Perchloride of Iron ordinarily contains an excess of Hydrochloric Acid.

LITMUS.

Litmus is a vegetable substance prepared from various lichens, which are principally collected on rocks adjoining the sea. The colouring matter is extracted by a peculiar process, and afterwards made up into a paste with chalk, plaster of Paris, etc.

Litmus occurs in commerce in the form of small cubes of a fine violet colour. In using it for the preparation of test-papers, it is digested in hot water, and sheets of porous paper are soaked in the blue liquid so formed. The red papers are prepared at first in the same manner, but afterwards placed in water which has been rendered faintly acid with Sulphuric or Hydrochloric Acid.

MERCURY, BICHLORIDE OF.

Symbol, HgCl₂. Atomic weight, 274.

This salt, also called Corrosive Sublimate, and sometimes Chloride of Mercury (the atomic weight of Mercury being halved), may be formed by heating Mercury in excess of Chlorine, or more economically, by subliming a mixture of Persulphate of Mercury and Chloride of Sodium.

Properties.—A very corrosive and poisonous salt, usually sold in semi-transparent, crystalline masses, or in the state of powder. Soluble in 16 parts of cold, and in 3 of hot water; more abundantly so in Alcohol, and also in Ether. The solubility in water may be increased by the addition of free Hydrochloric Acid, or of Chloride of Ammonium.

The Protochloride of Mercury is an insoluble white powder, commonly known under the name of Calomel.

METHYLIC ALCOHOL.

This liquid, known also by the names of wood naphtha and pyroxylic spirit, is one of the volatile products of the destructive distillation of wood. It is very volatile and limpid, with a pungent odour.

By a recent excise regulation, ordinary Spirit mixed with ten per cent, of wood naphtha is sold free of duty, under the name of "Methylated Spirit."

MILK.

The Milk of herbivorous animals contains three principal constituents—Fatty matter, Caseine, and Sugar; in addition to these, small quantities of the Chloride of Potassium, and of Phosphates of Lime and Magnesia, are present.

The fatty matter is contained in small cells, and forms the greater part of the cream which rises to the surface of the milk on standing; hence shimmed milk is to be preferred for Photographic use.

The second constituent, Caseine, is an organic principle somewhat analogous to Albumen in composition and properties. Its aqueous solution however does not, like Albumen, coagulate on boiling, unless an acid be present, which probably removes a small portion of alkali with which the Caseine was previously combined. The substance termed "rennet," which is the dried stomach of the calf, possesses the property of coagulating Caseine, but the exact mode of its action is unknown. Sherry-wine is also commonly employed to curdle Milk; but brandy and other spirituous liquids, when free from acid and astringent matter, have no effect.

In all these cases a portion of the Caseine usually remains in a soluble form in the whey; but when the Milk is coagulated by the addition of acids, the quantity so left is very small, and hence the use of the rennet is to be preferred, since the presence of Caseine facilitates the reduction of the sensitive Silver salts.

Caseine combines with Oxide of Silver in the same manner as Albumen, forming a white coagulum, which becomes brick-red on exposure to light.

Sugar of Milk, the third principal constituent, differs from both cane and grape sugar; it may be obtained by evaporating whey until crystallization begins to take place. It is hard and gritty, and only slightly sweet; slowly soluble, without forming a syrup, in about two and a half parts of boiling, and six of cold water. It does not ferment and form Alcohol on the addition of yeast, like grape sugar, but by the action of decomposing animal matter is converted into Lactic Acid.

When skimmed Milk is exposed to the air for some hours, it gradually becomes sour, from Lactic Acid formed in this way; and if then heated to ebullition, the Caseine coagulates very perfectly.

NITRIC ACID.

Symbol, NO₅. Atomic weight, 54.

Nitric Acid, or Aqua-fortis, is prepared by adding Sulphuric Acid to Nitrate of Potash, and distilling the mixture in a retort. Sulphate of Potash and free Nitric Acid are formed, the latter of which, being volatile, distils over in combination with one atom of water previously united with the Sulphuric Acid.

Properties.—Anhydrous Nitric Acid is a solid substance, white and crystalline, but it cannot be prepared except by an expensive and complicated process.

The concentrated liquid Nitric Acid contains 1 atom of water, and has a sp. gr. of about 1·5; if perfectly pure, it is colourless, but usually it has a slight yellow tint, from partial decomposition into Peroxide of Nitrogen: it fumes strongly in the air.

