Iodide of Silver.(SeeSilver, Iodide of.)
Iron, Protosulphate of.
Symbol, FeO SO{3} + 7 HO. Atomic weight, 139.
This salt, often termedcopperasorgreen 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 abasicpersulphate of iron, that is, a persulphate containing an excess of the oxide orbase. By the addition of sulphuric or acetic acid to the solution, the formation of adepositis prevented, the brown powder being soluble in acid liquids.
The crystals of sulphate of iron include a large quantityof water of crystallization, a part of which they lose by exposure to dry air. By a higher temperature, the salt may be rendered perfectlyanhydrous, in which state it forms a white powder.
Aqueous solution of sulphate of iron absorbs thebinoxide 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 evaporationin vacuoover 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 thetinctura ferri sesquichloridiof the Pharmacopœia. Commercial perchloride of iron ordinarily contains an excess of hydrochloric acid.
Litmus.
Litmus is a vegetable substance, prepared from variouslichens, 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 sometimeschloride 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 ofcalomel.
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. Henceskimmedmilk 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,coagulateon boiling, unlessan acidbe present, which probably removes a small portion of alkali with which the caseine was previously combined. The substancetermed "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 thewhey; 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 becomesbrick-redon exposure to light.
Sugar of milk, the third principal constituent, differs from both cane and grape sugar; it may be obtained by evaporatingwheyuntil 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 ofdecomposing animal matteris converted into lactic acid.
When skimmed milk is exposed to the air for some hours it gradually becomessour, 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, oraqua-fortis, is prepared by adding sulphuricacid 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 yellownitrous 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 asulphate 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 whichare soluble in water. Hence its presence cannot be determinedby any precipitating re-agent, in the same manner as that of hydrochloric and sulphuric acid.
Impurities of Commercial Nitric Acid.—These are principallychlorineandsulphuric 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,—amilkiness, 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 ina single dropof solution of chloride of barium; if sulphuric acid be present, an insoluble precipitate of sulphate of baryta will be formed.
Nitrous Acid.(SeeSilver, Nitrate of.)
Nitrate of Potash.
Symbol, KO NO{5}. Atomic weight, 102.
This salt, also termednitreorsaltpetre, 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 isan 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 areanhydrous. 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, inexcessof 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. TheIodideof lead is also very sparingly soluble in water.
Nitrate of Silver.(SeeSilver, 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 thepower 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 ofoxygen in the gaseous form. It is thenascentoxygen 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 strikingthan 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, beinginsolublein 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 andresinous 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 Pharmacopœia has a sp. gr. of 1·063, and contains about 5 per cent; of real potash. It is usually contaminated withcarbonateof 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, termedpearlash, 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 termpyroprefixed to gallic acid implies that the new substance is obtained by theaction of heatupon 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 anacid, this substance is strictlyneutral; 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 asmetagallic acid, which is formed when the heat is raised above the proper temperature in the process of manufacture.
Sel D'or.(SeeGold, Hyposulphite of.)
Silver.
Symbol, Ag. Atomic Weight, 108.
This metal, thelunaordianaof the alchemists, is foundnative 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 residuefusedstrongly; 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, withoutreference 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, termedfulminating 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 ammoniais necessarily formed. Thus:—
Chloride of ammonium + oxide of silver in ammonia = chloride of silver + ammonia + water.
Silver, Oxide of.
Symbol, AgO. Atomic weight, 116.
If a little potash or ammonia be added to solution of nitrate of silver, a brown substance is formed, which, on standing, collects at the bottom of the vessel. This is oxide of silver, displaced from its previous state of combination with nitric acid by the stronger oxide, potash. Oxide of silver is solubleto a very minute extentin pure water, the solution possessing an alkaline reaction to litmus; it is easily dissolved by nitric or acetic acid, forming a neutral nitrate or acetate; also soluble in ammonia (ammonio-nitrate of silver), and in nitrate of ammonia hyposulphite of soda, and cyanide of potassium. Long exposure to light converts it into a black substance, which is probably a suboxide.
Properties of the Suboxide of Silver.—Suboxide of silver bears the same relation to the ordinary brown protoxide of silver that subchloride bears to protochloride of silver.
It is a black powder, which assumes the metallic lustre on rubbing, and when treated with dilute acids is resolved into protoxide of silver which dissolves, and metallic silver.
Silver, Chloride of.
Symbol, AgCl. Atomic weight, 144.
Preparation of Chloride of Silver by double decomposition.—In order to illustrate this, take a solution in water of chloride of sodium or "common salt," and mix it with a solution containing nitrate of silver; immediately a dense, curdy, white precipitate falls, which is the substance in question.
In this reaction the elements change places; the chlorine leaves the sodium with which it was previously combined, and crosses over to the silver; the oxygen and nitric acid are released from the silver, and unite with the sodium: thus
Chloride of sodium + nitrate of silver = Chloride of silver + nitrate of soda.
