Chapter III (2)

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 CO{2} + 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. 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 coloring substance being separated, but not actually _destroyed_, as it is by _chlorine_ employed as a bleaching agent. This power of absorbing coloring 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.

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 odor; soluble to a considerable extent in water, the solution possessing the odor and color of the gas. It is nearly 2-1/2 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 color 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.

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, NH{4}Cl. Atomic weight, 54.

This salt, also known as muriate or hydrochlorate of ammonia, occurs in commerce in the form of colorless 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+2HO. 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 color 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.

_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 recrystallization, 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. If the citrate of silver be 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 color. 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, K, C{2}N, 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.

Ether.

Symbol, C{4}H{5}O. Atomic weight, 37.

Ether is obtained by distilling a mixture of sulphuric acid and alcohol. If the formula of alcohol (C{4}H{6}O{2}) 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._--It is neither acid nor alkaline to test-paper. Specific gravity, at 60°, about ·720. Boils at 98° Fahrenheit. The vapor 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 odor; 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 odor.

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 flour 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, C{2}HO{3}. 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 odor, 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.

Gelatine.

Symbol, C{13}H{10}O{5}N{2}. 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 or 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 f cold water will retain about three grains of isinglass without gelatinizing; but much depends upon the temperature, a few degrees greatly affecting the result.

Gelatine forms no compound with oxide of silver analogous to the albuminate of silver; which fact explains the difference in the photographic properties of albumen and gelatine.

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.

Gold, Chloride of.

Symbol, AuCl{3}. 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 color.

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 solution then contains chloride of gold in a neutral state, and free from copper and silver, with which the metallic gold is alloyed in the standard coin of the realm.

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 therefore 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 in 1 part of gold); collect the precipitated gold, which is now free from copper; re-dissolve in aqua-regia, and evaporate to dryness on a water bath.

Avoid using ammonia to neutralize chloride of gold, as it would be liable to occasion a deposit of "fulminating gold," the properties of which are described immediately following.

_Properties of Chloride of Gold._--As sold in commerce it usually contains excess of hydrochloric acid, and is then of a bright yellow color; 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 about to 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 S{2}O{2}. Atomic Weight, 253.

Hyposulphite of gold is produced by the reaction of chloride of gold upon hyposulphite of soda.

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.

_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, C{24}H{28}O{28}. Atomic weight, 366.

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 1-1/2 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 lights The action however in the case of _nitrate of silver_ is slow, unless the temperature be somewhat elevated. _Cane_ sugar does not possess these properties to an equal extent, and hence it is readily distinguished from the other variety.

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.

Hydrochloric; Acid.

Symbol, HCl. Atomic weight, 37.

Hydrochloric acid is a volatile gas, Which may be liberated from the salts termed chlorides by the action of sulphuric acid. The acid, by its superior affinities, removes the base; thus,--

NaCl + HO SO{3} = NaO SO{3} + 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 colorless, and fumes in the air. The yellow color of the commercial acid depends upon the presence of traces of perchloride of iron or 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, colorless 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 weighty 17.

This substance, also known as sulphuretted hydrogen, is a gaseous compound of sulphur and hydrogen, analogous in composition to hydrochloric and hydriodic acids. It is usually prepared by the action of dilute sulphuric acid upon sulphuret of iron, the decomposition being similar to that involved in the preparation of the hydrogen acids generally:--

FeS + HO SO{3} = FeO SO{3} + HS.

_Properties._--Cold water absorbs three times its bulk of hydrosulphuric acid, and acquires the peculiar putrid odor 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, NH{4}S HS. Atomic weight, 51.

The liquid known by this name, and formed by 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,--Colorless 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 S{2}H{2} + 5 HO. Atomic weight, 125.

The hyposulphite of soda commonly employed by photographers is a neutral combination of hyposulphurous acid and the alkali soda. It is selected as being more economical in preparation than any other hyposulphite adapted for fixing.

Hyposulphite of soda occurs in the form of large translucent groups of crystals, which include five atoms of water. These crystals are soluble in water almost to any extent, the solution being attended with the production of cold; they have a nauseous and bitter taste.

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 by 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 color 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-colored 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 color. 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 color 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 acid saturated with peroxide of nitrogen. The presence of alcohol or ether interferes to a certain extent with the result.

Iodide of Ammonium.

Symbol, NH{4}I. Atomic weight, 144.

This salt may be prepared by adding carbonate of ammonia to iodide of iron, but more easily by the following process:--A strong solution of hydrosulphate of ammonia is first made, by passing sulphuretted hydrogen gas into liquor ammoniæ To this liquid iodine is added until the whole of the sulphuret of ammonium has been converted into iodide. When this point is reached, the solution at once colors brown from solution of free iodine. On the first addition of the iodine, an escape of sulphuretted hydrogen gas and a dense deposit of sulphur take place. After the decomposition of the hydrosulphate of ammonia is complete, a portion of hydriodic acid--formed by the mutual reaction of sulphuretted hydrogen and iodine--attacks any carbonate of ammonia which may be present, and causes an effervescence. The effervescence being over, the liquid is still acid to test-paper, from excess of hydriodic acid; it is to be cautiously neutralized with ammonia, and evaporated by the heat of a water-bath to the crystallizing point.