The strength of commercial Nitric Acid is subject to much variation. An acid of sp. gr. 1·42, containing about 4 atoms of water, is commonly met with. If the specific gravity is much lower than this (less than 1·36), it will scarcely be adapted for the preparation of Pyroxyline. The yellow Nitrous Acid, so called, is a strong Nitric Acid partially saturated with the brown vapours of Peroxide of Nitrogen; it has a high specific gravity, but this is somewhat deceptive, being caused in part by the presence of the Peroxide. On mixing with Sulphuric Acid, the colour disappears, a compound being formed which has been termed a Sulphate of Nitrous Acid.

In the Appendix a Table is given which exhibits the quantity of real anhydrous Nitric Acid contained in samples of different densities.

Chemical Properties.—Nitric Acid is a powerful oxidizing agent (see page 13); it dissolves all the common metals, with the exception of Gold and Platinum. Animal substances, such as the cuticle, nails, etc., are tinged of a permanent yellow colour, and deeply corroded by a prolonged application. Nitric Acid forms a numerous class of salts, all of which are soluble in water. Hence its presence cannot be determined by any precipitating reagent, in the same manner as that of Hydrochloric and Sulphuric Acid.

Impurities of Commercial Nitric Acid.—These are principally Chlorine and Sulphuric Acid; also Peroxide of Nitrogen, which tinges the acid yellow, as already described. Chlorine is detected by diluting the acid with an equal bulk of distilled water, and adding a few drops of Nitrate of Silver,—a milkiness, which, is Chloride of Silver in suspension, indicates the presence of Chlorine. In testing for Sulphuric Acid, dilute the Nitric Acid as before, and drop in a single drop of solution of Chloride of Barium; if Sulphuric Acid be present, an insoluble precipitate of Sulphate of Baryta will be formed.

NITROUS ACID. See Silver, Nitrite of.

NITRATE OF POTASH.

Symbol, KO NO₅. Atomic weight, 102.

This salt, also termed Nitre, or Saltpetre, is an abundant natural product, found effloresced upon the soil in certain parts of the East Indies. It is also produced artificially in what are called Nitre-beds.

The properties of Nitrate of Potash are described as far as necessary at page 190.

NITRATE OF BARYTA.

Symbol, BaO NO₅. Atomic weight, 131.

Nitrate of Baryta forms octahedral crystals, which are anhydrous. It is considerably less soluble than the Chloride of Barium, requiring 12 parts of cold and 4 of boiling water for solution. It may be substituted for the Nitrate of Lead in the preparation of Protonitrate of Iron.

NITRATE OF LEAD.

Symbol, PbO NO₅. Atomic weight, 166.

Nitrate of Lead is obtained by dissolving the metal, or the Oxide of Lead, in excess of Nitric Acid, diluted with 2 parts of water. It crystallizes on evaporation in white anhydrous tetrahedra and octahedra, which are hard, and decrepitate on being heated; they are soluble in 8 parts of water at 60°.

Nitrate of Lead forms with Sulphuric Acid, or soluble Sulphates, a white precipitate, which is the insoluble Sulphate of Lead. The Iodide of Lead is also very sparingly soluble in water.

NITRATE OF SILVER, See Silver, Nitrate or.

NITRO-GLUCOSE.

When 3 fluid ounces of cold Nitro-Sulphuric Acid, consisting of 2 ounces of Oil of Vitriol and 1 ounce of highly concentrated Nitric Acid, are mixed with 1 ounce of finely powdered Cane Sugar, there is formed at first a thin, transparent, pasty mass. If it is stirred with a glass rod for a few minutes without interruption, the paste coagulates as it were, and separates from the liquid as a thick tenacious mass, aggregating into lumps, which can easily be removed from the acid mixture.

This substance has a very acid and intensely bitter taste. Kneaded in warm water until the latter no longer reddens litmus-paper, it acquires a silver colour and a beautiful silky lustre. It may be used in Photography to confer intensity upon newly mixed Collodion; but is inferior to Glycyrrhizine employed for the same purpose.

NITRO-HYDROCHLORIC ACID.

Symbol, NO₄ + Cl.