This interchange of elements is termed by chemistsdouble decomposition.
The essential requirements in two salts intended for the preparation of chloride of silver, are simply that the first should contain chlorine, the second silver, and that both should be soluble in water; hence the chloride of potassium or ammonium may be substituted for the chloride ofsodium, and the sulphate or acetate for the nitrate of silver.
In preparing chloride of silver by double decomposition, the white clotty masses which first form must be washed repeatedly with water, in order to free them from soluble nitrate of soda, the other product of the change. When this is done, the salt is in a pure state, and may be dried, etc., in the usual way.
Properties of Chloride of Silver.—Chloride of silver differs in appearance from the nitrate of silver. It is not met with in crystals, but forms a soft white powder resembling common chalk or whiting. It is tasteless and insoluble in water; unaffected by boiling with the strongest nitric acid, but sparingly dissolved by concentrated hydrochloric acid.
Ammonia dissolves chloride of silver freely, as do solutions of hyposulphite of soda and cyanide of potassium. Concentrated solutions of alkaline chlorides, iodides, and bromides are likewise solvents of chloride of silver, but to a limited extent.
Dry chloride of silver heated to redness fuses, and concretes on cooling into a tough and semi-transparent substance, which has been termedhorn silverorluna cornea.
Placed in contact with metallic zinc or iron acidified with dilute sulphuric acid, chloride of silver is reduced to the metallic state, the chlorine passing to the other metal under the decomposing influence of the galvanic current which is established.
Preparation and properties of the Subchloride of Silver.—If a plate of polished silver be dipped in solution of perchloride of iron, or of bichloride of mercury, ablack stainis produced, the iron or mercury salt losing a portion ofchlorine, which passes to the silver and converts it superficially into subchloride of silver. This compound differs from the white chloride of silver in containing less chlorine and more of the metallic element; the composition of the latter being represented by the formula AgCl, that of the former may perhaps be written as Ag{2}Cl. (?)
Subchloride of silver is interesting to the photographer as corresponding in properties and composition with the ordinary chloride of silver blackened by light. It is a pulverulent substance of a bluish-black color, which is decomposed by ammonia, hyposulphite of soda, and cyanide of potassium, into chloride of silver which dissolves, and insoluble metallic silver.
Silver, Bromide of.
Symbol, AgBr. Atomic weight, 186.
This substance so closely resembles the corresponding salts containing, chlorine and iodine, that a short notice of it will suffice.
Bromide of silver is prepared by exposing a silvered plate to the vapor of bromine, or by adding solution of bromide of potassium to nitrate of silver. It is an insoluble substance, slightly yellow in color, and distinguished from iodide of silver by dissolving in strong ammonia and in chloride of ammonium. It is freely soluble in hyposulphite of soda and in cyanide of potassium.
Silver, Citrate of.(SeeCitric Acid.)
Silver, Iodide of.
Symbol, AgI. Atomic weight, 234.
Preparation and Properties of Iodide of Silver.—Iodide of silver may be formed in an analogous manner to thechloride, viz. by the direct action of the vapor of iodine upon metallic silver, or by double decomposition between solutions of iodide of potassium and nitrate of silver.
When prepared by the latter mode it forms an impalpable powder, the color of which varies slightly with the manner of precipitation. If the iodide of potassium be in excess, the iodide of silver falls to the bottom of the vessel nearly white; but with an excess of nitrate of silver it is of a straw-yellow tint. This point may be noticed, because the yellow salt is the one adapted for photographic use, the other being insensible to the influence of light.
Iodide of silver is tasteless and inodorous; insoluble in water and in dilute nitric acid. It is scarcely dissolved by ammonia, which serves to distinguish it from the chloride of silver, freely soluble in that liquid. Hyposulphite of soda and cyanide of potassium both dissolve iodide of silver; it is also soluble in solutions of the alkaline bromides and iodides.
Silver, Fluoride of.
Symbol, AgF. Atomic weight, 127.
This compound differs from those just 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 hydrosulphateof 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.
Silver, Nitrate of.
Symbol, AgO NO{5}. Atomic weight, 170.
Nitrate of silver is prepared by dissolving metallic silver in nitric acid. Nitric acid is a powerfully acid and corrosive substance, containing two elementary bodies united in definite proportions. These are nitrogen and oxygen; the latter being present in greatest quantity.