The crystals should be thoroughly dried over a dish of sulphuric acid, and then sealed in small tubes containing each about half a drachm of the salt; by this means it will be preserved colorless.

Iodide of ammonium is very soluble in alcohol, but it is not advisable to keep it in solution, from the rapidity with which it decomposes and becomes brown.

The most common impurity of commercial iodide of ammonium is sulphate of ammonia; it is detected by its sparing insolubility in alcohol.

Iodide of Cadmium.

Symbol, CdI. Atomic weight, 182.

This salt is formed by heating filings of metallic cadmium with iodine, or by mixing the two together with addition of water. It is useful in iodizing collodion intended for keeping, since it does not become brown from liberation of free iodine with the same rapidity as the alkaline iodides.

Iodide of cadmium is very soluble both in alcohol and water; the solution yielding on evaporation large six-sided tables of a pearly lustre, which are permanent in the air. The crystalline form of this salt is a sufficient criterion of its purity.

Iodide of Iron.

Symbol, FeI. Atomic weight, 154.

Iodide of iron, in a fit state for photographic use, is easily obtained by dissolving a drachm of iodine in an ounce of _proof spirit_--that is, a mixture of equal bulks of spirits of wine and water--and adding an excess of iron filings. After a few hours, a green solution is obtained without the aid of heat. The presence of metallic iron in excess prevents the liberation of iodine and deposit of peroxide of iron which would otherwise speedily occur. 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 protoiodide 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 color; a mixture of iodide of potassium and _iodate of potash_ (KO IO{5}) 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 60°; 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 colored brown by free chlorine; also very rapidly by peroxide of nitrogen; ordinary acids, however, act less quickly, hydriodic acid being first formed, and subsequently decomposing spontaneously.

Iodide of potassium, as sold in the shops, is often contaminated with various impurities. The first and most remarkable is _carbonate of potash_. When a sample of iodide of potassium contains much carbonate of potash, it forms small and imperfect crystals, which are strongly alkaline to test-paper, and become moist on exposure to the air, from the deliquescent nature of the alkaline carbonate. _Sulphate of potash_ is also a common impurity; it may be detected by chloride of barium.

_Chloride of potassium_ is another impurity; it is detected as follows:--Precipitate the salt by an equal weight of nitrate of silver, and treat the yellow mass with solution of ammonia; if any chloride of silver is present, it dissolves in the ammonia, and after nitration is re-precipitated in white curds by the addition of an excess of pure nitric acid. If the nitric acid employed is not pure, but contains traces of free chlorine, the iodide of silver must be well washed with distilled water before treating it with ammonia, or the excess of free nitrate of silver dissolving in the ammonia would, on neutralizing, produce chloride of silver, and so cause an error.

_Iodide of potash_ is a fourth impurity often found in iodide of potassium: to detect it, add a drop of dilute sulphuric acid, or a crystal of citric acid, to the solution of the iodide; when, if much iodate be present, the liquid will become yellow from liberation of free iodine. The rationale of this reaction is as follows:--The sulphuric acid unites with the base of the salt, and liberates hydriodic acid (HI), _a colorless compound_; but if iodic acid (IO{5}) be also present, it decomposes the hydriodic acid first formed, oxidizing the hydrogen into water (HO), and setting free the iodine. The immediate production of a yellow color on adding a weak acid to aqueous solution of iodide of potassium is, therefore, a proof of the presence of an iodate. As iodate of potash is thought to render collodion insensitive (?), this point should be attended to.

Iodide of potassium may be rendered very pure by recrystallizing from spirit, or by dissolving in strong alcohol of sp. gr. ·823, in which sulphate, carbonate, and iodate of potash are insoluble. The proportion of iodide of potassium contained in saturated alcoholic solutions varies with the strength of the spirit.

Solution of chloride of barium is commonly used to detect impurities in iodide of potassium; it forms a white precipitate if carbonate, iodate, or sulphate be present. In the two former cases the precipitate dissolves on the addition of _pure_ dilute nitric acid, but in the latter it is insoluble. The commercial iodide, however, is rarely so pure as to remain quite clear on the addition of chloride of barium, a _mere opalescence_, therefore, may be disregarded.

Iodide of Silver. (_See_ Silver, Iodide of.)

Iron, Protosulphate of.

Symbol, FeO SO{3} + 7 HO. Atomic weight, 139.

This salt, often termed _copperas_ or _green vitriol_, is a most abundant substance, and used for a variety of purposes in the arts. Commercial sulphate of iron, however, being prepared on a large scale, requires recrystallization to render it sufficiently pure for photographic purposes.

Pure sulphate of iron occurs in the form of large, transparent prismatic crystals, of a delicate green color: by exposure to the air they gradually absorb oxygen and become rusty on the surface. Solution of sulphate of iron, colorless at first, afterwards changes to a red tint, and deposits a brown powder; this powder is a _basic_ persulphate of iron, that is, a persulphate containing an excess of the oxide or _base_. By the addition of sulphuric or acetic acid to the solution, the formation of a _deposit_ is prevented, the brown powder being soluble in acid liquids.