This liquid is the Aqua-Regia of the old alchemists. It is produced by mixing Nitric and Hydrochloric Acids: the Oxygen contained in the former combines with the Hydrogen of the latter, forming water and liberating Chlorine, thus:—

NO₅ + HCl = NO₄ + HO + Cl.

The presence of free Chlorine confers on the mixture the power of dissolving Gold and Platinum, which neither of the two acids possesses separately. In preparing Aqua-Regia it is usual to mix one part, by measure, of Nitric Acid with four of Hydrochloric Acid, and to dilute with an equal bulk of water. The application of a gentle heat assists the solution of the metal; but if the temperature rises to the boiling point, a violent effervescence and escape of Chlorine takes place.

NITRO-SULPHURIC ACID.

For the chemistry of this acid liquid, see page 77.

OXYGEN.

Symbol, O. Atomic weight, 8.

Oxygen gas may be obtained by heating Nitrate of Potash to redness, but in this case it is contaminated with a portion of Nitrogen. The salt termed Chlorate of Potash (the composition of which is closely analogous to that of the Nitrate, Chlorine being substituted for Nitrogen) yields abundance of pure Oxygen gas on the application of heat, leaving behind Chloride of Potassium.

Chemical Properties.—Oxygen combines eagerly with many of the chemical elements, forming Oxides. This chemical affinity however is not well seen when the elementary body is exposed to the action of Oxygen in the gaseous form. It is the nascent Oxygen which acts most powerfully as an oxidizer. By nascent Oxygen is meant Oxygen on the point of separation from other elementary atoms with which it was previously associated; it may then be considered to be in the liquid form, and hence it comes more perfectly into contact with the particles of the body to be oxidized.

Illustrations of the superior chemical energy of nascent Oxygen are numerous, but none perhaps are more striking than the mild and gradual oxidizing influence exerted by atmospheric air, as compared with the violent action of Nitric Acid and bodies of that class which contain Oxygen loosely combined.

OXYMEL.

This syrup of Honey and Vinegar is prepared as follows. Take of

Stand the pot containing the Honey in boiling water until a scum rises to the surface, which is to be removed two or three times. Then add the Acetic Acid and water, and skim once more if required. Allow to cool, and it will be fit for use.

POTASH.

Symbol, KO + HO. Atomic weight, 57.

Potash is obtained by separating the Carbonic Acid from Carbonate of Potash by means of Caustic Lime. Lime is a more feeble base than Potash, but the Carbonate of Lime, being insoluble in water, is at once formed on adding Milk of Lime to a solution of Carbonate of Potash (see page 314).

Properties.—Usually met with in the form of solid lumps, or in cylindrical sticks, which are formed by melting the Potash and running it into a mould. It always contains one atom of water, which cannot be driven off by the application of heat.

Potash is soluble almost to any extent in water, much heat being evolved. The solution is powerfully alkaline (p. 308), and acts rapidly upon the skin; it dissolves fatty and resinous bodies, converting them into soaps. Solution of Potash absorbs Carbonic Acid quickly from the air, and should therefore be preserved in stoppered bottles; the glass stoppers must be wiped occasionally, in order to prevent them from becoming immovably fixed by the solvent action of the Potash upon the Silica of the glass.

The Liquor PotassÆ of the London Pharmacopoeia has a sp. gr. of 1·063, and contains about 5 per cent, of real Potash. It is usually contaminated with Carbonate of Potash, which causes it to effervesce on the addition of acids; also, to a less extent, with Sulphate of Potash, Chloride of Potassium, Silica, etc.

POTASH, CARBONATE OF.

Symbol, KO CO₂. Atomic weight, 70.

The impure Carbonate of Potash, termed Pearlash, is obtained from the ashes of wood and vegetable matter, in the same manner as Carbonate of Soda is prepared from the ashes of seaweeds. Salts of Potash and of Soda appear essential to vegetation, and are absorbed and approximated by the living tissues of the plant. They exist in the vegetable structure, combined with organic acids in the form of salts, like the Oxalate, Tartrate, etc., which, when burned are converted into Carbonates.

Properties.—The Pearlash of commerce contains large and variable quantities of Chloride of Potassium, Sulphate of Potash, etc. A purer Carbonate is sold, which is free from Sulphates, and with only a trace of Chlorides. Carbonate of Potash is a strongly alkaline salt, deliquescent, and soluble in twice its weight of cold water; insoluble in Alcohol, and employed to deprive it of water (see page 196).