Nitric acid is a powerful solvent for the metallic bodies generally. To illustrate its action in that particular, as contrasted with other acids, place pieces of silver foil in two test-tubes, the one containing dilute sulphuric, the other dilute nitric acid; on the application of heat a violent action soon commences in the latter, but the former is unaffected. In order to understand the cause of the difference, it must be borne in mind that when a metallic substance dissolves in an acid, the nature of the solution is unlike that of anaqueoussolution of salt or sugar. If you take salt water, and boil it down until the whole of the water has evaporated, you obtain the salt again, with properties the same as at first; but if a similar experiment be made with a solution of silver in nitric acid, the result is different: in that case you do not get metallicsilver on evaporation, but silvercombined with oxygenandnitric acid, both of which are tightly retained, being, in fact, in a state of chemical combination with the metal.
If we closely examine the effects produced by treating silver with nitric acid, we find them to be of the following nature:—first, a certain amount of oxygen is imparted to the metal, so as to form anoxide, and afterwards this oxide dissolves in another portion of the nitric acid, producingnitrateof the oxide, or, as it is shortly termed, nitrate of silver.
It is therefore theinstabilityof nitric acid, its proneness to part with oxygen, which renders it superior to sulphuric acid in the experiment of dissolving silver. Nitric acid stands high in the list of "oxidizing agents," and it is important that the photographer should bear this fact in mind.
Properties of Nitrate of Silver.—In the preparation of nitrate of silver, when the metal has dissolved, the solution is boiled down in order to drive off the excess of nitric acid, and set aside to crystallize. The salt, however, as so obtained is still acid to test-paper, and requires either recrystallization, or a careful heating to about 300° Fahrenheit, to render it perfectly neutral.
Pure nitrate of silver occurs in the form of white crystalline plates, which are very heavy and dissolve readily in an equal weight of cold water. The solubility is much lessened by the presence of free nitric acid, and in theconcentratednitric acid the crystals are almost insoluble. Boiling alcohol takes up about one-fourth part of its weight of the crystallized nitrate, but deposits nearly the whole on cooling. Nitrate of silver has an intensely bitter and nauseous taste; acting as a caustic, and corroding theskin by a prolonged application. Its aqueous solution is perfectly neutral to test-paper.
Heated in a crucible the salt melts, and when poured into a mould and solidified, forms thelunar causticof commerce. At a still higher temperature it is decomposed, and bubbles of oxygen gas are evolved. The melted mass, cooled and dissolved in water, leaves behind a black powder, and yields a solution which is faintly alkaline to test-paper. The alkalinity depends upon the presence ofnitriteof silver associated with excess of oxide, in the form probably of a basic orsub-nitrite of silver.[B]
[B]Nitrite of silver differs from the nitrate in containing less oxygen, and is formed from it by the abstraction of two atoms of that element.
[B]Nitrite of silver differs from the nitrate in containing less oxygen, and is formed from it by the abstraction of two atoms of that element.
Solution of nitrate of silver is decomposed by iron, zinc, copper, mercury, etc., the nitric acid and oxygen passing to the other metal, and metallic silver being precipitated.
Silver, Nitrite of.
Symbol, AgO NO{3}. Atomic weight, 154.
Nitrite of silver is a compound of nitrous acid, or NO{3}, 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 nitrate 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 ofpure 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, NO{4}, is also decomposed by water and yields the same products.
Silver, Acetate of.
Symbol, AgO (C{4}H{3}O{3}). 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. Ifacetic acidbe 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.
Silver, Hyposulphite of.
Symbol, AgO S{2}O{3} . Atomic weight, 164.
In order to understand, more fully howdecompositionof hyposulphite of silver may affect the process of fixing, thepeculiar properties of this salt should be studied. With this view nitrate of silver and hyposulphite of soda may be mixed in equivalent proportions, viz. about twenty-one grains of the former salt to sixteen grains of the latter, first dissolving each in separate vessels in half an ounce of distilled water. These solutions are to be added to each other and well agitated; immediately a dense deposit forms, which is hyposulphite of silver.
At this point a curious series of changes commences. The precipitate, at first white and curdy, soon alters in color: it becomes canary-yellow, then of a rich orange-yellow, afterwards liver-color, and finally black. Therationaleof these changes is explained to a certain extent by studying the composition of the hyposulphite of silver.
The formula for this substance is as follows:—
AgO S{2}O{2},
But AgO S{2}O{2} plainly equals AgS, or sulphuret of silver, and SO{3}, or sulphuric acid. The acid reaction assumed by the supernatant liquid is due therefore to sulphuric acid, and the black substance formed is sulphuret of silver. The yellow and orange-yellow compounds are earlier stages of the decomposition, but their exact nature is uncertain.
The instability of hyposulphite of silver is principally seen when, it is in an isolated state: the presence of an excess of hyposulphite of soda renders it more permanent, by forming a double salt.