The crystals of sulphate of iron include a large quantity of water of crystallization, a part of which they lose by exposure to dry air. By a higher temperature, the salt may be rendered perfectly _anhydrous_, in which state it forms a white powder.

Aqueous solution of sulphate of iron absorbs the _binoxide of nitrogen_, acquiring a deep olive-brown color: 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 NO{5} + 7 HO. Atomic weight, 153.

This salt, by careful evaporation _in vacuo_ over sulphuric acid, forms transparent crystals, of a light green color, 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 following process is commonly followed for preparing protonitrate of iron:--

Take of nitrate of baryta 300 grains; powder and dissolve by the aid of heat in three ounces of water; then throw in, by degrees, with constant stirring, crystallized sulphate of iron, _powdered_, 320 grains. Continue to stir for about five or ten minutes. Allow to cool, and filter from the white deposit, which is the insoluble sulphate of baryta.

In place of nitrate of baryta, the nitrate of lead may be used (sulphate of lead being an insoluble salt), but the quantity required will be different. The atomic weights of nitrate of baryta and nitrate of lead are as 131 to 166; consequently 300 grains of the former are equivalent to 380 grains of the latter.

Iron, Perchloride of.

Symbol, Fe{2}Cl{3}. 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 coloring matter is extracted by a peculiar process, and afterwards made up into a paste with chalk, plaster of Paris, &c.

Litmus occurs in commerce in the form of small cubes, of a fine violet color. 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, HgCl{2}. 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 almost to any extent by the addition of free hydrochloric acid.

The protochloride of mercury is an insoluble white powder, commonly known under the name of _calomel_.

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 _skimmed_ 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 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 proportion 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, NO{5}. 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 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 colorless, 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 peroxyline. The yellow _nitrous acid_, so called, is a strong nitric acid partially saturated with the brown vapors 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 color disappears, a compound being formed which has been termed a _sulphate of nitrous acid_.

_Chemical properties._--Nitric acid is a powerful oxidizing agent; 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 color, 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 re-agent, 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, Nitrate of.)

Nitrate of Potash.

Symbol, KO NO{5}. 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.

Nitrate of potash is _an anhydrous salt_,--it contains simply nitric acid and potash, without any water of crystallization; still, in many cases, a little water is retained mechanically between the interstices of the crystals, and therefore it is better to dry before use. This may be done by laying it in a state of fine powder upon blotting-paper, close to a fire, or upon a heated metallic plate.

Nitrate of Baryta.

Symbol, BaO NO{5}. 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 NO{5}. 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 of.)

Nitro-Hydrochloric Acid.

Symbol, NO{4} + 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:--

NO{5} + HCl = NO{4} + 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.

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

Honey 1 pound.
Acid, acetic, fortiss. (Beaufoy's acid) 11 drachms.
Water 13 drachms.

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.

_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 contain some atoms 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 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 CO{2}. 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.

Pyrogallic Acid.

Symbol, C{8}H{4}O{4} (Stenhouse). Atomic weight. 84.

The term _pyro_ prefixed to gallic acid implies that the new substance is obtained by the _action of heat_ upon that body. At a temperature of about 410° Fahr., gallic acid is decomposed, and a white sublimate forms, which condenses in lamellar Crystals; this is pyrogallic acid.

Pyrogallic acid is very soluble in cold water, and in alcohol and ether; the solution decomposes and becomes brown by exposure to the air. It gives an indigo blue color with protosulphate of iron, which changes to dark green if any persulphate be present.

Although termed an _acid_, this substance is strictly _neutral_; it does not redden litmus-paper, and forms no salts. The addition of potash or soda decomposes pyrogallic acid, at the same time increasing the attraction for oxygen; hence this mixture may conveniently be employed for absorbing the oxygen contained in atmospheric air. The compounds of silver and gold are reduced by pyrogallic acid even more rapidly than by gallic acid, the reducing agent absorbing the oxygen, and becoming converted into carbonic acid and a brown matter insoluble in water.

Commercial pyrogallic acid is often contaminated with empyreumatic oil, and also with a black insoluble substance known as _metagallic acid_, which is formed when the heat is raised above the proper temperature in the process of manufacture.

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. It may be converted into nitrate of silver, sufficiently pure for photographic purposes, by dissolving it in nitric acid and evaporating the solution to the crystallizing point: or, if the quantity be small, the solution may be boiled down to complete dryness, and the residue _fused_ strongly; which decomposes the nitrate of copper, but leaves the greater portion of the silver salt unaffected. (N. B. Nitrate of silver which has undergone fusion contains nitrite of silver, and will require the addition of acetic acid if used for preparing the collodion sensitive film.)

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:--

Chloride of ammonium + oxide of silver in ammonia
= chloride of silver + ammonia + water.

Silver, Oxide of.

Symbol, AgO. Atomic weight, 116.