PYROGALLIC ACID.

Symbol, C₈H₄O₄ (Stenhouse). Atomic weight, 84.

The chemistry of Pyrogallic Acid has been described at page 28.

SEL D'OR. See Gold, Hyposulphite of.

SILVER.

Symbol, Ag. Atomic weight, 108.

This metal, the Luna or Diana of the alchemists, is found native in Peru and Mexico; it occurs also in the form of Sulphuret of Silver.

When pure it has a sp. gr. of 10·5, and is very malleable and ductile; melts at a bright red heat. Silver does not oxidize in the air, but when exposed to an impure atmosphere containing traces of Sulphuretted Hydrogen, it is slowly tarnished from formation of Sulphuret of Silver. It dissolves in Sulphuric Acid, but the best solvent is Nitric Acid.

The standard coin of the realm is an alloy of Silver and Copper, containing about one-eleventh of the latter metal.

To prepare pure Nitrate of Silver from it, dissolve in Nitric Acid and evaporate until crystals are obtained. Then wash the crystals with a little dilute Nitric Acid, redissolve them in water, and crystallize by evaporation a second time. Lastly, fuse the product at a moderate heat, in order to expel the last traces of Nitric and Nitrous Acids.

SILVER, AMMONIO-NITRATE OF.

Crystallized Nitrate of Silver absorbs Ammoniacal gas rapidly, with production of heat sufficient to fuse the resulting compound, which is white, and consists of 100 parts of the Nitrate + 29·5 of Ammonia. The compound however which Photographers employ under the name of Ammonio-Nitrate of Silver may be viewed more simply as a solution of the Oxide of Silver in Ammonia, without reference to the Nitrate of Ammonia necessarily produced in the reaction.

Very strong Ammonia, in acting upon Oxide of Silver, converts it into a black powder, termed Fulminating Silver, which possesses the most dangerous explosive properties. Its composition is uncertain. In preparing Ammonio-Nitrate of Silver by the common process, the Oxide first precipitated occasionally leaves a little black powder behind, on re-solution; this does not appear however, according to the observations of the Author, to be Fulminating Silver.

In sensitizing salted paper by the Ammonio-Nitrate of Silver, free Ammonia is necessarily formed. Thus—

SILVER, OXIDE OF.

Symbol, AgO. Atomic weight, 116.

This compound has already been described in Part I., page 17.

SILVER, CHLORIDE OF.

Symbol, AgCl. Atomic weight, 144.

The preparation and properties of Chloride of Silver are given in Part I. page 14.

SILVER, BROMIDE OF.

Symbol, AgBr. Atomic weight, 186.

See Part I. page 17.

SILVER, CITRATE OF. See Citric Acid.

SILVER, IODIDE OF.

Symbol, AgI. Atomic weight, 234.

See Part I. page 16.

SILVER, FLUORIDE OF.

Symbol, AgF. Atomic weight, 127.

This compound differs from those last described in being soluble in water. The dry salt fuses on being heated, and is reduced by a higher temperature, or by exposure to light.

SILVER, SULPHURET OF.

Symbol, AgS. Atomic weight, 124.

This compound is formed by the action of Sulphur upon metallic Silver, or of Sulphuretted Hydrogen or Hydrosulphate of Ammonia upon the Silver salts; the decomposition of Hyposulphite of Silver also furnishes the black Sulphuret.

Sulphuret of Silver is insoluble in water, and nearly so in those substances which dissolve the Chloride, Bromide, and Iodide, such as Ammonia, Hyposulphites, Cyanides, etc.; but it dissolves in Nitric Acid, being converted into soluble Sulphate and Nitrate of Silver. (For a further account of the properties of the Sulphuret of Silver, see page 146.)

SILVER, NITRATE OF.

Symbol, AgO NO₅. Atomic weight, 170.

The preparation and properties of this salt have been explained at pages 12 and 362.

SILVER, NITRITE OF.

Symbol, AgO NO₃. Atomic weight, 154.