In fixing photographic prints this brown deposit of sulphuret of silver is very liable to form in the bath and upon the picture; particularly so when thetemperatureis high. To obviate it observe the following directions:—It isespecially in the reaction betweennitrate of silverand hyposulphite of soda that the blackening is seen; the chloride and otherinsolublesalts of silver being dissolved, even to saturation, without any decomposition of the hyposulphite first formed. Hence, if the print be washed in water to remove the soluble nitrate, a very much weaker fixing bath than usual may be employed. This plan, however, involving a little additional trouble, is, on that account, often objected to, and, when such is the case, aconcentratedsolution of hyposulphite of soda must be used, in order to dissolve off the white hyposulphite of silver before it begins to decompose. When the proofs are taken at once from the printing frame and immersed in adilutebath of hyposulphite (one part of the salt to six or eight of water),a shade of brownmay often be observed to pass over the surface of the print, and a large deposit of sulphuret of silver soon forms as the result of this decomposition. On the other hand, with a strong hyposulphite bath there is little or no discoloration and the black deposit is absent.
But even if, by a preliminary removal of the nitrate of silver, the danger of blackening be in a great measure obviated, yet the print must not be taken out of the fixing bath too speedily, or some appearance of brown patches, visible by transmitted light, may occur.
Each atom of nitrate of silver requiresthreeatoms of hyposulphite of soda to form thesweet and soluble double salt, and hence, if the action be not continued sufficiently long, another compound will be formed almost tasteless and insoluble. Even immersion in a new bath of hyposulphite of soda does not fix the print when once the yellow stage of decomposition has been established. Thisyellow salt is insoluble in hyposulphite of soda, and consequently remains in the paper.
Sugar of Milk.(SeeMilk.)
Sulphuretted Hydrogen.(SeeHydrosulphuric Acid.)
Sulphuric Acid.
Symbol, SO{3}. Atomic weight, 40.
Sulphuric acid maybe 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 (SO{2}), 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 SO{2} into SO{3}, 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.
Thismono-hydratedsulphuric acid, represented by the formula HO SO{3}, 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 notfumein the same manner as nitric or hydrochloric acid. The concentrated acid may be cooled down even to zero without solidifying; but a weaker compound,containing twice the quantity of water, and termedglacialsulphuric 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 known as nitro-sulphuric acid.
Sulphuric acid possesses intense chemical powers, and displaces the greater number of ordinary acids from their salts. Itcharsorganic substances, by removing the elements of water, and converts alcohol into ether in a similar manner. Thestrengthof a given sample of sulphuric acid may generally be calculated from its specific gravity, and a table is given by Dr. Ure for that purpose.
Impurities of Commercial Sulphuric Acid.—The liquid acid sold asoil of vitriolis tolerably constant in composition, and seems to be as well adapted for photographic use as thepuresulphuric 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.
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 acrystalline precipitate will form, caused by the sparing solubility of the chloride of barium itself in acid solutions.
Sulphurous Acid.
Symbol, SO{2}. 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 odor, 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.
Water.
Symbol, H{2}O. Atomic weight, 9.
Water is an oxide of hydrogen, containing single atoms of each of the gases.
Distilled wateris 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.Puredistilled 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 discolors 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 ofammonia, 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, tinging it of a brown color and imparting an unpleasant smell, it must be rejected.
Springorriverwater, 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 ofpurenitric 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 adilutesolution 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. Thephotographic nitrate bathcannot 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. Thechlorides it contains are precipitated by the nitrate of silver, leaving solublenitratesin 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. 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.
Black Varnish.
Asphaltum, dissolved in Spirits or Oil of Turpentine.—The asphaltum may be coarsely pulverized and put into a bottle containing the turpentine, and in a few hours, if it be occasionally shaken, it will be dissolved and ready for use. It should be of about the consistency of thick paste.
I use the above, but will now give two more compositions, for any who may wish to adopt them:
Black Japan.—Boil together a gallon of boiled linseed oil, 8 ounces of amber, and 3 ounces of asphaltum. When sufficiently cool, thin it with oil of turpentine.
Brunswick Black.—Melt 4 lbs. of asphaltum, add 2 lbs. of hot boiled linseed oil, and when sufficiently cool, add a gallon of oil of turpentine.
The following is fromHumphrey's Journal, Vol. viii, number 16.
Black Varnish.—I generally purchase this from the dealer; but I have made an article which answered the purpose well, by dissolving pulverized asphaltum in spirits of turpentine. Any of the black varnishes can be improved by the addition of a little bees'-wax to it. It is less liable to crack and gives an improved gloss.
Before closing this chapter, it has been thought advisable to remark, that one of the most important departments of Photography is the practice of its chemistry. Many of the annoying failures experienced by those who are just engaging in the practice of the art, arise from the want of good and pure chemical agents, and the mostcertain way to avoid this, is to purchase them only from persons who thoroughly understand both their nature and mode of application. As many who may read this work might wish to know the prices of the various articles employed in the practice of the processes given, they can be informed by addressing the author, who will furnish them with a printed Price List.
PRACTICAL DETAILS
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POSITIVE
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AMBROTYPE PROCESS.