Nitrite of Silver is a compound of Nitrous Acid, or NO₃, with Oxide of Silver. It is formed by heating Nitrate of Silver, so as to drive off a portion of its Oxygen, or more conveniently, by mixing Nitrate of Silver and Nitrite of Potash in equal parts, fusing strongly, and dissolving in a small quantity of boiling water: on cooling, the Nitrite crystallizes out, and may be purified by pressing in blotting-paper. Mr. Hadow describes an economical method of preparing Nitrite of Silver in quantity, viz. by heating 1 part of Starch in 8 of Nitric Acid of 1·25 specific gravity, and conducting the evolved gases into a solution of pure Carbonate of Soda until effervescence has ceased. The Nitrite of Soda thus formed is afterwards added to Nitrate of Silver in the usual way.

Properties.—Nitrite of Silver is soluble in 120 parts of cold water; easily soluble in boiling water, and crystallizes, on cooling, in long slender needles. It has a certain degree of affinity for Oxygen, and tends to pass into the condition of Nitrate of Silver; but it is probable that its Photographic properties depend more upon a decomposition of the salt and liberation of Nitrous Acid.

Properties of Nitrous Acid.—This substance possesses very feeble acid properties, its salts being decomposed even by Acetic Acid. It is an unstable body, and splits up, in contact with water, into Binoxide of Nitrogen and Nitric Acid. The Peroxide of Nitrogen, NO₄, is also decomposed by water, and yields the same products.

SILVER, ACETATE OF.

Symbol, AgO (C₄H₃O₃). Atomic weight, 167.

This is a difficultly soluble salt, deposited in lamellar crystals when an Acetate is added to a strong solution of Nitrate of Silver. If Acetic Acid be used in place of an Acetate, the Acetate of Silver does not fall so readily, since the Nitric Acid which would then be liberated impedes the decomposition. Its properties have been sufficiently described at page 89.

SILVER, HYPOSULPHITE OF.

Symbol, AgO S₂O₂. Atomic weight, 164.

This salt is fully described in Part I. page 129. For the properties of the soluble double salt of Hyposulphite of Silver and Hyposulphite of Soda, see page 43.

SUGAR OF MILK. See Milk.

SULPHURETTED HYDROGEN. See Hydrosulphuric Acid.

SULPHURIC ACID.

Symbol, SO₃. Atomic weight, 40.

Sulphuric Acid may be formed by oxidizing Sulphur with boiling Nitric Acid; but this plan would be too expensive to be adopted on a large scale. The commercial process for the manufacture of Sulphuric Acid is exceedingly ingenious and beautiful, but it involves reactions which are too complicated to admit of a superficial explanation. The Sulphur is first burnt into gaseous Sulphurous Acid (SO₂), and then by the agency of Binoxide of Nitrogen gas, an additional atom of Oxygen is imparted from the atmosphere, so as to convert the SO₂ into SO₃, or Sulphuric Acid.

Properties.—Anhydrous Sulphuric Acid is a white crystalline solid. The strongest liquid acid always contains one atom of water, which is closely associated with it, and cannot be driven off by the application of heat.

This mono-hydrated Sulphuric Acid, represented by the formula HO SO₃, is a dense fluid, having a specific gravity of about 1·845; boils at 620°, and distils without decomposition. It is not volatile at common temperatures, and therefore does not fume in the same manner as Nitric or Hydrochloric Acid. The concentrated acid maybe cooled down even to zero without solidifying; but a weaker compound, containing twice the quantity of water, and termed glacial Sulphuric Acid, crystallizes at 40° Fahr. Sulphuric Acid is intensely acid and caustic, but it does not destroy the skin or dissolve metals so readily as Nitric Acid. It has an energetic attraction for water, and when the two are mixed, condensation ensues, and much heat is evolved; four parts of acid and one of water produce a temperature equal to that of boiling water. Mixed with aqueous Nitric Acid, it forms the compound know a as Nitro-Sulphuric Acid.

Sulphuric Acid possesses intense chemical powers, and displaces the greater number of ordinary acids from their salts. It chars organic substances, by removing the elements of water, and converts Alcohol into Ether in a similar manner. The strength of a given sample of Sulphuric Acid may be calculated, nearly, from its specific gravity, and a Table is given by Dr. Ure for that purpose. (See Appendix.)

Impurities of Commercial Sulphuric Acid.—The liquid acid sold as Oil of Vitriol is tolerably constant in composition, and seems to be as well adapted for Photographic use as the pure Sulphuric Acid, which is far more expensive. The specific gravity should be about 1·836 at 60°. If a drop, evaporated upon Platinum-foil, gives a fixed residue, probably Bisulphate of Potash is present. A milkiness, on dilution, indicates Sulphate of Lead (see page 186).

Test for Sulphuric Acid.—If the presence of Sulphuric Acid, or a soluble Sulphate, be suspected in any liquid, it is tested for by adding a few drops of dilute solution of Chloride of Barium, or Nitrate of Baryta. A white precipitate, insoluble in Nitric Acid, indicates Sulphuric Acid. If the liquid to be tested is very acid, from Nitric or Hydrochloric Acid, it must be largely diluted before testing, or a crystalline precipitate will form, caused by the sparing solubility of the Chloride of Barium itself in acid solutions.

SULPHUROUS ACID.

Symbol, SO₂. Atomic weight, 32.

This is a gaseous compound, formed by burning Sulphur in atmospheric air or Oxygen gas: also by heating Oil of Vitriol in contact with metallic Copper, or with Charcoal.

When an acid of any kind is added to Hyposulphite of Soda, Sulphurous Acid is formed as a product of the decomposition of Hyposulphurous Acid, but it afterwards disappears from the liquid by a secondary reaction, resulting in the production of Trithionate and Tetrathionate of Soda.

Properties.—Sulphurous Acid possesses a peculiar and suffocating odour, familiar to all in the fumes of burning Sulphur. It is a feeble acid, and escapes with effervescence, like Carbonic Acid, when its salts are treated with Oil of Vitriol. It is soluble in water.

TETRATHIONIC ACID.

Symbol, S₄O₅. Atomic weight, 104.

The chemistry of the Polythionic Acids and their salts will be found described in the First Part of this Work, page 157.

WATER.

Symbol, HO. Atomic weight, 9.

Water is an Oxide of Hydrogen, containing single atoms of each of the gases.

Distilled water is water which has been vaporized and again condensed; by this means it is freed from earthy and saline impurities, which, not being volatile, are left in the body of the retort. Pure distilled water leaves no residue on evaporation, and should remain perfectly clear on the addition of Nitrate of Silver, even when exposed to the light; it should also be neutral to test-paper.

The condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter, which discolours Nitrate of Silver, and is therefore injurious.

Rain-water, having undergone a natural process of distillation, is free from inorganic salts, but it usually contains a minute portion of Ammonia, which gives it an alkaline reaction to test-paper. It is very good for Photographic purposes if collected in clean vessels, but when taken from a common rain-water tank should always be examined, and if much organic matter be present, tingeing it of a brown colour and imparting an unpleasant smell, it must be rejected.

Spring or River water, commonly known as "hard water," usually contains Sulphate of Lime, and Carbonate of Lime dissolved in Carbonic Acid; also Chloride of Sodium in greater or less quantity. On boiling the water, the Carbonic Acid gas is evolved, and the greater part of the Carbonate of Lime (if any is present) deposits, forming an earthy incrustation on the boiler.

In testing water for Sulphates and Chlorides, acidify a portion with a few drops of pure Nitric Acid, free from Chlorine (if this is not at hand, use pure Acetic Acid); then divide it into two parts, and add to the first a dilute solution of Chloride of Barium, and to the second, Nitrate of Silver,—a milkiness indicates the presence of Sulphates in the first case or of Chlorides in the second. The Photographic Nitrate Bath cannot be used as a test, since the Iodide of Silver it contains is precipitated on dilution, giving a milkiness which might be mistaken for Chloride of Silver.

Common hard water can often be used for making a Nitrate Bath when nothing better is at hand. The Chlorides it contains are precipitated by the Nitrate of Silver, leaving soluble Nitrates in solution, which are not injurious. The Carbonate of Lime, if any is present, neutralizes free Nitric Acid, rendering the Bath alkaline in the same manner as Carbonate of Soda. (See page 89.) Sulphate of Lime, usually present in well water, is said to exercise a retarding action upon the sensitive Silver Salts, but on this point the writer is unable to give certain information.

Hard water is not often sufficiently pure for the developing fluids. The Chloride of Sodium it contains decomposes the Nitrate of Silver upon the film, and the image cannot be brought out perfectly. The New River water, however, supplied to many parts of London, is almost free from Chlorides, and answers very well. In other cases a few drops of Nitrate of Silver solution may be added, to separate the Chlorine, taking care not to use a large excess.